Abstract:
In the present fault diagnosis method and the fault diagnosis system for an engine, an external diagnosis device: obtains continuous data on the incidences of misfire by misfiring cylinders from the vehicle&#39;s side with regard to the actually occurring misfire or past incidences of misfire; detects a misfire pattern, which is to be classified according to the presence of continuous incidences of misfire, on the basis of the continuous data; and narrows down a fault site in accordance with the misfire pattern.

Description:
TECHNICAL FIELD 
     The present invention relates to a fault diagnosing method, a fault diagnosing system, and a fault diagnosing machine (device) for engines, which track down a possible faulty region in the event of a misfire occurring in a multicylinder engine that includes a misfire detecting function. 
     BACKGROUND ART 
     There has been proposed a diagnostic apparatus for conducting a test to track down a faulty region in a misfiring engine. See, Japanese Laid-Open Patent Publication No. 05-202801 (hereinafter referred to as “JP05-202801A”″). According to JP05-202801A, the diagnostic apparatus detects an air-fuel ratio in a given cylinder, approximately at a time when fuel stops being injected into the cylinder, and determines which one of a fuel injection system and an ignition system is responsible for a misfire in the cylinder based on the detected air-fuel ratio (Abstract). More specifically, according to JP05-202801A, if the fuel injection system is responsible for a misfire and fuel is not properly injected, then the air-fuel ratio remains essentially unchanged approximately at the time that fuel stops being injected into the cylinder. However, if the ignition system is responsible for a misfire and fuel is injected, then the air-fuel ratio changes approximately at the time that fuel stops being injected into the cylinder. Based on such a finding, the diagnostic apparatus can determine which one of the fuel injection system and the ignition system is suffering from a fault (see paragraph “0009”). 
     SUMMARY OF INVENTION 
     A test for confirming operational states of various parts (function test), as disclosed in JP05-202801A, is highly tedious and time-consuming and hence is of poor efficiency, because the test needs to be conducted on each region that may possibly be responsible for a misfire. In addition, in actuality, such a test tends to make it difficult to perform a diagnosis if the misfire is not repeated or is difficult to repeat. 
     The present invention has been made in view of the above problems. It is an object of the present invention to provide a fault diagnosing method, a fault diagnosing system, and a fault diagnosing machine for engines, which are capable of greatly reducing the number of man-hours required to identify or confirm a faulty region responsible for a misfire. 
     According to the present invention, there is provided a fault diagnosing method for diagnosing an engine by tracking down a faulty region in the event of a misfire that occurs in a multicylinder engine having a misfire detecting function, using an external diagnosing machine that communicates with an engine control unit, comprising the steps of reading a diagnostic fault code representative of a misfiring cylinder from the engine control unit into the external diagnosing machine, detecting misfire patterns classified depending on whether or not successive misfires occur in the misfiring cylinder, and tracking down faulty regions according to the detected misfire patterns. 
     According to the present invention, a misfire pattern of misfires, which are occurring at present or have occurred in the past, is detected, and a faulty region is tracked down according to the detected misfire pattern. Consequently, it is possible to significantly reduce the number of man-hours required to identify or confirm a faulty region. 
     The external diagnosing machine may display, as the misfire patterns, a graph having a horizontal axis representing time and a vertical axis representing accumulated values of number of misfires. Therefore, the operator can easily identify a misfire pattern by visually confirming the graph. Hence, the operator can easily grasp the situation in relation to misfires that actually are occurring, and classify the misfires as successive misfires or not. Further, the operator can easily grasp an actual diagnostic work technique in relation to judging whether or not successive misfires are occurring, and the result of the diagnostic work technique used to indicate whether or not successive misfires are occurring. Accordingly, the operator can perform work with increased efficiency. In addition, the external diagnosing machine provides a high learning capability for improving the skills of inexperienced operators. 
     The step of detecting the misfire patterns may comprise the steps of restarting the engine in accordance with an instruction from the external diagnosing machine in an attempt to repeat a misfire in the misfiring cylinder, and if a misfire is not repeated in the misfiring cylinder, detecting the misfire pattern based on successive data generated upon occurrence of a misfire in the misfiring cylinder, which are stored in the engine control unit at a time that the diagnostic fault code is generated. Therefore, even if a misfire is not repeated upon restarting the engine, it is possible to track down faulty regions by using successive data generated upon occurrence of a misfire at a time that the diagnostic fault code is generated. 
     The step of detecting misfire patterns may comprise the steps of idling the engine in accordance with an instruction from the external diagnosing machine in an attempt to repeat a misfire in the misfiring cylinder, and if a misfire is not repeated in the misfiring cylinder, detecting the misfire pattern by causing a misfire to occur again in the misfiring cylinder by repeating an operating state of the engine, which is stored in the engine control unit at a time that the diagnostic fault code is generated. Therefore, even if it is difficult to repeat a misfire while the engine is idling, it is possible to track down faulty regions by positively causing a misfiring state to occur again. 
     The fault diagnosing method may further comprise the steps of, if a misfire is repeated in the misfiring cylinder when the engine is restarted in accordance with an instruction from the external diagnosing machine, cranking the engine to rotate a crankshaft while canceling fuel explosion in cylinders of the engine, detecting variations in angular velocity of the crankshaft while the engine is being cranked, and determining a cylinder, which has variations in angular velocity that are equal to or smaller than a predetermined value, as a compression pressure shortage cylinder that suffers from a shortage of compression pressure, and successively changing air-fuel ratios of fuel supplied to the cylinders while the engine is not under a load, and judging an air-fuel ratio failure based on a degree of occurrence of a misfire in the cylinders, thereby further tracking down faulty regions. Accordingly, it is possible, from among faulty regions that have already been tracked down based on the misfire pattern, to further track down significantly limited faulty regions, by determining a compression pressure shortage cylinder and an air-fuel ratio failure. Consequently, it is possible to further reduce the number of man-hours required to identify or confirm a faulty region. 
     The misfire patterns may represent successive misfires in a single cylinder, successive misfires in plural cylinders, random misfires in a single cylinder, and random misfires in plural cylinders. Therefore, it is possible to easily classify the misfire patterns and to efficiently track down a faulty region. 
     According to the present invention, there also is provided a fault diagnosing system for diagnosing an engine, comprising an engine control unit for detecting a misfire occurring in an engine having a plurality of cylinders and storing a diagnostic fault code representative of a misfiring cylinder, and an external diagnosing machine for tracking down faulty regions responsible for the misfire, wherein the external diagnosing machine acquires the diagnostic fault code from the engine control unit and provides guidance concerning a diagnostic work technique that depends on the diagnostic fault code, and the external diagnosing machine detects misfire patterns classified depending on whether or not successive misfires occur in the misfiring cylinder, and tracks down and displays faulty regions according to the detected misfire patterns. 
     According to the present invention, there is further provided a fault diagnosing machine for diagnosing an engine having a plurality of cylinders by tracking down faulty regions responsible for a misfire that occurs in the engine, wherein successive data upon occurrence of a misfire in a misfiring cylinder are acquired directly or indirectly from a vehicle, misfire patterns, which are classified depending on whether or not successive misfires occurs in the misfiring cylinder, are detected based on the successive data, and faulty regions are tracked down according to the detected misfire patterns. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a block diagram showing a general configuration of a fault diagnosing system according to an embodiment of the present invention; 
         FIG. 2  is a view showing a general internal structure of an engine; 
         FIG. 3  is a flowchart of a processing sequence, which is carried out when a misfire occurs in the engine while a vehicle powered by an engine is traveling normally (while the engine is operating normally); 
         FIG. 4  is a first flowchart of a processing sequence for diagnosing the engine in the event of a misfire; 
         FIG. 5  is a second flowchart of a processing sequence for diagnosing the engine in the event of a misfire; 
         FIG. 6  is a view showing positions of areas of a work guidance screen displayed upon diagnosis of the engine; 
         FIG. 7  is a diagram showing by way of example an overall work flowchart, which is displayed in an overall work display area; 
         FIG. 8  is a diagram showing by way of example a plurality of misfire patterns with corresponding misfire details, and possible faulty regions displayed in a specific work display area; 
         FIG. 9  is a diagram showing by way of example test results and possible faulty regions displayed in the specific work display area; 
         FIG. 10  is a diagram showing by way of example a relationship between strokes of a piston in each cylinder, and the magnitude of a load applied to the crankshaft as the piston operates during cranking of the engine, when the cylinder is operating normally, and when the cylinder suffers from a shortage of compression pressure; 
         FIG. 11  is a diagram showing by way of example a relationship between crankshaft angles and crankshaft angular velocities, together with strokes (intake, compression, power, and exhaust strokes) of cylinders when first through fourth cylinders are operating normally, and when the first cylinder suffers from a misfire during cranking of the engine; 
         FIG. 12  is a diagram showing a relationship between the crankshaft angles and the angular velocity variations shown in  FIG. 11 , together with power strokes of the cylinders; 
         FIG. 13  is a diagram showing by way of example individual average values of angular velocity variations in the case that a tappet clearance of the first cylinder is normal, in the case that a deviation of the tappet clearance is small, in the case that a deviation of the tappet clearance is large, and in the case that the compression pressure is zero at a time that the first cylinder is abnormal and the second through fourth cylinders are normal; 
         FIG. 14  is a diagram showing ratios of the individual average values to a total average value, based on the individual average values of the cylinders shown in  FIG. 13 ; 
         FIG. 15  is a diagram showing at an enlarged scale a portion of the ratios shown in  FIG. 14 ; and 
         FIG. 16  is a timing chart showing a relationship between engine rotational speed, air-fuel ratios of respective cylinders, and accumulated values of number of misfires occurring in the respective cylinders at a time that an air-fuel ratio failure diagnostic test is conducted. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     A. Embodiment 
     1. Arrangement 
     (1) Overall Configuration 
       FIG. 1  is a block diagram showing the general configuration of an engine diagnosing system  10  (hereinafter referred to simply as a “system  10 ”) according to an embodiment of the present invention. The system  10  includes a vehicle  12 , which incorporates an engine  16  as a target to be diagnosed, and an external diagnosing machine  14  (hereinafter also referred to as a “diagnosing machine  14 ”) for diagnosing the engine  16 . 
     (2) Vehicle  12   
     (a) Overall Configuration 
     The vehicle  12  includes, in addition to the engine  16 , an exhaust gas filter  18  (hereinafter also referred to as a “filter  18 ”) for purifying exhaust gases emitted from the engine  16 , an engine electronic control unit  20  (hereinafter referred to as an “engine ECU  20 ” or an “ECU  20 ”) for controlling operation of the engine  16 , an ignition switch  22  (hereinafter referred to by “IGSW  22 ”), an accelerator pedal  24 , a depressed angle sensor  26  for detecting a depressed angle Bap of the accelerator pedal  24 , and a warning lamp  28 . 
     (b) Engine  16   
       FIG. 2  shows a general internal structure of the engine  16 . As shown in  FIG. 2 , the engine  16  comprises a so-called in-line four-cylinder engine having an amount-of-intake-air sensor  30 , a throttle valve  32 , a degree-of-opening sensor  34 , first through fourth cylinders  36   a  through  36   d  (hereinafter referred to collectively as “cylinders  36 ”), intake valves  38 , exhaust valves  40 , fuel injection valves  42 , spark plugs  44 , an air-fuel-ratio sensor  46 , a crankshaft  48 , pistons  50 , a starter motor  52 , a crankshaft angle sensor  54 , top-dead-center sensors  56 , and a temperature sensor  58 . The intake valves  38 , the exhaust valves  40 , and the spark plugs  44  are disposed in facing relation to combustion chambers  60  of the cylinders  36   a  through  36   d.    
     The amount-of-intake-air sensor  30  detects an amount of air (hereinafter referred to as “an amount of intake air Qaf”) that is drawn into the engine  16  depending on a degree of opening θth [°] of the throttle valve  32 , and outputs the detected amount of intake air Qaf to the engine ECU  20 . The throttle valve  32  is disposed in an intake manifold  62 . The degree-of-opening sensor  34  detects the degree of opening θth of the throttle valve  32 , and outputs the detected degree of opening θth to the engine ECU  20 . The fuel injection valves  42  and the spark plugs  44  are disposed in facing relation to the combustion chambers  60  of the cylinders  36   a  through  36   d . The air-fuel-ratio sensor  46 , which includes a non-illustrated oxygen sensor, is disposed in an exhaust manifold  64 . The air-fuel-ratio sensor  46  detects an air-fuel ratio (hereinafter referred to as a “total air-fuel ratio Raf_total”) of the engine  16  in its entirety, and outputs the detected total air-fuel ratio Raf_total to the engine ECU  20 . 
     The starter motor  52  actuates the crankshaft  48  based on electric power supplied from a battery, not shown. The crankshaft angle sensor  54  detects a rotational angle (hereinafter referred to as a “crankshaft angle Ac”) [°] of the crankshaft  48 , and outputs the detected crankshaft angle Ac to the engine ECU  20 . Each of the top-dead-center sensors  56  detects a top dead center position of a corresponding piston  50 , and outputs the detected top dead center position to the engine ECU  20 . The temperature sensor  58  detects a temperature Tw [° C.] of an engine coolant, not shown, and outputs the detected temperature Tw to the engine ECU  20 . The temperature sensor  58  may detect the temperature To [° C.] of an engine oil, not shown. 
     (c) Exhaust Gas Filter  18   
     The exhaust gas filter  18 , which is disposed downstream (on the exhaust side) from the exhaust manifold  64 , purifies exhaust gases from the engine  16  and discharges the purified exhaust gases. According to the present embodiment, the exhaust gas filter  18  includes a three-way catalyst for purifying exhaust gases. 
     (d) Engine ECU  20   
     The engine ECU  20  serves to control operations of the engine  16 . As shown in  FIG. 1 , the engine ECU  20  has an input/output unit  70 , a processor  72 , and a memory  74 . 
     According to the present embodiment, the engine ECU  20  includes and carries out an engine rotational speed calculating function, a misfire counting function, a throttle valve controlling function, a fuel injection valve controlling function, and a spark plug controlling function, for example. 
     The engine rotational speed calculating function is a function to calculate a rotational speed (engine rotational speed NE) [rpm] of the engine  16  based on output signals from the top-dead-center sensors  56 . According to the present embodiment, the engine rotational speed calculating function is combined with the top-dead-center sensors  56  in order to provide an engine rotational speed sensor. Alternatively, an engine rotational speed sensor may be provided independently of the engine ECU  20 , and an output signal from the engine rotational speed sensor may be sent to the engine ECU  20 . 
     The misfire counting function is a function to judge whether or not a misfire is occurring in each of the cylinders  36   a  through  36   d  based on output signals from the crankshaft angle sensor  54 , and to count misfires that have been judged as occurring in each of the cylinders  36   a  through  36   d . According to the present embodiment, the misfire counting function is combined with the crankshaft angle sensor  54  in order to provide a misfire counter. A misfire may be judged as occurring by known means. For example, a combustion pressure at a given crankshaft angle may be detected, and a misfire may be judged as occurring if the detected combustion pressure is equal to or lower than a predetermined value. Alternatively, a misfire may be judged as occurring if a combustion pressure at a given crankshaft angle is equal to or lower than a predetermined value. 
     The throttle valve controlling function is a function to control the output power of the engine  16  by controlling a degree of opening θth of the throttle valve  32 , based on the depressed angle θap of the accelerator pedal  24  or the like. 
     The fuel injection controlling function is a function to control the output power of the engine  16  by controlling an injected amount of fuel Qfi (target value) from the fuel injection valve  42  based on the depressed angle θap of the accelerator pedal  24  or the like. 
     The spark plug controlling function is a function to control the output power of the engine  16  by controlling an ignition timing of each of the spark plugs  44  based on the depressed angle θap of the accelerator pedal  24  or the like. 
     (3) External Diagnosing Machine  14   
     The external diagnosing machine  14  serves to diagnose the engine  16  for faults. As shown in  FIG. 1 , the external diagnosing machine  14  includes a cable  82 , which is connected to the engine ECU  20  through a data link connector  80  on the vehicle  12  for inputting and outputting intravehicular data, an input/output unit  84  to which the cable  82  is connected, an operating unit  86  in the form of a keyboard, a touch pad, etc., not shown, a processor  88  for controlling various components and judging each of the cylinders  36   a  through  36   d  for a malfunction, a memory  90  for storing various data together with various programs including a control program used by the processor  88  and a fault diagnosing program, and a display unit  92  for displaying various items of information. 
     The external diagnosing machine  14  may be made up of hardware in the form of a commercially available laptop computer, a tablet computer, or a smartphone, for example. 
     For diagnosing the respective cylinders  36   a  through  36   d  for faults using the external diagnosing machine  14 , an operator (user) connects one end of the cable  82  to the input/output unit  84  and the other end of the cable  82  to the data link connector  80 , which is mounted on an instrument panel, not shown, of the vehicle  12 . Thereafter, the operator operates the operating unit  86  in order to instruct the external diagnosing machine  14  to diagnose the respective cylinders  36   a  through  36   d  for faults. The external diagnosing machine  14  causes the engine ECU  20  to operate the engine  16 . Details of a process, which is carried out by the external diagnosing machine  14  in order to diagnose the respective cylinders  36   a  through  36   d  for faults, will be described later. 
     2. Diagnosis Upon the Occurrence of a Misfire 
     (1) Processing Sequence Carried Out When a Misfire Occurs 
       FIG. 3  is a flowchart of a processing sequence, which is carried out when a misfire occurs in the engine  16  while the vehicle  12  is traveling normally (while the engine  16  is operating normally). In step S 1 , the engine ECU  20  detects, by way of the misfire counter, a misfire that occurs in the engine  16 . 
     In step S 2 , the engine ECU  20  stores in the memory  74  a diagnostic fault code representative of the occurrence of a misfire, and one of the cylinders  36   a  through  36   d  that has suffered from the misfire. For example, if a misfire has occurred in the first cylinder  36   a , then the engine ECU  20  stores a diagnostic fault code “P0301” in the memory  74 . If a misfire has occurred in the second cylinder  36   b , then the engine ECU  20  stores a diagnostic fault code “P0302” in the memory  74 . If a misfire has occurred in the third cylinder  36   c , then the engine ECU  20  stores a diagnostic fault code “P0303” in the memory  74 . If a misfire has occurred in the fourth cylinder  36   d , then the engine ECU  20  stores a diagnostic fault code “P0304” in the memory  74 . If multiple cylinders  36   a  through  36   d  have misfired, then the engine ECU  20  stores a diagnostic fault code “P0300” in the memory  74 . 
     Data, which is available for 15 seconds immediately before the engine ECU  20  judges that a misfire has occurred, are acquired at intervals of 0.2 seconds as relevant data in relation to the occurrence of a misfire. The acquired data include data representing a vehicle speed V [km/h], an engine rotational speed NE [rpm], a temperature Tw [° C.] of the coolant of the engine  16 , a depressed angle bap of the accelerator pedal  24 , and a number of misfires that have occurred in the cylinders  36   a  through  36   d  within a predetermined period (measurement period Pm, to be described later). 
     In step S 3 , the engine ECU  20  issues a warning by turning on the warning lamp  28 , thereby indicating to the user the occurrence of a fault (misfire). In response to the warning, the user takes the vehicle  12  to a repair shop or the like, for example. 
     (2) Processing Sequence for Diagnosing Engine  16  in the Event of a Misfire 
       FIGS. 4 and 5  show first and second flowcharts, respectively, of a processing sequence for diagnosing the engine  16  in the event of a misfire. In step S 11 , an operator (technician), having confirmed that the warning lamp  28  on the vehicle  12  is turned on, connects the external diagnosing machine  14  to the engine ECU  20  through the cable  82  and the data link connector  80 . In step S 12 , the processor  88  of the external diagnosing machine  14  reads a diagnostic program from the memory  90  in response to an action made by the operator, and activates the diagnostic program. In step S 13 , the operator carries out an operation, using the operating unit  86  of the external diagnosing machine  14 , in order to read data and a diagnostic fault code (DTC) into the external diagnosing machine  14  upon the occurrence of a fault in the engine ECU  20 . 
     In step S 14 , the external diagnosing machine  14  displays a work guidance screen for diagnostic work, which corresponds to the read diagnostic fault code.  FIG. 6  is a view showing positions of areas of a work guidance screen  100 , which are displayed when the engine  16  is diagnosed. As shown in  FIG. 6 , the work guidance screen  100  primarily is made up of a toolbar area  102 , an overall work display area  104 , and a specific work display area  106 . 
     The toolbar area  102  is an area for displaying a tool bar, not shown, including icons “SAVE”, “PRINT”, etc. The overall work display area  104  is an area for displaying an overall work flowchart  110  (see  FIG. 7 ), which depends on the diagnostic fault code read in step S 13 . The specific work display area  106  is an area for displaying specific work details, which depend on a work title (work item) selected in the overall work flowchart  110 . 
       FIG. 7  shows by way of example the overall work flowchart  110  displayed in the overall work display area  104 . As shown in  FIG. 7 , the overall work flowchart  110  has a start box  112 , first through third work title boxes  114   a  through  114   c  (hereinafter collectively referred to as “work title boxes  114 ”), a plurality of vertical arrows  116 , a plurality of horizontal arrows  118 , first through fourth check result boxes  120   a  through  120   d  (hereinafter referred to collectively as “check result boxes  120 ”), first through third skip boxes  122   a  through  122   c  (hereinafter referred to as “skip boxes  122 ”), a first scroll bar  124 , and a second scroll bar  126  (see  FIG. 6 ). 
     The start box  112  is a field for displaying a diagnostic fault code or symptom. According to the present embodiment, the diagnostic fault codes include codes such as “P0300”, “P0301”, “P0302”, “P0303”, “P0304” (indicative of the occurrence of a misfire in the engine  16 ). 
     The work title boxes  114  are fields for displaying work titles (work items). The work titles (work items) include a diagnostic work item and a verifying work item. The diagnostic work item is a work item for identifying a faulty region, and the verifying work item is a work item for repairing or replacing the identified faulty region. In  FIG. 7 , the first work title box  114   a , which has a pentagonal shape, represents an overall work item, and the second and third work title boxes  114   b ,  114   c , which have rectangular shapes, represent individual work items. 
     The vertical arrows  116  and the horizontal arrows  118  point to work title boxes  114   a  and skip boxes  122 , which are proceeded to in a subsequent step. 
     The check result boxes  120  are fields for displaying a check result, i.e., “OK” (repeated or normal) or “NG” (not repeated or abnormal). Since the check result boxes  120  have a transparent background and a transparent frame, only the letters in the check result boxes  120  are illustrated in  FIG. 7 . 
     The skip boxes  122  are fields for displaying a skip from one work title box  114  to another work title box  114 . 
     The first scroll bar  124  ( FIG. 6 ) is a scroll bar for vertically scrolling the overall work flowchart  110  displayed in the overall work display area  104 . If the overall work flowchart  110  cannot be displayed in its entirety within the overall work display area  104  shown in  FIG. 6 , then the operator can move the first scroll bar  124  in order to vertically scroll the overall work flowchart  110 , so that the operator can see the overall work flowchart  110  in its entirety. 
     The second scroll bar  126  ( FIG. 6 ) is a scroll bar for vertically scrolling the specific work details displayed in the specific work display area  106 . If the specific work details cannot be displayed in their entirety within the specific work display area  106  shown in  FIG. 6 , then the operator can move the second scroll bar  126  in order to vertically scroll the specific work details (see  FIG. 8 , for example), so that the operator can see the specific work details in their entirety. 
     The work guidance screen  100  may have a scroll bar for horizontally scrolling the displayed information, in addition to the first scroll bar  124  and the second scroll bar  126 , which serve to vertically scroll the displayed information. 
     A work title box  114  to be handled, i.e., any work item to be dealt with, has a frame thereof displayed in bold, so that the display of the work title box  114  to be handled is highlighted. For example, if the second work title box  114   b  (“REPEATABILITY TEST”) is to be handled, then the second work title box  114   b  is highlighted with the frame thereof displayed in bold. Alternatively, if the work guidance screen  100  is displayed in color, then the background color of any work title box  114  may be changed from blue to orange, for example, so that the display of the work title box  114  is highlighted. 
     In step S 15  of  FIG. 4 , the operator judges whether or not the read diagnostic fault code represents the occurrence of a misfire. If the diagnostic fault code does not represent the occurrence of a misfire (S 15 : NO), then in step S 16 , the operator carries out a diagnostic process in accordance with the diagnostic fault code. 
     If the diagnostic fault code represents the occurrence of a misfire (S 15 : YES), then in step S 17 , the external diagnosing machine  14  keeps the engine  16  idling, and conducts a misfire repeatability test on all of the cylinders  36   a  through  36   d . In the misfire repeatability test of step S 17 , a misfire is judged as being repeated when the warning lamp  28 , which indicates the occurrence of a misfire, is turned on while the engine  16  is idling. The operator selects one of the skip boxes  122  from within the overall work flowchart  110  in order to enter the result of the misfire repeatability test. Thereafter, the external diagnosing machine  14  guides the operator to perform a work process depending on the entered result of the misfire repeatability test. Alternatively, rather than the operator entering the result of the misfire repeatability test, the external diagnosing machine  14  may judge whether or not a misfire is repeated. More specifically, the external diagnosing machine  14  keeps the engine  16  idling while measuring with the misfire counter an accumulated value Tmf_cyl_n of the number of misfires that have occurred in the respective cylinders  36   a  through  36   d  during a measurement period Pm. The variable “n” in “Tmf_cyl_n” represents the number of a cylinder in question. For example, “Tmf_cyl_ 1 ” represents an accumulated value with respect to the first cylinder  36   a . The external diagnosing machine  14  judges that a misfire is repeated when the accumulated value “Tmf_cyl_n” with respect to at least one cylinder exceeds a given threshold value for judging the occurrence of a misfire. 
     If a misfire is repeated while the engine  16  is idling (S 18 : YES), then control proceeds to step S 22 . If a misfire is not repeated while the engine  16  is idling (S 18 : NO), then in step S 19 , the external diagnosing machine  14  conducts a misfire repeatability test on all of the cylinders  36   a  through  36   d  while the fault that has occurred is being repeated. The misfire repeatability test in step S 19  is the same as the misfire repeatability test in step S 17 , except that the step is performed while the fault that has occurred is being repeated, rather than while the engine  16  is idling. More specifically, the external diagnosing machine  14  displays, within the specific work display area  106 , a message which requests the operator to repeat the occurrence of the fault, and to acquire data upon occurrence of a misfire, together with the data that was read upon the occurrence of the fault in step S 13 . The data upon occurrence of the fault include data representative of a vehicle speed V [km/h], an engine rotational speed NE [rpm], a temperature Tw [° C.] of the coolant of the engine  16 , and a depressed angle θap of the accelerator pedal  24 , for example. Based on the message and the data, which are displayed in the specific work display area  106 , the operator repeats the occurrence of the fault. If a misfire is repeated at this time, then the warning lamp  28  is turned on in order to indicate the occurrence of a misfire. Depending on whether or not the warning lamp  28  is turned on, the operator judges whether a misfire is repeated, and selects one of the skip boxes  122  from within the overall work flowchart  110  in order to enter the result of the misfire repeatability test. Alternatively, in the same manner as in step S 17 , the external diagnosing machine  14  may operate autonomously to judge whether or not a misfire is repeated. 
     If a misfire is repeated while the fault that has occurred is repeated (S 20 : YES), then control proceeds to step S 22 . If a misfire is not repeated while the fault that has occurred is repeated (S 20 : NO), then in step S 21 , the external diagnosing machine  14  displays a list of misfire-related fault data in the past, together with details of the misfire-related fault data in the past selected from the list displayed on the display unit  92 . The operator can now identify a faulty region based on the misfire-related fault data in the past. The misfire-related fault data in the past are stored in the memory  90 . 
     In step S 22 , the external diagnosing machine  14  displays misfire patterns of the cylinders  36   a  through  36   d  on the display unit  92 .  FIG. 8  is a diagram showing by way of example a plurality of misfire patterns together with corresponding misfire details and possible faulty regions that are displayed in the specific work display area  106 . 
     In  FIG. 8 , the misfire patterns include a misfire pattern  1  in which a single cylinder suffers from a succession of misfires (SINGLE CYLINDER—SUCCESSIVE MISFIRES), a misfire pattern  2  in which a plurality of cylinders suffer from a succession of misfires (PLURAL CYLINDERS—SUCCESSIVE MISFIRES), a misfire pattern  3  in which a single cylinder randomly misfires (SINGLE CYLINDER—RANDOM MISFIRES), and a misfire pattern  4  in which a plurality of cylinders randomly misfire (PLURAL CYLINDERS—RANDOM MISFIRES). 
     The misfire pattern  1  (SINGLE CYLINDER—SUCCESSIVE MISFIRES) is combined with a field “MISFIRE DETAILS (MISFIRE SYMPTOM)” which displays a statement “MISFIRING CYLINDER MISFIRES EVERY TIME AND MISFIRE COUNTER COUNTS UP SUCCESSIVELY (LINEARLY)” and a statement “MISFIRE COUNTER COUNTS UP FOR ONE CYLINDER ONLY” while also displaying a graph of the misfire pattern  1  (SINGLE CYLINDER—SUCCESSIVE MISFIRES) the horizontal axis of which represents time (MEASURING TIME) and the vertical axis of which represents an accumulated value Tmf_cyl_n of the number of misfires (MISFIRE COUNTER [COUNT]). The misfire pattern  1  also is combined with a field “POSSIBLE FAULTY REGION” which displays “IG (ignition) COIL”, “SPARK PLUG”, “COMPRESSION” (compression failure), “TAPPET CLEARANCE” (valve clearance), and “INJECTOR”, as regions that potentially are suffering from a fault according to the misfire pattern  1 . 
     The misfire pattern  2  (PLURAL CYLINDERS—SUCCESSIVE MISFIRES) is combined with a field “MISFIRE DETAILS (MISFIRE SYMPTOM)” which displays a statement “MISFIRING CYLINDERS MISFIRE EVERY TIME AND MISFIRE COUNTER COUNTS UP SUCCESSIVELY (LINEARLY)” and a statement “MISFIRE COUNTER COUNTS UP FOR PLURAL CYLINDERS”, and also displays a graph of the misfire pattern  2  (PLURAL CYLINDERS—SUCCESSIVE MISFIRES) the horizontal axis of which represents time and the vertical axis of which represents an accumulated value Tmf_cyl_n of the number of misfires. The misfire pattern  2  also is combined with a field “POSSIBLE FAULTY REGION” which displays “IG COIL”, “SPARK PLUG”, “COMPRESSION”, “TAPPET CLEARANCE”, and “INJECTOR”, as regions that potentially are suffering from a fault according to the misfire pattern  2 . 
     The misfire pattern  3  (SINGLE CYLINDER—RANDOM MISFIRES) is combined with a field “MISFIRE DETAILS (MISFIRE SYMPTOM)” which displays a statement “MISFIRING CYLINDERS MISFIRE RANDOMLY (IRREGULARLY) AND OPERATES NORMALLY AND MISFIRE COUNTER COUNTS UP IRREGULARLY”, a statement “CYLINDER OCCASIONALLY OPERATES NORMALLY AND MISFIRE COUNTER OCCASIONALLY COUNTS UP”, and a statement “MISFIRE COUNTER COUNTS UP FOR ONE CYLINDER ONLY”, and also displays a graph of the misfire pattern  3  (SINGLE CYLINDER—RANDOM MISFIRES) the horizontal axis of which represents time and the vertical axis of which represents an accumulated value Tmf_cyl_n of the number of misfires. The misfire pattern  3  also is combined with a field “POSSIBLE FAULTY REGION” which displays “IG COIL”, “CRANKSHAFT PULSER”, “CRANKSHAFT SENSOR”, “VTEC” (variable valve timing/valve lifting mechanism), “COMPRESSION”, and “TAPPET CLEARANCE”, as regions that potentially are suffering from a fault according to the misfire pattern  3 . 
     The misfire pattern  4  (PLURAL CYLINDERS—RANDOM MISFIRES) is combined with a field “MISFIRE DETAILS (MISFIRE SYMPTOM)” which displays a statement “MISFIRING CYLINDERS MISFIRE RANDOMLY (IRREGULARLY) AND OPERATES NORMALLY AND MISFIRE COUNTER COUNTS UP IRREGULARLY”, a statement “CYLINDERS OCCASIONALLY OPERATE NORMALLY AND MISFIRE COUNTER OCCASIONALLY COUNTS UP”, and a statement “MISFIRE COUNTER COUNTS UP FOR ALL CYLINDERS OR PLURAL CYLINDERS”, and also displays a graph of the misfire pattern  4  (PLURAL CYLINDERS—RANDOM MISFIRES) the horizontal axis of which represents time and the vertical axis of which represents an accumulated value Tmf_cyl_n of the number of misfires. The misfire pattern  4  also is combined with a field “POSSIBLE FAULTY REGION” which displays “CRANKSHAFT PULSER”, “CRANKSHAFT SENSOR”, “EGR VALVE”, “VTEC”, “COMPRESSION”, “TAPPET CLEARANCE”, “FUEL CHAMBER”, “FUEL SYSTEM” (FUEL PRESSURE, PUMP, FILTER), and “ENGINE BODY” as regions that potentially are suffering from a fault according to the misfire pattern  4 . 
     The displayed information shown in  FIG. 8  is used as a guideline for the operator to judge the misfire pattern. In addition to the displayed information shown in  FIG. 8 , the external diagnosing machine  14  displays, in the specific work display area  106 , graphs (not shown, identical to those shown in  FIG. 8 ) of the accumulated values Tmf_cyl_n of the present number of misfires occurring in the respective cylinders  36   a  through  36   d . If a misfire is repeated while the engine  16  is idling (S 18 : YES), then data acquired up to that time are used as the accumulated values Tmf_cyl_n of the present number of misfires. If a misfire is not repeated while the engine  16  is idling (S 18 : NO), then data upon the occurrence of the misfire are used as the accumulated values Tmf_cyl_n of the present number of misfires. In addition, misfire pattern options are displayed in the specific work display area  106 . 
     Based on the displayed information shown in  FIG. 8  and the graphs of the accumulated values Tmf_cyl_n of the present number of misfires, the operator selects one of the misfire pattern options using the operating unit  86 . In this manner, a misfire pattern is selected. 
     In step S 23  shown in  FIG. 5 , the external diagnosing machine  14  conducts a compression pressure failure judgment test. The compression pressure failure judgment test is a test for judging a compression pressure failure in the cylinders  36   a  through  36   d , and is capable of determining whether or not a mechanical system fault is occurring, as will be described in detail later. 
     In step S 24 , the external diagnosing machine  14  conducts an air-fuel-ratio failure diagnostic test. The air-fuel-ratio failure diagnostic test is a test for diagnosing an air-fuel-ratio failure in the cylinders  36   a  through  36   d , and is capable of determining a lean failure in which the air-fuel ratio is excessively low, and a rich failure in which the air-fuel ratio is excessively high. 
     In step S 25 , the external diagnosing machine  14  tracks down regions (hereinafter referred to as “possible faulty regions”) where a fault possibly is occurring based on the results of the misfire repeatability test (S 17 , S 19 ), the compression pressure failure judgment test (S 23 ), and the air-fuel-ratio failure diagnostic test (S 24 ), and displays the possible faulty region along with the results of the tests. 
       FIG. 9  is a diagram showing by way of example test results and possible faulty regions, which are displayed in the specific work display area  106 . In  FIG. 9 , the possible faulty regions are displayed depending on the misfire patterns  1  through  4 , the result of the compression pressure failure judgment test, and the result of the air-fuel-ratio failure diagnostic test. 
     More specifically, if the misfire pattern is “1” (SINGLE CYLINDER—SUCCESSIVE MISFIRES) or “2” (PLURAL CYLINDERS—SUCCESSIVE MISFIRES) and the result of the compression pressure failure judgment test indicates “NOT PASSED” (mechanical fault), then “COMPRESSION” and “TAPPET CLEARANCE (LARGE)” are displayed as possible faulty regions. If the misfire pattern is “1” or “2”, the result of the compression pressure failure judgment test indicates 
     “PASSED” (no mechanical fault), and the result of the air-fuel-ratio failure diagnostic test indicates “LEAN FAILURE”, then “IG COIL”, “SPARK PLUG”, and “INJECTOR” are displayed as possible faulty regions. If the misfire pattern is “1” or “2”, the result of the compression pressure failure judgment test indicates “PASSED” (no mechanical fault), and the result of the air-fuel-ratio failure diagnostic test indicates “RICH FAILURE”, then “INJECTOR” is displayed as a possible faulty region. 
     If the misfire pattern is “ 3 ” (SINGLE CYLINDER—RANDOM MISFIRES) and the result of the compression pressure failure judgment test indicates “NOT PASSED” (mechanical fault), then “COMPRESSION” and “TAPPET CLEARANCE (LARGE)” are displayed as possible faulty regions. If the misfire pattern is “3”, the result of the compression pressure failure judgment test indicates “PASSED” (no mechanical fault), and the result of the air-fuel-ratio failure diagnostic test indicates “LEAN FAILURE”, then “IG COIL”, “TAPPET CLEARANCE (SMALL)”, and “INJECTOR” are displayed as possible faulty regions. If the misfire pattern is “ 3 ”, the result of the compression pressure failure judgment test indicates “PASSED”, and the result of the air-fuel-ratio failure diagnostic test indicates “RICH FAILURE”, then “INJECTOR” is displayed as a possible faulty region. If the misfire pattern is “3”, and the result of the compression pressure failure judgment test and the result of the air-fuel-ratio failure diagnostic test indicate “PASSED”, then “IG COIL”, “CRANKSHAFT PULSER”, “CRANKSHAFT SENSOR”, and “VTEC” are displayed as possible faulty regions. 
     If the misfire pattern is “4” (PLURAL CYLINDERS—RANDOM MISFIRES) and the result of the compression pressure failure judgment test indicates “NOT PASSED” (mechanical fault), then “COMPRESSION” and “TAPPET CLEARANCE (LARGE)” are displayed as possible faulty regions. If the misfire pattern is “4”, the result of the compression pressure failure judgment test indicates “PASSED” (no mechanical fault), and the result of the air-fuel-ratio failure diagnostic test indicates “LEAN FAILURE”, then “TAPPET CLEARANCE (SMALL)”, “FUEL CHAMBER”, “FUEL SYSTEM (FUEL PRESSURE, PUMP, FILTER)”, “EGR OPENING FAILURE”, and “EGR LEAKAGE” are displayed as possible faulty regions. If the misfire pattern is “4”, the result of the compression pressure failure judgment test indicates “PASSED”, and the result of the air-fuel-ratio failure diagnostic test indicates “RICH FAILURE”, then “INJECTOR” is displayed as a possible faulty region. If the misfire pattern is “4”, and the result of the compression pressure failure judgment test and the result of the air-fuel-ratio failure diagnostic test indicate “PASSED”, then “CRANKSHAFT PULSER”, “CRANKSHAFT SENSOR”, “VTEC SWITCHING DELAY”, and “EGR FOLLOW-UP DELAY” are displayed as possible faulty regions. 
     In step S 26  of  FIG. 5 , the operator judges a fault of each of the possible faulty regions. For example, the specific possible faulty regions (compression, IG coil, etc.) shown in  FIG. 9  are linked together with data concerning specific work details. The operator enters one of the options (possible faulty regions) using the operating unit  86 . Thereafter, the external diagnosing machine  14  displays, in the specific work display area  106 , an overall work flowchart  110  depending on the entered option (possible faulty region) in the overall work display area  104 , and also displays specific work details according to the work title box  114  selected in the overall work flowchart  110 . The operator refers to the displayed information in order to confirm whether or not a fault actually is occurring in the possible faulty region. If a fault is actually occurring in the possible faulty region, then the operator repairs the possible faulty region. 
     The compression pressure failure judgment test and the air-fuel-ratio failure diagnostic test according to the present embodiment will be described below. 
     (3) Compression Pressure Failure Judgment Test 
     As described above, the compression pressure failure judgment test is a test for judging a compression pressure failure in the cylinders  36   a  through  36   d , and is capable of judging whether or not a mechanical system fault is occurring. In the compression pressure failure judgment test, the engine  16  is cranked to rotate the crankshaft  48  while at the same time canceling fuel explosion in the cylinders  36   a  through  36   d , and those of the cylinders  36   a  through  36   d  that are suffering from a compression pressure failure (compression pressure failure cylinder) are judged based on an angular velocity variation Δω [rad/s] of the crankshaft  48 . According to the present embodiment, when the engine  16  is cranked, both the fuel supply system (fuel injection valve  42 , etc.) and the ignition system (spark plug  44 , etc.) are disabled. 
       FIG. 10  is a diagram showing a model representation of a relationship between strokes of the piston  50  in each of the cylinders  36   a  through  36   d  and the magnitude of a load L 1  applied to the crankshaft  48  during operation of the piston  50  upon cranking the engine, at a time that the cylinder  36  is operating normally and when the cylinder  36  is suffering from a shortage of compression pressure. The load L 1  induces a reduction in engine rotational speed NE [rpm], i.e., a reduction in the angular velocity ω [rad/s] of the crankshaft  48 . Since fuel explosion is canceled in the cylinders  36   a  through  36   d  when the engine is cranked, explosion of fuel does not actually occur in the power stroke shown in  FIG. 10 . Stated otherwise, the power stroke shown in  FIG. 10  represents a stroke having the same range of the crankshaft angle Ac as the power stroke during normal operation of the engine. 
     In the example shown in  FIG. 10 , the load L 1  applied when the cylinder  36  is operating normally and the load L 1  applied when the cylinder  36  is suffering from a shortage of compression pressure are compared with each other. The difference between the compared loads L 1  is particularly large during the compression stroke compared with during the intake stroke, the power stroke, and the exhaust stroke. This is because the compressive load is small when a gas leakage occurs somewhere in the cylinder  36 . 
     With the engine  16  having the plural cylinders  36   a  through  36   d , strokes of the cylinders  36   a  through  36   d  are kept out of phase with each other in order to produce regular angular velocity variations Δω which allow the engine  16  to be cranked stably during normal operation thereof. However, when a compression failure occurs in either one of the cylinders  36   a  through  36   d , the compressive load is not applied as required, thereby causing a disturbance in angular velocity variations Δω. 
     According to the present embodiment, which is based on the above observations, the difference between the loads L 1  in the compression stroke is not used directly, but rather, angular velocity variations Δω in the power stroke are used to judge a shortage of compression pressure. More specifically, this is based on the fact that, when the engine  16  having the cylinders  36   a  through  36   d , which include one cylinder that suffers from a shortage of compression pressure, is cranked, the crankshaft angular velocity ω increases during the compression stroke of the cylinder that suffers from a shortage of compression pressure, but in reaction thereto, decreases in the next stroke, i.e., the power stroke, of the same cylinder. It is thus possible to judge whether or not a shortage of compression pressure has occurred based on a reduction (variation) in the angular velocity co during the power stroke. Accordingly, if the crankshaft angular velocity variation Δω is used in the logic for judging a misfire, then the same logic used for judging a misfire can also be used as a logic for determining a cylinder that suffers from a shortage of compression pressure. 
       FIG. 11  shows a model representation of the relationship between crankshaft angles Ac and crankshaft angular velocities ω, along with strokes (intake, compression, power, and exhaust strokes) of the cylinders  36   a  through  36   d  when the cylinders  36   a  through  36   d  operate normally and when the first cylinder  36   a  is suffering from a misfire while the engine is being cranked. In  FIG. 11 , the solid-line curve  130  represents a relationship between crankshaft angles Ac and crankshaft angular velocities ω at a time that the cylinders  36   a  through  36   d  are operating normally, whereas the broken-line curve  132  represents a relationship between crankshaft angles Ac and crankshaft angular velocities ω at a time that the first cylinder  36   a  is misfiring. 
     In the example shown in  FIG. 11 , the angular velocity ω drops sharply due to a rotational disturbance in the power stroke, which occurs subsequent to the compression stroke of the first cylinder  36   a.    
       FIG. 12  shows a relationship between crankshaft angles Ac and angular velocity variations Δω, which correspond to the data shown in  FIG. 11  and the power strokes of the cylinders  36   a  through  36   d . In  FIG. 12 , the solid-line curve  140  represents a relationship between crankshaft angles Ac and angular velocity variations Δω at a time that the cylinders  36   a  through  36   d  are operating normally, whereas the broken-line curve  142  represents a relationship between crankshaft angles Ac and angular velocity variations Δω at a time that the first cylinder  36   a  is misfiring. In the example shown in  FIG. 12 , in order to clearly show a compression pressure failure, compression leakages (zero compression pressure) are shown which occur during compression strokes of the first cylinder  36   a . In the example shown in  FIG. 12 , angular velocity variations Δω are reduced in the power stroke of the first cylinder  36   a . This is because a compressive load is not applied while a corresponding increase occurs in the angular velocity variation Δω during the compression stroke of the first cylinder  36   a , and then as a reaction thereto, the angular velocity variation Δω decreases in the power stroke of the cylinder  36   a . It is thus possible to judge whether or not a shortage of compression pressure has occurred in the first cylinder  36   a  based on a comparison of angular velocity variations Δω during respective power strokes. 
     More specifically, the external diagnosing machine  14  calculates individual average values AVEr, a total average value AVEt, and ratios R 1  based on the acquired angular velocity variations Δω. The individual average values AVEr represent average values of angular velocity variations Δω that occur in respective power strokes of the cylinders  36   a  through  36   d . The total average value AVEt is an average of the individual average values AVEr of all of the cylinders  36   a  through  36   d . The ratios R 1  are calculated as a quotient of the individual average values AVEr divided by the total average value AVEt (AVEr/AVEt). 
     The external diagnosing machine  14  judges whether or not there is a mechanical fault in the cylinders  36   a  through  36   d  based on the previously read diagnostic fault code and the ratios R 1 , and displays the judgment result on the display unit  92 . 
     In particular, the external diagnosing machine  14  judges whether or not there is a shortage of compression pressure in a misfiring cylinder based on the ratios R 1  with respect to the cylinders  36   a  through  36   d . More specifically, if the ratio R 1  with respect to a misfiring cylinder is smaller than a threshold value by which it is judged whether or not there is a shortage of compression pressure (compression force shortage judging threshold value TH 2 ), then the ECU  20  judges that there is a shortage of compression pressure in the misfiring cylinder. According to the present embodiment, the threshold value TH 2  is 100%. 
       FIG. 13  shows by way of example individual average values AVEr in the case that the tappet clearance TC of the first cylinder  36   a  is normal (e.g., TC=0.23 mm), in the case that the deviation of the tappet clearance deviation TC is small (e.g., TC=0.13 mm), in the case that the deviation of the tappet clearance TC is large (e.g., TC=0.05 mm), and in the case that the compression pressure is zero at a time that the first cylinder  36   a  is abnormal and the second through fourth cylinders  36   b  through  36   d  are normal. 
     The solid-line curve  150  represents individual average values AVEr in the case that the tappet clearance TC of the first cylinder  36   a  is normal (e.g., TC=0.23 mm). The broken-line curve  152  represents individual average values AVEr in the case that the deviation of the tappet clearance deviation TC is small (e.g., TC=0.13 mm). The dot-and-dash-line curve  154  represents individual average values AVEr in the case that the deviation of the tappet clearance TC is large (e.g., TC=0.05 mm). The two-dot-and-dash-line curve  156  represents individual average values AVEr in the case that the compression pressure is zero. 
       FIG. 14  shows ratios R 1  (=Avr/AVEt) of the individual average values AVEr to the total average value AVEt, based on individual average values AVEr of the cylinders  36   a  through  36   d  shown in  FIG. 13 .  FIG. 15  is a diagram showing, at an enlarged scale, a portion of the ratios shown in  FIG. 14 . In  FIGS. 14 and 15 , the solid-line curve  160  corresponds to the first cylinder  36   a , the broken-line curve  162  corresponds to the second cylinder  36   b , the dot-and-dash-line curve  164  corresponds to the third cylinder  36   c , and the two-dot-and-dash-line curve  166  corresponds to the fourth cylinder  36   d.    
     According to the above process, the external diagnosing machine  14  judges a compression pressure failure of the cylinders  36   a  through  36   d  in order to determine whether or not a mechanical fault has occurred. 
     (4) Air-Fuel-Ratio Failure Diagnostic Test 
     As described above, the air-fuel-ratio failure diagnostic test is a test for diagnosing an air-fuel-ratio failure in the cylinders  36   a  through  36   d , and is capable of judging the occurrence of a lean failure in which the air-fuel ratio is excessively low, and a rich failure in which the air-fuel ratio is excessively high. 
     (a) Air-Fuel Ratio Control Process According to the Present Embodiment 
     First, an air-fuel ratio control process according to the present embodiment will be described below. According to the present embodiment, an ordinary air-fuel ratio control process generally is used during operation of the engine  16 . The ordinary air-fuel ratio control process is a control process, which is used in the fuel injection valve controlling function of the engine ECU  20 . Such a process comprises a combination of a basic fuel injection control process and an air-fuel-ratio feedback control process (hereinafter referred to as an “air-fuel-ratio FB control process”). 
     According to the present embodiment, the basic fuel injection control process is a control process for bringing ratios of the air and fuel (gasoline) in an air-fuel mixture supplied to the cylinders  36   a  through  36   d  (hereinafter referred to as “cylinder air-fuel ratios Raf_ 1  through Raf_ 4 ” or collectively as “cylinder air-fuel ratios Raf_n” or simply “air-fuel ratios Raf_n”) into a stoichiometric air-fuel ratio (fuel:air=1:14.7). 
     More specifically, according to a basic fuel injection control process, a relationship between amounts of intake air Qaf, which are detected by the amount-of-intake-air sensor  30 , and target values for the injected amounts of fuel Qfi from the fuel injection valves  42  are established beforehand as a map, and the fuel injection valves  42  are controlled depending on a target value for the injected amounts of fuel Qfi, which correspond to a detected amount of intake air Qaf. 
     However, due to various factors including variations in operation timings (tappet clearances TC) of the intake valves  38  and the exhaust valves  40  of the cylinders  36   a  through  36   d , as well as aging of the fuel injection valve  42 , the air-fuel ratios Raf_n of the cylinders  36   a  through  36   d  and the total air-fuel ratio Raf_total of the engine  16  overall may not necessarily be equivalent to the stoichiometric air-fuel ratio. 
     According to the present invention, the air-fuel ratio FB control process is a control process for equalizing the total air-fuel ratio Raf_total to the stoichiometric air-fuel ratio under a feedback control. More specifically, if the detected value from the air-fuel ratio sensor  46  is not equivalent to the stoichiometric air-fuel ratio, then injected amounts of fuel Qfi from all of the fuel injection valves  42  are increased or reduced in order to equalize the detected value with the stoichiometric air-fuel ratio. At this time, the injected amounts of fuel Qfi are corrected using a corrective value Pc. The corrective value Pc is a corrective value for later-described fuel injection ratios Rfi_n [%] (target values) of the cylinders  36   a  through  36   d . However, the corrective value Pc may be a corrective value for a different parameter, for example, the injected amounts of fuel Qfi (target values) or the total air-fuel ratio Raf_total. 
     According to the present embodiment, since only one air-fuel ratio sensor  46  is disposed downstream of the exhaust manifold  64 , air-fuel ratios Raf_n of each of the respective cylinders  36   a  through  36   d  are not detected. In the basic fuel injection control process, therefore, the air-fuel ratios Raf_n of respective cylinders  36   a  through  36   d  cannot be made to converge to the stoichiometric air-fuel ratio, although the total air-fuel ratio Raf_total of the engine  16  can be made to converge to the stoichiometric air-fuel ratio. 
     (b) Air-Fuel-Ratio Failure Diagnostic Test 
       FIG. 16  is a timing chart showing a relationship between the engine rotational speed NE, the air-fuel ratios of the respective cylinders  36   a  through  36   d  (cylinder air-fuel ratios Raf_n), and the accumulated values Tmf_cyl_n [counts] of the number of misfires occurring in the respective cylinders  36   a  through  36   d  at the time the air-fuel ratio failure diagnostic test is conducted. 
     In  FIG. 16 , the period from time t 1  to time t 2  defines a warming-up period of the engine  16 , the period from time t 3  to time t 4  defines a test period for diagnosing the first cylinder  36   a , the period from time t 4  to time t 5  defines a test period for diagnosing the second cylinder  36   b , the period from time t 5  to time t 6  defines a test period for diagnosing the third cylinder  36   c , and the period from time t 6  to time t 7  defines a test period for diagnosing the fourth cylinder  36   d.    
     In the air-fuel-ratio failure diagnostic test, as shown in  FIG. 16 , the cylinders  36   a  through  36   d  are operated one by one while changing the cylinder air-fuel ratios Raf_n in a stepwise manner with respect to the cylinders  36   a  through  36   d , which serve as targets to be diagnosed (hereinafter referred to as “target cylinders”). At this time, accumulated values Tmf_cyl_n are detected. The accumulated values Tmf_cyl_n represent accumulated values of the number of misfires of the target cylinders during a measurement period Pm [sec]. The variable “n” in “Tmf_cyl_n” is indicative of the number of a given cylinder in question from among the cylinders  36   a  through  36   d . For example, “Tmf_cyl_ 1 ” represents an accumulated value with respect to the first cylinder  36   a . Cylinders  36   a  through  36   d  other than the target cylinder are controlled according to the basic fuel injection control process. 
     The measurement period Pm is a period for switching between corrective values Cfi_n, i.e., a period for switching between the fuel injection ratios Rfi_n and the cylinder air-fuel ratios Raf_n (see  FIG. 16 ). Therefore, the accumulated values Tmf_cyl_n represent accumulated values of the number of misfires of the target cylinders each time that the corrective values Cfi_n are switched. 
     If the cylinder air-fuel ratios Raf_n are not abnormal, then the accumulated values Tmf_cyl_n are zero when the cylinder air-fuel ratios Raf_n are 0%, and the accumulated values Tmf_cyl_n increase as the cylinder air-fuel ratios Raf_n differ progressively away from 0%. It is possible to judge whether or not an air-fuel ratio failure is occurring in the cylinders  36   a  through  36   d  by confirming a relationship between the cylinder air-fuel ratios Raf_n and the accumulated values Tmf_cyl_n for the respective cylinders  36   a  through  36   d . More specifically, if the accumulated values Tmf_cyl_n of the number of misfires are smaller when the cylinder air-fuel ratios Raf_n are negative values (e.g., −20%) than when the cylinder air-fuel ratios Raf_n are 0%, then the cylinders  36   a  through  36   d  can be the to be suffering from a rich failure in which fuel is injected in an excessive rich state. If a misfire has already occurred when the cylinder air-fuel ratios Raf_n are 0% (e.g., the accumulated values Tmf_cyl_n of the number of misfires are greater than a predetermined threshold value) and the accumulated values Tmf_cyl_n of the number of misfires are greater when the cylinder air-fuel ratios Raf_n are negative values (e.g., −20%), then the cylinders  36   a  through  36   d  can be the to be suffering from a lean failure in which fuel is injected in an excessively lean state. 
     Therefore, the external diagnosing machine  14  is capable of judging whether or not the cylinders  36   a  through  36   d  are suffering from a rich failure or a lean failure, based on the relationship between the cylinder air-fuel ratios Raf_n and the accumulated values Tmf_cyl_n of the number of misfires. 
     3. Advantages of the Present Embodiment 
     According to the present embodiment, as described above, a misfire pattern of misfires which actually are occurring or that have occurred in the past is detected, and a faulty region is tracked down according to the detected misfire pattern ( FIGS. 8 and 9 ). Consequently, it is possible to significantly reduce the number of man-hours required to identify and confirm a faulty region. 
     According to the present embodiment, the external diagnosing machine  14  displays as a misfire pattern a graph having a horizontal axis representing time and a vertical axis representing the accumulated values Tmf_cyl_n, (see  FIG. 8 ). Therefore, the operator can easily identify the misfire pattern by visually confirming the graph. The operator can thus easily grasp the situation in relation to misfires that actually are occurring, and to classify the misfires as successive misfires or not. The operator can easily grasp an actual diagnostic work technique concerning judgment of whether successive misfires are occurring or not, and the result of the diagnostic work technique, which is indicative of whether or not successive misfires are occurring, so that the operator can perform work with increased efficiency. In addition, the external diagnosing machine  14  enables even inexperienced operators to upgrade their skill with a high learning capability. 
     According to the present embodiment, the external diagnosing machine  14  instructs restarting of the engine  16  in an attempt to repeat a misfire in a misfiring cylinder (S 17  or S 19  in  FIG. 4 ). If a misfire is not repeated (S 18 : NO or S 20 : NO), then the external diagnosing machine  14  detects a misfire pattern based on data that was produced upon the occurrence of a misfire (S 21 ). Therefore, even if misfiring is not repeated when the engine  16  is restarted, it is possible to track down faulty regions by using successive data collected upon the occurrence of a misfire at the time that the diagnostic fault code was generated. 
     According to the present embodiment, the external diagnosing machine  14  instructs the engine  16  to idle in an attempt to repeat a misfire in a misfiring cylinder (S 17  in  FIG. 4 ). If a misfire is not repeated (S 18 : NO), then the external diagnosing machine  14  repeats the situation that took place upon the occurrence of a misfire, based on data that was produced upon the occurrence of a misfire, in order to repeat a misfire in the misfiring cylinder, whereupon the external diagnosing machine  14  detects a misfire pattern (S 19 ). Therefore, even if it is difficult to repeat a misfire while the engine  16  is idling, it is possible to track down faulty regions by positively repeating the condition that took place upon misfiring. 
     According to the present embodiment, if a misfire is repeated in a misfiring cylinder when the external diagnosing machine  14  instructs restarting of the engine  16  (S 18 : YES or S 20 : YES), the external diagnosing machine  14  further tracks down a faulty region by determining a compression pressure shortage cylinder using the compression pressure failure judgment test, and by judging an air-fuel ratio failure using the air-fuel ratio failure diagnostic test. Thus, by determining a compression pressure shortage cylinder and judging an air-fuel ratio failure, it is possible to further track down, from the faulty regions that have been tracked down based on the misfire pattern, considerably limited faulty regions. Consequently, it is possible to further reduce the number of man-hours required to identify and confirm a faulty region. 
     According to the present embodiment, the misfire patterns include successive misfires that take place in a single cylinder and in plural cylinders, random misfires in a single cylinder, and random misfires in plural cylinders ( FIG. 8 ). Therefore, it is possible to easily classify the misfire patterns and efficiently track down a faulty region. 
     B. Modifications 
     The present invention is not limited to the above embodiment, but may employ various additional or alternative arrangements based on the above disclosure of the present invention. For example, the present invention may employ the following arrangements. 
     1. External Diagnosing Machine  14   
     In the above embodiment, the external diagnosing machine  14  is used to diagnose the engine  16  of the vehicle  12 . However, the external diagnosing machine  14  may be used in connection with other systems having internal combustion engines, e.g., mobile objects such as ships or the like. In the above embodiment, the external diagnosing machine  14  communicates with the engine ECU  20  from an external location outside of the vehicle  12 . However, the external diagnosing machine  14  may be incorporated in the vehicle  12 . Stated otherwise, the engine ECU  20  may include the functions of the external diagnosing machine  14 . 
     2. Engine  16   
     In the above embodiment, the engine  16  is an in-line four-cylinder engine. However, the layout and number of the cylinders  36  are not limited to those that make up parts of an in-line four-cylinder engine. The engine  16  may be a V-shaped six-cylinder engine. If the engine  16  is a V-shaped six-cylinder engine, the strokes, i.e., the intake, compression, power, and exhaust strokes, of the six cylinders occur successively in two revolutions (720°) of the crankshaft  48 . Consequently, crankshaft angles Ac, which are incremented by 120° (=720°/6), are assigned respectively to the power strokes of the cylinders. 
     3. Air-Fuel Ratio Sensor  46   
     In the above embodiment, the engine  16  in its entirety has one air-fuel ratio sensor  46 . However, plural air-fuel ratio sensors  46  may be combined with the respective cylinders  36 . 
     4. Misfire Repeatability Test 
     In the above embodiment, two types of misfire repeatability tests (while the engine is idling, and during repetition of a situation upon occurrence of a misfire) are conducted. However, either one of these two types of misfire repeatability tests may be conducted. 
     In the above embodiment, the misfire patterns include “SINGLE CYLINDER—SUCCESSIVE MISFIRES”, “PLURAL CYLINDERS—SUCCESSIVE MISFIRES”, “SINGLE CYLINDER—RANDOM MISFIRES”, and “PLURAL CYLINDERS—RANDOM MISFIRES”. However, the misfire patterns may include two types of misfire patterns, e.g., “SUCCESSIVE MISFIRES” and “RANDOM MISFIRES”, or three types of random misfires, e.g., “SUCCESSIVE MISFIRES”, “SINGLE CYLINDER—RANDOM MISFIRES”, and “PLURAL CYLINDERS—RANDOM MISFIRES”. 
     In the above embodiment, the operator judges misfire patterns. However, the external diagnosing machine  14  may judge misfire patterns automatically. If the external diagnosing machine  14  automatically judges misfire patterns, then a first misfire threshold value for distinguishing between the absence of a misfire and random misfires, and a second misfire threshold value for distinguishing between random misfires and successive misfires may be established in advance. In this case, the external diagnosing machine  14  compares the accumulated value Tmf_cyl_n of the number of misfires with respect to the cylinders  36   a  through  36   d  with the first misfire threshold value and the second misfire threshold value, thereby judging whether each of the cylinders  36   a  through  36   d  is not suffering from a misfire, or suffers from random or successive misfires. 
     In the above embodiment, both the displayed information shown in  FIG. 8  and the graph of the accumulated values Tmf_cyl_n of the present number of misfires are displayed in the specific work display area  106 , as guidelines for enabling the operator to judge a misfire pattern. However, only the accumulated values Tmf_cyl_n of the present number of misfires may be displayed in the specific work display area  106  as a guideline for enabling the operator to judge a misfire pattern. 
     In the above embodiment, the external diagnosing machine  14  acquires data upon the occurrence of a misfire directly from the vehicle  12  (engine ECU  20 ). However, the external diagnosing machine  14  may acquire data indirectly through a relay device upon the occurrence of a misfire. The relay device may be a wireless relay unit, which communicates with the engine ECU  20  via a wired link, and communicates with the external diagnosing machine  14  via a wireless link. Alternatively, the external diagnosing machine  14  may acquire data upon the occurrence of a misfire through a car navigation system having a wireless communication function, or a portable terminal such as a smartphone or the like. 
     5. Compression Pressure Failure Judgment Test 
     In the above embodiment, upon cranking the engine  16 , both the fuel supply system (fuel injection valve  42 , etc.) and the ignition system (spark plug  44 , etc.) are disabled. Insofar as explosion of fuel does not occur in the cylinders  36   a  through  36   d , only the fuel supply system may be disabled upon cranking the engine  16 . 
     In the above embodiment, a compression pressure failure is judged using angular velocity variations Δω occurring during the power stroke. However, a compression pressure failure may also be judged using angular velocity variations Δω that occur during the compression stroke. 
     6. Air-Fuel Ratio Failure Diagnostic Test 
     In the above embodiment, the processor  88  of the external diagnosing machine  14  switches between the cylinder air-fuel ratios Ra_n (the fuel injection ratios Rfi_n and the corrective values Cfi_n) through operation of the engine ECU  20 . However, the processor  88  may directly control the fuel injection valves  42  in order to switch between the cylinder air-fuel ratios Raf_n (the fuel injection ratios Rfi_n and the corrective values Cfi_n). Alternatively, the engine ECU  20  may include the functions of the external diagnosing machine  14 , so that the engine ECU  20  (the external diagnosing machine  14 ) can directly control the fuel injection valves  42  in order to switch between the cylinder air-fuel ratios Raf_n (the fuel injection ratios Rfi_n and the corrective values Cfi_n). 
     In the above embodiment, the corrective values Cfi_n are reduced in order to reduce the air-fuel ratios Raf_n of the respective cylinders  36  from the stoichiometric air-fuel ratio to a leaner air-fuel ratio ( FIG. 16 ). However, the corrective values Cfi_n may be changed in a stepwise manner from a positive value to a negative value, so as to change the air-fuel ratios Raf_n in a stepwise manner from a richer air-fuel ratio to a leaner air-fuel ratio. Alternatively, the corrective values Cfi_n may be changed in a stepwise manner from a negative value to a positive value, so as to change the air-fuel ratios Raf_n in a stepwise manner from a leaner air-fuel ratio to a richer air-fuel ratio. In such cases, the corrective values Cfi_n are not necessarily a combination of ±0%, −10%, −20%, −30%, and −40%, but may be changed in other appropriate ways. 
     In the above embodiment, the cylinders  36   a  through  36   d , other than the target cylinder, are controlled according to the basic fuel injection control process, whereas the air-fuel ratio Raf_n (the corrective value Cfi_n) of the target cylinder is changed gradually. However, with respect to cylinders  36   a  through  36   d  other than the target cylinder, the basic fuel injection control process may be canceled in order to stop injecting and igniting fuel. 
     7. Other Arrangements 
     In the above embodiment, a possible faulty region is tracked down by combining the results of the misfire repeatability test, the compression pressure failure judgment test, and the air-fuel ratio failure diagnostic test. However, it is possible to track down a possible faulty region by conducting either one of the compression pressure failure judgment test and the air-fuel ratio failure diagnostic test in addition to the misfire repeatability test.