Diagnostic system for detecting faults in engine air intake check valves

A system is disclosed for diagnosing faulty check valve operation in an internal combusiton engine having a back flow preventing check valve positioned in each intake air passage leading to an engine cylinder. The system measures the pressure in the engine air induction system, upstream of each check valve and downstream from an air intake throttle valve. The system produces an indication of faulty check valve operation, when an abnormal fluctuation in the measured pressure is detected. Preferably, the diagnostic function is performed when the engine operates in a defined mode, at idle speeds with light engine loading, where the pressure differential appearing across each check valve is essentially maximized. The engine cylinder associated with a faulty check valve is identified based upon the sensed rotational position of the engine when an abnormal fluctuation in pressure is detected.

BACKGROUND OF THE INVENTION 
This invention relates to a system for diagnosing faulty check valve 
operation in an internal combustion engine having a back flow preventing 
check valve positioned in each air intake passage leading to an engine 
cylinder. 
It is generally known that at high operating speeds, the performance of a 
four-stroke internal combustion engine can be improved by advancing the 
opening and retarding the closing of cylinder intake valves during the 
engine operating cycle. Opening a cylinder intake valve early, while its 
respective exhaust valve is still open (known as cross-over), facilitates 
the discharge of gaseous combustion products from the cylinder at high 
engine speeds, while delaying the closing of the intake valve, until after 
beginning of cylinder compression, improves cylinder filling. 
Both of the above valve timing modifications improve high speed engine 
performance, however, the torque produced at low speeds is significantly 
reduced. This occurs because the inertia of the intake air inducted into 
the engine decreases at low engine speeds. As a result, a portion of the 
air-fuel charge in each cylinder is driven back into the air induction 
system at low engine speeds, due to the delayed intake valve closing. In 
addition, when the engine is operated under light loading conditions, 
exhaust products are able to flow from the exhaust system back into the 
engine cylinders and air induction system during the cross-over period. 
This can result in cylinder misfires and rough engine idling. 
It is also generally known that the above low speed drawbacks can be 
obviated by placing check valves in the engine air induction system. These 
check valves are typically placed downstream of the air intake throttle 
valve, in each air passage leading to an engine cylinder. Each check valve 
is positioned to allow air flow in a direction toward its associated 
cylinder, but prevent back flow in the opposite direction, away from the 
cylinder. Consequently, engine volumetric efficiency and torque output are 
greatly improved at low speeds and combustion stability is improved under 
light engine loading conditions. 
In this type of engine, if one of the intake passage check valves becomes 
damaged or malfunctions, the engine will not perform properly at low 
speeds. The back flow of exhaust gas into the associated cylinder can 
cause misfires and rough engine idling. Also, the decreased volumetric 
efficiency of the cylinder reduces the output torque. In addition, the 
cylinder associated with the faulty check valve will receive less air, 
while the other cylinders receive excess air. This produces incorrect 
cylinder air-fuel mixtures and increases engine exhaust emissions. 
Consequently, there exists a need for a system, which is capable of 
diagnosing and indicating faulty check valve operation in engines equipped 
with such valves, to ensure proper engine performance and low exhaust 
emissions. 
SUMMARY OF THE INVENTION 
Accordingly, the general object of the present invention is to provide a 
system for diagnosing faulty check valve operation in engines having back 
flow preventing check valves positioned in air intake passages leading to 
each engine cylinder. The system measures the pressure of air in the 
induction system, upstream of each check valve and downstream from the 
induction system throttle valve. An indication of faulty check valve 
operation is then produced, if an abnormal fluctuation in the measured 
pressure is detected. Since modern engine computer control systems 
generally include a sensor for measuring the pressure in the air induction 
system and a warning indicator to alert an operator when the engine 
malfunctions, the present invention can be implemented in such control 
systems by simply making computer software changes, without requiring any 
additional hardware. 
Preferably the system performs the diagnostic function when the engine is 
operating in a defined mode at engine idle speeds with light engine 
loading. In this operating mode, the pressure differential appearing 
across each check valve is essentially maximized. Consequently, a 
malfunctioning check valve will produce larger fluctuations in the 
measured pressure, when the engine is operating in this mode, making 
detection easier and more certain. 
In one embodiment of the present invention, a peak value associated with 
the measured pressure in the induction system is obtained and compared 
with a predetermined threshold value. If the peak value exceeds the 
threshold value, the system indicates that an abnormal fluctuation in the 
measured pressure has occurred. Preferably, the peak value is obtained by 
sampling the measured pressure to obtain a set of sampled pressure values, 
and selecting a largest value from among the set of sampled pressure 
values to obtain the peak value associated with the measured pressure. 
In another embodiment, a peak-to-peak value associated with the measured 
induction system pressure is obtained and compared with a predetermined 
threshold value. If this peak-to-peak value exceeds the threshold value, 
the system indicates that an abnormal fluctuation in the measure pressure 
has occurred. Preferably, the peak-to-peak value is obtained by sampling 
the measured pressure to obtain a set of sampled pressure values, 
selecting a largest and a smallest value from among the set of sample 
pressure values, and then subtracting the largest and smallest values to 
obtain the peak-to-peak value associated with the measured pressure. 
In both of the above embodiments, it is preferable that the diagnostic 
system sample the measured pressure over more than one engine operating 
cycle to obtain a predetermined number of sample pressure values for the 
set. Since an abnormal fluctuation in measured pressure repeats once every 
engine cycle, excessive sampling rates are not then required to obtain 
accurate peak or peak-to-peak values. Additionally, the sampling does not 
have to be synchronized with the rotation of the engine, when the sampling 
is extended over more than one engine cycle. 
According to one aspect of the invention, the diagnostic system is provided 
with a means for determining an engine rotational position corresponding 
to the occurrence of the abnormal fluctuation in the measured pressure 
caused by a faulty check valve. The diagnostic system then identifies and 
indicates the cylinder associated with the faulty check valve based upon 
the determined engine rotational position. Consequently, a substantial 
reduction in the time required to identify and repair faulty check valves 
can be realized with the use of the present invention. 
According to another aspect of the invention, an indication of faulty check 
valve operation is produced, only after a predetermined number of 
sequential abnormal fluctuations are detected in measured pressure. This 
reduces the chance of an incorrect diagnosis caused by the occurrence of 
measurement noise or pressure transients. 
These and other aspects and advantages of the invention may be best 
understood by reference to the following detailed description of the 
preferred embodiments when considered in conjunction with the accompanying 
drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring now to the drawings, and in particular to FIG. 1, there is shown 
schematically an internal combustion engine generally designated as 10, 
which includes an air induction system, generally referred to as 12, for 
supplying the engine 10 with air for the combustion process. Engine 10 
further includes a piston 14 disposed in a cylinder 16, and exhaust port 
18 and intake port 22, with exhaust valve 20 and intake valve 24 seated 
respectively therein. 
The air induction system 12 of engine 10 includes an air passage 26 leading 
to the intake port 22 of cylinder 16, an air intake manifold 28 
communicating with the air passage 26, and an adjustable air throttle 
valve 30 disposed within the intake manifold 28 for controlling the 
quantity of air flowing into engine 10. A check valve 32 is positioned in 
the air passage 26 leading to cylinder 16, downstream from the throttle 
valve 30. The check valve 32 allows air to flow in a direction toward 
cylinder 16, but prevents back flow in the reverse direction, away from 
cylinder 16. As shown, check valve 32 is a reed type valve, however, any 
other kind of back flow preventing valve known in the art may also be 
used. 
Although only a single engine cylinder 16 is illustrated in FIG. 1, engine 
10 can have multiple cylinders with intake manifold 28 communicating in 
parallel with the air passages and check valves leading to the engine 
cylinders. 
Also shown in FIG. 1 is a conventional electronic control unit (ECU) 34, 
which is customarily used for controlling the operation of engine 10 to 
achieve desired performance characteristics. The ECU 34 generally includes 
a central processing unit, random access memory, read only memory, 
analog-to-digital and digital-to-analog converters, input/output 
circuitry, and clock circuitry, as will be recognized by those skilled in 
the art of computer engine control. 
In controlling engine 10, the ECU 34 receives input signals from several 
standard engine sensors. Typically, a temperature sensor 38 provides ECU 
34 with a TEMP input signal, related to the engine coolant temperature. 
The TEMP input signal is sequentially sampled by the analog-to-digital 
circuitry within ECU 34, with the most recent Kth sample value being 
stored in random access memory as a variable TEMP(K). 
Additionally, the ECU 34 is generally provided with a POS input signal for 
deriving the rotational position of the engine. The POS input signal can 
be obtained from any conventional rotational sensor, such as the 
electromagnetic sensor 40 and accompanying toothed wheel 42 shown in FIG. 
1. The electromagnetic sensor 40 detects the passage of teeth on wheel 42 
as it is rotated by the engine and produces corresponding pulses in the 
POS input signal. The asymmetrical tooth on wheel 42 provides a reference 
position in the engine cycle (for example top dead center in the exhaust 
stroke), with the symmetrically spaced teeth corresponding to known 
rotational angles from the reference position. One complete rotational 
cycle in a four-stroke engine requires 720.degree. or two complete 
revolutions of the engine crankshaft (not shown). Wheel 42 can be rotated 
by the engine camshaft (not shown), which rotates one revolution each 
engine cycle, so that the rotational angle of the engine in its 
720.degree. cycle can be determined. The ECU 34 computes the current 
rotational angle of the engine by counting symmetrically spaced pulses, in 
relation to the one asymmetrical pulse, and interpolating between counted 
pulses. The computed value is stored in random access memory as the 
variable ANG, which represents the rotational angle of the engine in its 
720.degree. cycle. 
Based upon the POS input signal, the ECU 34 also derives a value for a 
SPEED variable, which is stored in random access memory. The SPEED 
variable represents the current rotational speed of the engine, and its 
value is normally computed by counting the number of symmetrical pulses in 
the POS input signal that occur during a fixed time interval, and 
multiplying that count by an appropriate constant to obtain the current 
rotational speed of the engine (in rpm). 
A MAP (manifold absolute pressure) input signal is provided for the ECU 34 
by a standard pressure sensor 44, which is positioned to measure the 
pressure in the air induction system 12 upstream of each check valve 32 
and downstream from the throttle valve 30. In a standard fashion, the 
analog MAP signal is sampled by the analog-to-digital circuitry within ECU 
34, and the corresponding sample values are then stored in random access 
memory, with the most recent Kth sample value being designated as MAP(K). 
It is well known in the engine control art that the MAP signal can be used 
to derive an indication of the current load acting on the engine. Usually 
this is accomplished by low passing filtering the MAP(K) sample values 
using a conventional digital filter having a first order lag 
characteristic. The output samples generated from this filtering process 
will be designated as AVEMAP(K), which approximate the average value of 
the MAP input signal. These AVEMAP(K) samples are also stored in memory 
and are substantially proportional to the current load acting on engine 
10. It will also be recognized that other known techniques for obtaining 
engine load may also be used in the present invention, as for example, 
those based upon throttle valve position or engine mass air flow. 
In most conventional computer engine control systems, it is also common 
practice to include a means for warning an operator when certain engine 
malfunctions are detected. For example, if a detected engine operating 
parameter is found to be outside an expected range of values, the ECU 34 
will issue a WARN output signal to a warning indicator 46. This warning 
indicator 46 is typically a light bulb or light emitting diode (LED) that 
provides a visual warning, although a speaker and tone generator could be 
used to provide an audio alarm. 
In this type of warning system, it is also common for the ECU 34 to store a 
predetermined binary WARNING CODE related to the WARN signal, for use in 
identifying the particular engine malfunction. The code can be read out, 
for example, by closing a switch 48, which is coupled to the ECU 34. After 
sensing the closure of switch 48, the ECU 48 outputs the value of each bit 
in the WARNING CODE by sequentially turning the warning indicator either 
on or off using the WARN output signal, depending on the particular value 
of each bit in the WARNING CODE. Alternatively, ECU 48 could be provided 
with a port for connecting to an auxiliary computer, which could then read 
the stored binary WARNING CODE for diagnostic purposes. 
Many other sensors, actuators, ECU input signals, and ECU output signals 
are generally present in a conventional engine control system, however, 
these have not been specifically shown in FIG. 1, since they are not 
required in describing the present invention, and their inclusion would 
tend to make the present description overly complex. 
As is generally known, the high speed performance of the four-stroke 
internal combustion engine 10 can be improved by advancing the opening and 
retarding the closing of each cylinder intake valve 24 during the engine 
operating cycle. Opening the intake valve 24 early, while exhaust valve 20 
is still open (known as cross-over), facilitates the discharge of gaseous 
combustion products from cylinder 16 at high engine speeds, while delaying 
the closing of intake valve 24, until after the beginning of compression 
in cylinder 16, improves cylinder filling. 
When these modifications to valve timing are implemented, low speed engine 
performance is improved by placing a check valve 32 in each intake passage 
26 leading to an engine cylinder 16. Without such check valves, exhaust 
products flow back into the engine cylinders during the cross-over event 
under light engine loading, and portions of the cylinder air-fuel charges 
flow back into the air intake manifold 28, at low engine speeds. 
In this type of engine, if a check valve 32 becomes damaged or 
malfunctions, the engine 10 will not perform properly at low speeds. The 
back flow of exhaust gas into the cylinder 16 can cause misfires and rough 
engine idling. The decreased volumetric efficiency of cylinder 16 reduces 
the engine output torque. In addition, cylinder 16 will receive less air, 
while the other engine cylinders receive excess air. This results in 
incorrect cylinder air-fuel mixtures, which in turn increases engine 
exhaust emissions. 
Consequently, there exists a need for a system, which can diagnose and 
indicate faulty check valve operation in engines equipped with such 
valves, to ensure proper engine performance at low engine speeds. 
Referring now to FIG. 2(A)-(C), there is shown graphical representations of 
the absolute pressure (in kPa) at locations in the air induction system 12 
as a function of the rotational position of engine 10 (in degrees), during 
operation at idle speeds with light engine loading. At the angles of 
0.degree., 720.degree., and 1440.degree., piston 14 is at top dead center 
after completion of the exhaust stroke in cylinder 16. 
Graph (A) depicts the absolute pressure in the intake manifold 28, as 
measured by the pressure sensor 44, with each check valve functioning 
properly. Normally, the peak-to-peak variations in the intake manifold 
pressure are in the order of 3 to 5 kPa when the engine is operated at 
idle speeds with light engine loading. 
Graph (B) depicts the absolute pressure in the air passage 26 between 
intake valve 24 and check valve 32. The large fluctuations in this 
pressure results during cross-over, when the intake valve 24 is opened 
with the exhaust valve 20 not yet fully closed. During this cross-over 
period, the pressure in cylinder 16 and air passage 26 becomes essential 
equal to the exhaust back pressure appearing at the open exhaust port 18, 
which is basically at atmospheric pressure (approximately 100 kPa). 
Once the exhaust stroke is complete, piston 14 begins its intake stroke, 
drawing in air from the intake manifold 28 through the check valve 32. 
When this occurs, the pressure in the air intake passage 26 is quickly 
equalized to that in the intake manifold 28. Also, at high engine loads, 
the increased pressure of the air inducted into the air passage 26 
substantially reduces the amplitude of the large pressure fluctuations 
occurring in the air passage 26. Consequently, the largest pressure 
differential appears across check valve 32 when the pressure in the intake 
manifold 28 is at its lowest value and the pressure in air passage 26 is 
at its highest value. This occurs when engine 10 is operated at idling 
speeds with light engine loading, where throttle valve 30 is essentially 
in its closed idle position. 
Graph (C) of FIG. 2 depicts the pressure measured in intake manifold 28 by 
pressure sensor 44, when check valve 32 is faulty and allows back flow 
from the air passage 26 into the intake manifold 28. As a result, an 
abnormal fluctuation in the intake manifold pressure occurs every engine 
cycle during the cross-over period, when the pressure in the intake 
passage 26 would normally be at its maximum value. Measurements indicate 
that the peak value of the intake manifold pressure fluctuation is in the 
order of 50 kPa and the peak-to-peak variation is in the order of 18 kPa, 
when check valve 32 completely malfunctions. 
The present invention is directed toward utilizing the abnormal 
fluctuations in the intake manifold pressure, as depicted in graph (C), to 
provide a system for diagnosing faulty check valve operation. The system 
measures the pressure of air in the induction system 12, upstream of each 
check valve 32 and downstream from the induction system throttle valve 30. 
An indication of faulty check valve operation is then produced, if an 
abnormal fluctuation is detected in the measured pressure. Since typical 
engine computer control systems generally include a manifold absolute 
pressure sensor 44 for measuring the pressure in the intake manifold 28 
and a warning indicator 46 to alert an operator to engine malfunctions, 
the present invention can be easily implemented in such control systems by 
simply making computer software changes, without requiring any additional 
hardware. 
Preferably the system performs the diagnostic function when the engine is 
operating in a defined mode, at engine idle speeds with light loading. As 
discussed previously, the pressure differential appearing across each 
check valve is essentially maximized when the engine operates in this 
mode. Consequently, the largest fluctuation in the measure manifold 
pressure also occurs when the engine is operating in this mode, making it 
easier to detect such abnormal fluctuations. 
Referring now to FIGS. 3A-B, there is shown a flow diagram representative 
of the steps executed by ECU 34 in diagnosing faulty check valve operation 
based upon a peak value associated with abnormal pressure fluctuations in 
the engine air induction system. This Peak Value Diagnostic Routine forms 
a portion of the main looped control program, which is continuously 
executed by ECU 34 in controlling the operation of engine 10. After engine 
start up, all counters, registers, and timers within the ECU are properly 
initialized before entering the main control program. 
Each time the looped main control program is executed, the Peak Value 
Diagnostic Routine is called at the appropriate location in the main 
control program. 
Execution of the Peak Value Diagnostic Routine begins at step 50, where the 
current values for the TEMP(K), ANG, MAP(K), SPEED, and AVEMAP(K) 
variables are obtained from locations in the random access memory of ECU 
34. As discussed previously, TEMP(K) indicates the temperature of the 
engine coolant; ANG represents the angular position of the engine in its 
operating cycle (from top dead center in the exhaust stroke); MAP(K) 
indicates the pressure in the intake manifold upstream of each engine 
check valve 32 and downstream form the air throttle valve 30; SPEED 
represents the rotational speed of the engine; and AVEMAP(K) indicates the 
average value of the pressure in the intake manifold. After obtaining the 
currently stored values for these variables, the routine proceeds to step 
52. 
At step 52, the routine determines whether the temperature of the engine 
coolant is greater than a first defined temperature T1 and less than a 
second defined temperature T2 (for example, 70.degree. C. and 105.degree. 
C., respectively). If the temperature TEMP(K) is within the defined range, 
the engine is considered to be operating in a normal warmed up mode, and 
the routine proceeds to step 54, otherwise the routine passes to step 58. 
When the engine is properly warmed up, the routine next proceeds to step 54 
to determines whether the engine is operating at a rotational speed 
between a first defined speed N1 and a second defined speed N2 (for 
example, 475 rpm and 525 rpm, respectively). If the engine speed is within 
the range extending from N1 to N2, the engine is considered to be 
operating at idle speeds, and the program proceeds to step 56, otherwise 
it passes to step 58. 
At step 56, the program determines whether the average manifold pressure 
indicated by AVEMAP(K) is between a first defined pressure P1 and a second 
defined pressure P2 (for example, 30 kPa and 35 kPa, respectively). If the 
average manifold pressure is within the range extending from P1 to P2, the 
engine is considered to be operating with light engine loading, and the 
routine proceeds to step 62, otherwise the routine passes to step 58. 
The sequence of decisions in steps 52, 54, and 56 basically determines 
whether the engine is operating in a defined mode at idle speeds with 
light engine loading with the engine operating in a warmed up condition. 
As discussed previously, it is preferable that the engine be operating in 
this defined mode, when diagnosing check valve operation, since abnormal 
fluctuations in manifold pressure due to faulty check valves are the 
largest and easiest to detect when the engine operates at idle speeds with 
light loading. Requiring that the engine operate in the warmed up 
condition assures that range defining idle speeds (between speed N1 and 
speed N2 at step 54) will remain constant. Normally, the engine control 
system sets the idle speed range high for a cold engine, and then 
decreases the idle speed range as a function of coolant temperature, until 
the engine is considered to be warmed up. Also, frictional engine loading 
is reduced when the engine warms up due to the decreased viscosity of the 
engine lubricating oil. 
When engine is found not to be operating in the defined mode, the routine 
proceeds to step 58, where two counters, ACOUNTER and BCOUNTER, along with 
a variable MAX are all reset to their initialized values, i.e. the values 
set prior to entering the routine for the first time (normally zero). From 
step 58, the routine is exited at point 60. 
When the engine is found to be operating in the defined mode, the routine 
proceeds from step 56 to step 62, where a decision is required as to 
whether the current value for the manifold pressure MAP(K) is greater than 
a defined value MAX. If MAP(K) is not greater than MAX the routine 
proceeds to step 66. However, if MAP(K) is greater than MAX, the routine 
first passes to step 64, before proceeding to step 66. 
At step 64, the variable MAX is set equal to MAP(K) and a variable MAXANG 
is set equal to the current rotational angle of the engine ANG. In this 
fashion, the maximum value for the manifold pressure is stored as the 
variable MAX, and the angle of rotation corresponding to this maximum 
pressure is stored as the variable MAXANG. 
At step 66, the current count of counter ACOUNTER is incremented by one. 
Next at step 68, the count of ACOUNTER is examined to determine whether it 
is equal to a predetermined count designated at COUNT1. If ACOUNTER has 
reached the predetermined COUNT1, the routine passes to step 70, otherwise 
it proceeds to exit the routine at point 60. 
When the counter ACOUNTER equals COUNT1, the routine passes to step 70, 
where a value PEAK is set equal to the current value of MAX determined at 
step 104. This PEAK value represents the maximum peak value of any 
fluctuation occurring in the measured manifold pressure as the ACOUNTER is 
incremented from its initial count (normally zero) to COUNT1. 
It will be recognized that the value of COUNT1 defines the number of values 
of manifold pressure MAP(K) in a set that are examined (at step 62), 
before determining the PEAK value associated with the measured manifold 
pressure (at step 70). It is preferable that the value of COUNT1 be chosen 
such that the MAP input signal is sampled over more than one engine 
operating cycle when obtaining the predetermined number of sample manifold 
pressure values in the set. Since any abnormal fluctuation caused by a 
faulty check valve repeats each engine cycle, if sampling is extended over 
more than one operating cycle, an excessive sampling rate is not required 
to obtain an accurate PEAK value representing the measured manifold 
pressure. In addition, the sampling will not have to be synchronized with 
the rotation of the engine (assuming that the sampling frequency is not an 
exact multiple of the engine firing frequency). The same also applies 
where a peak-to-peak value is obtained to represent fluctuations in 
measured manifold pressure (as will be described subsequently in 
discussing the embodiment related to FIGS. 4A-B). 
From step 70, the routine proceeds to step 72 where the ACOUNTER and the 
variable MAX are reset to their initialized values. 
Next at step 74, the value of PEAK is compared with a predetermined 
THRESHOLD value. If PEAK is not greater than THRESHOLD, then an abnormal 
fluctuation in the measured manifold pressure is considered not to have 
occurred, and the routine will proceed to step 76 to reset a counter 
designated as BCOUNTER to its initialized value (normally to zero) prior 
to exiting the routine at step 60. However, if PEAK exceeds the THRESHOLD 
value, this indicates that an abnormal fluctuation in the measured 
manifold pressure is considered to have occurred, and the routine proceeds 
to step 78. The value selected for THRESHOLD will vary from application to 
application, but it should be greater than the maximum expected variation 
in the manifold pressure (for example, 35 kPa) and less than the largest 
expected variation in manifold pressure caused by the complete failure of 
a check valve, which is approximately 50 kPa for the application 
illustrated in FIG. 2(C)). Thus, for this application, THRESHOLD can be 
set at 42 to 44 kPa to detect the abnormal pressure fluctuations depicted 
in FIG. 2(C). 
When an abnormal fluctuation is detected at step 74, the routine proceeds 
to step 78, where the count of BCOUNTER is incremented by one. 
Next at step 80, a decision is required as to whether the BCOUNTER has a 
count equal to the value COUNT2. If the count of BCOUNTER does not equal 
COUNT2, the routine exits at point 60. However, if the count of BCOUNTER 
equals COUNT2, this indicates that a predetermined number of sequential 
abnormal fluctuations (as determined by the value of COUNT2) in the 
measured manifold pressure have been detected. When this occurs, the 
routine passes to step 82, which indicates that the diagnostic system 
definitely considers a check valve to be faulty. By requiring the 
detection of more than one abnormal pressure fluctuation in sequence, 
before indicating the presence of a faulty check valve, the chance of an 
incorrect diagnosis caused by the occurrence of measurement noise or 
pressure transients is reduced. 
When a faulty check valve is diagnosed the routine passes to step 82. There 
the count of the BCOUNTER is reset to its initialized value (normally 
zero), before proceeding to step 84. 
At step 84, a CYLINDER number for the cylinder associated with the faulty 
check valve is identified by looking up the value of CYLINDER in a table 
as a function of the value of the variable MAXANG found at step 64. As 
described previously, the value of MAXANG represents the engine rotational 
angle corresponding to the most recently detected abnormal fluctuation in 
measured manifold pressure. As such, MAXANG can be used to identify the 
particular cylinder associated with the indicated faulty check valve. The 
look up table is established to provide a cylinder number based upon the 
value of the rotational angle MAXANG. For example, consider an engine 
having four cylinders with the firing order 1-2-3-4, with the rotational 
angle of 0.degree. corresponding to top dead center in the exhaust stroke 
for cylinder number 1. The look up table would then be configured to 
provide the CYLINDER number 1, 2, 3, or 4, whenever the value of MAXANG is 
within the respective range of rotation al angles defined by 
-90.degree..ltoreq.MAXANG&lt;90.degree., 90.degree..ltoreq.MAXANG&lt; 
270.degree., 270.degree..ltoreq.MAXANG&lt; 450.degree., or 
450.degree..ltoreq.MAXANG&lt;630.degree.. Of course, engines having different 
numbers of cylinders or different firing orders will have different 
CYLINDER numbers associated with different ranges of rotational angles in 
the look up table. After looking up the CYLINDER number based upon the 
value of MAXANG, the routine then proceeds to step 86. 
At step 86, the ECU 34 stores a WARNING CODE associated with the detection 
of a faulty check valve. The WARNING CODE includes a defined first portion 
indicating that a check valve is faulty, to distinguish it from other 
codes used to indicate other types of engine malfunctions. The CYLINDER 
number found at step 84 forms the second portion of the WARNING CODE, 
which identifies the particular cylinder having the faulty check valve. 
Next, at step 88, the ECU 34 issues a WARN output signal to activate the 
warning indicator 46. As discussed previously, the stored WARNING CODE may 
be read out by closing switch 48, to determine that a check valve has 
malfunctioned and identify the cylinder associated with the faulty check 
valve. As a consequence, a substantial reduction in the time required to 
identify and repair a faulty check valve can be realized with the use of 
the present invention. 
Referring now to FIGS. 4A-B, there is shown a flow diagram representative 
of the steps executed by ECU 34 in a second embodiment of the present 
invention. In this embodiment, the detection of faulty check valve 
operation is based upon a peak-to-peak value associated with abnormal 
pressure fluctuations in the engine air induction system, rather than the 
peak value used in the flow diagram presented in FIGS. 3A-B. 
The Peak-To-Peak Value Diagnostic Routine of FIGS. 4A-B forms a portion of 
the main looped control program that is continuously executed by ECU 34 in 
controlling the operation of engine 10. This routine contains many 
identically numbered steps that were previously discussed in describing 
the flow diagram for the Peak Value Routine of FIGS. 3A-B. Consequently, 
the present discussion will be limited to the difference between the steps 
of the two routines. 
After entering the Peak-To-Peak Value Routine, the same steps described in 
the previous routine are executed, until the new steps 102 and 104 are 
encountered. These new steps have been included in the present routine to 
detect a minimum value MIN associated with fluctuations in the measured 
manifold pressure. 
At step 102, a decision is required as to whether the current value of the 
sampled manifold pressure MAP(K) is less than a variable MIN, which would 
normally be initialized to have a value greater than the largest expected 
manifold pressure (for example, 120 kPa). If MAP(K) is not less than MIN, 
the routine passes to step 66. However, when MAP(K) is less than MIN, the 
routine passes to step 104, where the variable MIN is then set equal to 
the value of MAP(K). The routine then passes to step 66. 
Note also, that step 58 of the previous routine (FIGS. 3A-B) has been 
replaced with step 100 in the present routine. In addition to resetting 
the ACOUNTER, BCOUNTER, and variable MAX, as was done in step 58, the new 
step 100 includes the resetting of the MIN variable added by step 104 in 
the present routine. 
In addition, steps 70 to 74 in the previous routine (FIGS. 3A-B) have been 
replaced by new steps 106 to 110 in the present routine. At step 106, a 
peak-to-peak value PTP is computed by subtracting the value of MIN, found 
at step 104, from the value of MAX, found at step 64. This PTP value 
represents the largest peak-to-peak fluctuation obtained from the set of 
measured manifold pressure samples MAP(K) as the ACOUNTER is incremented 
from its initial count to COUNT1. 
From step 106, the routine proceeds to new step 108, where the ACOUNTER, 
the variable MAX, and the variable MIN are all reset to their initialized 
values. Note that new step 106 differs from step 72 of the previous 
routine, only by including MIN in the list of variable to be reset. 
From step 108, the present routine proceeds to new step 110, where the 
value of PTP is compared with a predetermined THRESHOLD value. If the PTP 
value is not greater than THRESHOLD, then an abnormal fluctuation in the 
measured manifold pressure is considered not to have occurred. In this 
case the routine proceeds to step 76. However, if PTP exceeds the 
THRESHOLD value, this indicates that an abnormal fluctuation in the 
measured pressure is considered to have occurred and the routine then 
passes to step 78. From either step 76 or 78, the remainder of the present 
routine is identical with that described earlier for the routine in FIGS. 
3A-B. 
The Peak Value Diagnostic Routine depicted in FIGS. 3A-B provides for the 
detection of faulty check valve based upon the peak or maximum value of 
the intake manifold pressure, and consequently requires fewer 
computational steps than the Peak-To-Peak Diagnostic Routine shown in 
FIGS. 4A-B. This reduces the execution time of the routine, which can be 
significant in engine control applications. On the other hand, when 
execution time is not an important factor, the PEAK-TO-PEAK Diagnostic 
Routine improves the capability of detecting faulty check valve operation 
since the peak-to-peak value of an abnormal pressure fluctuation is 
relatively larger than the peak value. 
In the above described embodiments of the present invention, the check 
valve diagnostic system was applied to a four-stroke engine. It will be 
recognized by those skilled in the art that the invention is equally 
applicable to two-stroke engines employing check valves in their air 
induction systems. For two-stroke engine applications, abnormal 
fluctuations in induction system pressure due to a faulty check valve 
would repeat with every rotation of the engine crankshaft, and would occur 
at defined times dependent upon the locations of the intake and exhaust 
ports on the cylinder walls. Thus, the CYLINDER look up table would need 
to be modified accordingly to provide the proper cylinder identification 
in two-stroke engines. 
It will also be recognized that in the two-stroke application, the engine 
rotational sensor can be rotated by the engine crankshaft, which rotates 
only once during the complete two-stroke engine cycle. Such a crankshaft 
rotational sensor could also be used in the four-stroke engine 
application, however, it would then only be possible to associate pairs of 
engine cylinders with a faulty check valve, since the precise rotational 
position in the engine cycle would not be available. 
Thus, the aforementioned description of the preferred embodiments of the 
invention is for the purpose of illustrating the invention, and is not to 
be considered as limiting or restricting the invention, since many 
modifications may be made by the exercise of skill in the art without 
departing from the scope of the invention.