Abstract:
A fuel system includes a pair of electronically controllable high pressure fuel pumps operable to supply high pressure fuel from a lower pressure fuel source to a high pressure fuel accumulator having a pressure sensor associated therewith. The fuel collection chamber feeds an electronically controllable valve operable to dispense the high pressure fuel to a fuel distribution unit supplying fuel to a number of fuel injectors. A control computer is provided for controlling the high pressure fuel pump and valve in response to requested fueling, engine speed and fuel pressure provided by the pressure sensor. The accumulator pressure signal is processed in accordance with the present invention for diagnosing erratic pressure sensor failures. The control computer is operable to compute error pressure values based on differences between peak accumulator pressure values and a target pressure value, and compute pressure error variance values based on subsets of the pressure error values. A fault code is logged and a limp home fueling algorithm is executed if a predefined number of variance values exceed a variance threshold.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to fuel system control techniques, and more specifically to techniques for diagnosing failures and fault conditions in a fuel system. 
     BACKGROUND OF THE INVENTION 
     Electronically controlled high pressure fuel systems are known and commonly used in the automotive and heavy duty truck industries. Such systems may include a fuel pump operable to provide high pressure fuel to a collection unit that supplies the pressurized fuel to one or more fuel injectors. One or more pressure sensors are typically provided for monitoring and controlling the fuel pressure throughout the system. 
     An example of one such system is described in U.S. Pat. No. 5,678,521 to Thompson et al., which is assigned to the assignee of the present invention. The Thompson et al. fuel system includes a pair of cam driven high pressure fuel pumps operable to pump fuel from a low pressure fuel source to an accumulator. The accumulator passes the high pressure fuel to a single injection control valve which is electronically controllable to supply the fuel to a distributor unit. The distributor, in turn, distributes the fuel to any of a number of fuel injectors. The accumulator includes a pressure sensor for monitoring accumulator pressure. An electronic control unit monitors accumulator pressure, throttle position and engine speed, and is operable to control the operation of the fuel system in accordance therewith. 
     High pressure fuel systems of the type just described, while having many advantages over prior mechanical systems, have certain drawbacks associated therewith. For example, failure of electrical and/or mechanical components of the system may result in total system failure, in which case the engine is often shut down leaving the vehicle and occupant stranded. In severe cases, failure of such components can lead to catastrophic destruction of fuel system components. 
     What is therefore needed is a system for diagnosing faults and failures in an electronically controlled fuel system of the type just described. Such a system should ideally log fault codes indicative of fuel system related failures, and pressure sensor failures in particular, to assist in repair efforts, and should additionally provide for a limp home fueling operational mode so that the vehicle can be driven out of danger and/or to a repair facility. 
     SUMMARY OF THE INVENTION 
     The foregoing shortcomings of the prior art are addressed by the present invention. In accordance with one aspect of the present invention, apparatus for diagnosing a fuel system of an internal combustion engine comprises an accumulator receiving pressurized fuel from a source of pressurized fuel, means for sensing fuel pressure within the accumulator and producing a pressure signal corresponding thereto, the pressure signal having peak values corresponding to peak pressures of fuel supplied thereto by the source of pressurized fuel, and a control computer sampling a number of first pressure values each near a separate one of the peak values and determining a number of pressure error values each between a separate one of the number of first pressure values and a corresponding reference pressure, the control computer determining a variance of at least some of the number of pressure error values and incrementing an error counter if the variance exceeds a variance threshold. 
     In accordance with another aspect of the present invention, a method of diagnosing a fuel system of an internal combustion engine comprises the steps of supplying fuel from a source of pressurized fuel to an accumulator based on a target fuel pressure value, measuring a number of peak pressure values within the accumulator each near corresponding actual peak pressures therein resulting from the supplying step, determining a number of error pressure values each between a separate one of the peak pressure values and the target fuel pressure value, determining a variance of at least some of the number of error pressure values, and incrementing an error counter if the variance exceeds a variance threshold. 
     In accordance with either of the foregoing aspects, the error counter is decremented, preferably not below a predefined count value, if the variance is less than the variance threshold. If the error counter exceeds a predefined count value, a fault code is logged and a limp home fueling algorithm is preferably executed. 
     One object of the present invention is to provide an apparatus and method for diagnosing erratic pressure sensor related failures in an electronically controlled fuel system of an internal combustion engine. 
     Another object of the present invention is to provide such an apparatus and method that logs a fault code and executed a limp home fueling algorithm upon detection of erratic pressure sensor behavior. 
     These and other objects of the present invention will become more apparent from the following description of the preferred embodiment. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagrammatic illustration of a fuel system for an internal combustion engine and associated control system, in accordance with the present invention. 
     FIG. 2 is a block diagram illustration of some of the internal features of the control computer of FIG. 1 under normal operation thereof, as they relate to the present invention. 
     FIG. 3 is composed of FIGS. 3A-3G and illustrates waveform diagrams of normal operation of the fuel system and associated control system of FIG.  1 . 
     FIG. 4 is a plot of pressure vs. crank angle of a normal pressure waveform associated with the accumulator of in FIG.  1 . 
     FIG. 5 is a plot of pressure vs. time of a normal pressure waveform and a target pressure waveform associated with the accumulator of FIG.  1 . 
     FIG. 6 is a plot of pressure vs. time of an accumulator pressure waveform sensed by an erratic pressure sensor as compared with a target pressure waveform. 
     FIG. 7 is a plot of pressure vs. number of samples of target pressure, measured pressure and pressure error waveforms indicative of normal operation of the fuel system of FIG.  1 . 
     FIG. 8 is a plot of pressure vs. number of samples of target pressure, measured pressure and pressure error waveforms indicative of erratic pressure sensor operation in the fuel system of FIG.  1 . 
     FIG. 9 is a plot of variance vs. number of samples of target pressure variance, measured pressure variance and pressure error variance waveforms indicative of normal operation of the fuel system of FIG.  1 . 
     FIG. 10 is a plot of variance vs. number of samples of target pressure variance, measured pressure variance and pressure error variance waveforms indicative of erratic pressure sensor operation of the fuel system of FIG.  1 . 
     FIG. 11 is a plot of variance vs. number of samples comparing pressure error variance of a normally operating pressure sensor and an erratic pressure sensor, in the fuel system of FIG.  1 . 
     FIG. 12 is a flowchart illustrating one preferred embodiment of a software algorithm for determining processed pressure error values, in accordance with the present invention. 
     FIG. 13 is a flowchart illustrating one preferred embodiment of a software algorithm for determining a variance of the processed pressure error values determined in accordance with FIG. 12, and for executing diagnostic features upon detection of erratic pressure sensor behavior, in accordance with the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     For the purposes of promoting an understanding of the principles of the invention, reference will now be made to one preferred embodiment illustrated in the drawings and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended, such alterations and further modifications in the illustrated embodiment, and such further applications of the principles of the invention as illustrated therein being contemplated as would normally occur to one skilled in the art to which the invention relates. 
     Referring now to FIG. 1, a fuel system and associated control system  10 , in accordance with the present invention, is shown. System  10  includes a fuel tank  12  or similar source of fuel  14  having a fuel flow path  15  extending into a low pressure fuel pump  16 . Preferably, low pressure pump  16  is a known gear pump having a manually gear mechanism  18  and fuel pressure regulator  20 . A fuel flow conduit  24   a  extends into a high pressure fuel pump  22  having a first (front) pump element  24   b  and a second (rear) pump element  24   c . Pump elements  24   b  and  24   c  are mechanically driven by an engine drive mechanism  28  via cams  26   a  and  26   b  respectively. Fuel flow conduit  24   a  feeds a first pump control valve  30   a  having an output fuel flow conduit  24   d  connected to pump element  24   b . Fuel flow conduit  24   a  is also connected to a fuel flow conduit  24   e  which feeds a second pump control valve  30   b  having an output fuel flow conduit  24   f  connected to pump element  24   c . The first pump element  24   b  is connected to a high pressure fuel accumulator  34  via conduit  36   a  with a check valve  32   a  disposed therebetween. Likewise, the second pump element  24   c  is connected to accumulator  34  via conduit  36   b  with a check valve  32   b  disposed therebetween. 
     High pressure accumulator  34  is connected to an injection control valve  38  via conduit  40 . Injection control valve  38  includes a drain conduit  42  and an output conduit  44  feeding an input  46  of a fuel distributor  48 . Distributor  48  includes a number of output ports, wherein six such output ports  50   1 — 50   6  are illustrated in FIG.  1 . It is to be understood, however, that distributor  48  may include any number of output ports for distributing fuel to a number of fuel injectors or groups of fuel injectors. In FIG. 1, one such fuel injector  52  is connected to output port  50   2  via fuel flow path  54 , wherein injector  52  has an injector output  56  for injecting fuel into an engine cylinder. 
     System  10  is electronically controlled by a control computer  58  in response to a number of sensor and engine/vehicle operating conditions. An accelerator pedal  60  preferably includes an accelerator pedal position sensor (not shown) providing a signal indicative of accelerator pedal position or percentage to input IN 1  of control computer  58  via signal path  62 , although the present invention contemplates utilizing any known sensing mechanism to provide control computer  58  with a fuel demand signal from accelerator pedal  60 . A known cruise control unit  64  provides a fuel demand signal to input IN 2  of control computer  58  via signal path  66  indicative of desired vehicle speed when cruise control operation is selected as is known in the art. 
     An engine speed sensor  68  is connected to an input IN 3  of control computer  58  via signal path  70 , providing control computer  58  with a signal indicative of engine speed position. In one embodiment, engine speed sensor  68  is a known HALL effect sensor, although the present invention contemplates using any known sensor operable to sense engine speed and preferably engine position, such as a variable reluctance sensor. High pressure accumulator  34  includes a pressure sensor  72  connected thereto which is operable to sense pressure within the accumulator  34 . Pressure sensor  72  provides a pressure signal indicative of accumulator pressure to input IN 4  of control computer  58  via signal path  74 . Preferably, pressure sensor  72  is a known pressure sensor, although the present invention contemplates utilizing any known device, mechanism or technique for providing control computer  58  with a signal indicative of fuel pressure within accumulator  34 , conduit  36   a , conduit  36   b  or conduit  40 . Control computer  58  also includes a first output OUT 1  connected to injection control valve  38  via signal path  76  and a second output  78  connected to pump control valves  30   a  and  30   b  via signal path  78 . The general operation of fuel system  10  and associated control system will be described with reference to FIGS. 1-4. 
     Referring to FIGS. 1 and 2, some of the internal features of control computer  58 , as they relate to the present invention, are illustrated. The accelerator pedal signal and cruise control signal enter control computer  58  via signal paths  62  and  66  respectively. As is known in the art, both signals are operator originated in accordance with desired fueling, and control computer  58  is responsive to either signal to correspondingly control the fuel system  10 . Hereinafter, the accelerator pedal and/or cruise control signal will be referred to generically as a fuel demand signal. In any case, the fuel demand signal is provided to a fueling request conversion block  90  which converts the fuel demand signal to a fueling request signal in accordance with known techniques. Typically, fueling request conversion block  90  includes a number of fuel maps and is responsive to a number of engine/vehicle operating conditions, in addition to the fuel demand signal, to determine an appropriate fueling request value. 
     The fueling request value is provided to a reference pressure calculation block  92  which is responsive to the fueling request value to determine a reference pressure indicative of a desired accumulator pressure set point. The reference pressure is provided to an accumulator pressure control loop which provides a pump command signal on signal path  78  based on the reference pressure value and accumulator pressure provided by pressure sensor  72  on signal path  74 . In one embodiment, the reference pressure value is provided to a positive input of a summing node Σ 1  which also has a negative input connected to signal path  74 . An output of summing node Σ 1  is provided to a governor block  96 , the output of which is connected to signal path  78 . In one embodiment, governor block  96  includes a known PID governor, although the present invention contemplates utilizing other known governors or governor techniques. 
     The fueling request value is also provided to a reference speed calculation block  94  which is responsive to the fueling request value to determine a reference speed indicative of a desired engine speed. The reference speed is provided to an engine speed control loop which produces a fuel command value in accordance therewith, as is known in the art, based on the reference speed and actual engine speed provided by engine speed sensor  68  on signal path  70 . In one embodiment, the reference speed value is provided to a positive input of a summing node Σ 2  which also has a negative input connected to signal path  70 . An output of summing node Σ 2  is provided to a governor block  98 , the output of which provides the fuel command value. In one embodiment, governor block  98  includes a known PID governor, although the present invention contemplates utilizing other known governors or governor techniques. 
     Control computer  58  also includes an ICV on time calculation block  100  which is operable to determine an “on time” for activating the injection control valve (ICV)  38  based on the actual accumulator pressure signal provided on signal path  74  and the fuel command provided by governor  98 . The ICV on time calculation block  100  produces a fuel signal on signal path  76  for controlling activation/deactivation of the injector control valve  38 . 
     Referring now to FIG. 3, which is composed of FIGS. 3A-3G, some of the general timing events of fuel system  10  are illustrated. Control computer  58  is operable to control fuel pressure within the accumulator  34  by controlling the pump control valves  24   b  and  24   c . Control of one of the valves  24   b  will now be described, although it is to be understood that operation thereof applies identically to valve  24   c . As the pump plunger retract within the pump element  24   b  under the action of cam  26   a , fuel supplied by low pressure fuel pump  16  flows into the trapped volume of fuel pump element  24   b  as long as valve  30   a  is not energized. If valve  30   a  remains de-energized as the pump plunger rises, fuel within the trapped volume flows back out to low pressure fuel pump  16 . When the pump control valve  30   a  is energized, the outward fuel flow path is closed and the fuel within the trapped volume of pump element  24   b  becomes pressurizes as the pump plunger rises. When the fuel pressure within the trapped volume reaches a specified pressure level, check valve  32   a  opens and the pressurized fuel within the trapped volume flows into the accumulator. Based upon a difference between the reference pressure (block  92  of FIG. 2) and the actual accumulator pressure (provided on signal path  74 ), the pressure control loop of FIG. 2 specifies the angle before pump plunger top dead center (TDC) at which the pump control valve  30   a  is energized. This angle will be referred to hereinafter as a valve close angle (VCA). 
     In one embodiment of fuel system  10 , as illustrated in FIGS. 3B-3G, pump plunger TDC (shown in FIGS. 3D and 3F as front and rear cam respectively) and cylinder TDC (FIG. 3B) are aligned  60  crank degrees apart (FIG.  3 C). The commanded VCA (pump command) may occur anywhere between zero and 120 degrees before pump plunger TDC (see FIGS.  3 D- 3 G). When the difference between the reference pressure and actual accumulator pressure is large, the respective commanded VCA is large and vice versa. Examples of different commanded VCA&#39;s are illustrated in FIGS. 3E and 3G wherein pump command activation times are shown as having a pump activation delay time A and a pump activation time B. VCA&#39;s corresponding to 65 degrees and 30 degrees are shown in FIG. 3E by C and F respectively, and a VCA of 120 degrees is shown in FIG. 3G by D. If the actual accumulator pressure is greater than the reference pressure, the commanded VCA is automatically set at zero degrees, corresponding to no energization of the pump control valve  30   a , as illustrated at E in FIG.  3 G. Control computer  58  is further operable to activate the injection control valve  38  (to control fuel timing) and deactivate valve  38  (to control fueling amount) between pump plunger TDC and cylinder TDC as illustrated in FIGS. 3A,  3 B,  3 D and  3 F. Further operational and structural details of fuel system  10  and associated control system are given in U.S. Pat. No. 5,678,521 to Thompson et al., which is assigned to the assignee of the present invention, the contents of which are incorporated herein by reference. 
     As fuel enters the accumulator  34 , accumulator pressure begins to rise and reaches the reference pressure (FIG. 2) approximately 30 degrees after pump plunger TDC. Thirty degrees after pump plunger TDC of each pumping event, control computer  58  samples accumulator pressure and maintains such samples as peak accumulator pressure samples. Approximately 45-75 degrees after pump plunger TDC, control computer  58  activates the injection control valve  38  (FIG. 3A) to begin an injection event. As fuel is drawn out of the accumulator  38  resulting from activation of the injection control valve  38 , the pressure in the accumulator decreases, and approximately 80 degrees after pump plunger TDC accumulator pressure reaches a minimum. Control computer  58  again samples accumulator pressure at 80 degrees after pump plunger TDC and maintains such samples valley accumulator pressure samples. A plot of accumulator pressure  110  vs. crank degrees, as contrasted with reference pressure  112 , is illustrated in FIG.  4 . FIG. 4 illustrates an accumulator pressure profile for one complete cam revolution of a six cylinder engine. As shown by waveform  110 , the front ( 24   b ) and rear ( 24   c ) pump elements alternate operation, and control computer  58  samples six peak pressure values and six valley pressure values each cam revolution. 
     In accordance the present invention, control computer  58  is operable to monitor the accumulator pressure waveform, an example of which is illustrated in FIG. 4, and diagnose various fuel system related faults and failure conditions; particularly faults and failures associated with the operation of the pressure sensor  72 . One example of such a fuel system fault or failure condition is a stuck in-range failure of pressure sensor  72 , the details of which are described in co-pending U.S. patent application Ser. No. 09/033,338 filed by Stavnheim et al., entitled APPARATUS FOR DIAGNOSING FAILURES AND FAULT CONDITIONS IN A FUEL SYSTEM OF AN INTERNAL COMBUSTION ENGINE and assigned to the assignee of the present invention, the contents of which are incorporated herein by reference. However, the Stavnheim et al. application, as it relates to the detection of pressure sensor related failures, is concerned mainly with stuck in range pressure sensor failures and is therefore not operable to detect intermittent or erratic pressure sensor failures. The present invention is directed to diagnosing such intermittent or erratic pressure sensor failures by monitoring accumulator pressure, and computing a variance in a difference between the sensed accumulator pressure and reference pressure (or pressure setpoint). If the variance between the sensed accumulator pressure and pressure setpoint exceeds a predefined variance threshold for a calibratable number of variance values, computer  58  is operable to log a fault code therein and execute a limp home fueling algorithm directed at pressure sensor-related failures. An example of one particular limp home fueling algorithm useful with the present invention is described in co-pending U.S. patent application Ser. No. 09/033,338 filed by Olson et al., entitled APPARATUS FOR CONTROLLING A FUEL SYSTEM OF AN INTERNAL COMBUSTION ENGINE and assigned to the assignee of the present invention, the contents of which are incorporated herein by reference. 
     Referring now to FIG. 5, a plot of accumulator pressure  122  over time, preferably provided to input IN 4  of control computer  58  via pressure sensor  72 , is shown compared to a reference pressure (accumulator pressure setpoint) waveform  120 , preferably provided by the reference pressure calculation block  92  of FIG.  2 . Waveforms  120  and  122  are indicative of a normally operating fuel system  10 , and it should be observed that actual accumulator pressure  122  tracks the reference pressure  120  fairly closely. Although the exact values of the set point waveform  120  and the measured pressure waveform  122  may be somewhat different at any given instant in time, the variability or rates at which they each change are very similar. 
     Referring now to FIG. 6, by contrast, a reference pressure waveform  130  is shown compared with an accumulator pressure waveform  132  indicative of a pressure sensor  72  exhibiting erratic operation. Erratic pressure sensor behavior is characterized by random and unpredictable transients which are independent of the pressure setpoint waveform  130 . The random and varying pressure measurements contribute higher frequency components to the measured pressure signal  132 . As seen in FIG. 6, it is the higher frequency components that distinguish waveform  132  from waveform  130 , thereby indicating erratic pressure sensor operation. In accordance with the present invention, control computer  58  is accordingly operable to measure and compare the variability of the pressure setpoint waveform with the variability of the measured accumulator pressure waveform. 
     Referring now to FIGS. 12 and 13, one embodiment of a pair of software algorithms  200  and  250  for diagnosing erratic pressure sensor behavior, in accordance with the present invention, are shown. Algorithms  200  and  250  are preferably included within control computer  58  and are executed thereby many times per second as is known in the art. Preferably, algorithms  200  and  250  are executed simultaneously, and are operable to share information. With the aid of the waveform illustrations of FIGS. 7-11, details of algorithms  200  and  250  will now be described in detail. 
     Referring to FIG. 12, algorithm  200  begins at step  202  and at step  204 , control computer  58  samples the current accumulator pressure setpoint, or reference pressure (REF), preferably provided by the reference pressure calculation block  92  of FIG.  2 . Thereafter at step  206 , control computer  58  is operable to sample the actual accumulator pressure (AP), preferably via the signal provided on signal path  74  by pressure sensor  72 , near the peak pressure value (see FIG. 4) for the present pumping event as described hereinabove. Thereafter at step  208 , control computer  58  is operable to compute a pressure error (PE) value based on the current pressure setpoint and current accumulator pressure values. Preferably, control computer  58  is operable to compute the PE value as an algebraic difference between the PE and AP values, although other more complicated difference formulas are contemplated by the present invention. 
     Algorithm execution continues from step  208  at step  210  where the pressure error value (PE) from step  208  is filtered to remove low frequency components therefrom. Preferably, control computer  58  is operable to provide such filtering in accordance with known software filtering techniques. In one embodiment, control computer  58  includes a high pass software filter having a cut off frequency that is set appropriately so as to remove any constant bias, yet pass the high frequency components indicative of erratic pressure sensor behavior. The remaining filtered pressure error signal (FPE) represents the high frequency components of the measured accumulator pressure signal which do not correspond with the computer commanded pressure setpoint. Thereafter at step  212 , control computer  58  is operable to compute an absolute value of the current FPE value determined in step  210 , resulting in an absolute valued filtered pressure error value (ABSFPE). Thereafter at step  214 , the control computer  58  stores the current ABSFPE value therein for further processing in accordance with the software algorithm  250  of FIG.  13 . From step  214 , algorithm execution loops back to step  204 . Algorithm  200  thus continuously produces a signal indicative of pre-processed (filtered and absolute valued) pressure error values. 
     Referring to FIG. 7, example waveforms of the sampled accumulator pressure setpoint values  140  (step  204  of algorithm  200 ), the sampled accumulator pressure values  142  (step  206  of algorithm  200 ) and the computed pressure error values  144  (step  208  of algorithm  200 ) are shown for a normally operating fuel system  10 . By contrast, FIG. 8 shows example waveforms of the sampled accumulator pressure setpoint values  150 , the sampled accumulator pressure values  152  and the computed pressure error values  154  for a pressure sensor  72  exhibiting erratic sensor behavior. 
     Referring now to FIG. 13, one embodiment of a software algorithm  250  for processing the stored ABSFPE values (step  214  of FIG.  12 ), is shown. Algorithm execution begins at step  252  and at step  254  control computer  58  sets a counter equal to an arbitrary value; zero in this case. Thereafter at step  256 , control computer  58  determines whether a predefined number, N, of ABSFPE values are available for processing. In one embodiment, control computer  58  includes a queue which holds the ten most recent ABSFPE values, and computer  58  is operable to determine the variance of the pressure error values based on these 10 ABSFPE values. It is to be understood, however, that any number of recent ABSFPE values can be used for the variance computation, and the actual number used is a matter of design choice. In any case, if control computer  58  determines at step  256  that less than N (e.g. 10) ABSFPE samples are available, algorithm execution loops back on step  256  until algorithm  200  provides N such values. When at least N ABSFPE values are available, algorithm execution continues a step  258 . 
     At step  258 , control computer  58  computes a variance (VAR) of the N most recent ABSFPE samples. In one embodiment, control computer  58  is operable at step  258  to compute VAR as a simplified variance by summing the N samples. However, the present invention contemplates computing VAR in accordance with other known variance equations at step  258 . By computing a simplified variance based on the  10  most recent ABSFPE samples, susceptibility to spurious noise is reduced; i.e. detection of erratic sensor behavior will require detection of a meaningful number of high frequency spikes. 
     Algorithm execution continues from step  258  at step  260  where control computer  58  tests the variance value VAR against a variance threshold TH, which is preferably calibratable. If, at step  260 , control computer  58  determines that VAR is greater than TH, algorithm execution continues at step  262  where control computer  58  increments the error counter. If, at step  260 , control computer  58  determines that VAR is less than or equal to the threshold TH, algorithm execution continues at step  264  where the control computer  58  decrements the error counter (preferably not below zero, however). From either of steps  260  or  264 , algorithm execution continues at step  266 . 
     At step  266 , control computer  58  compares the error counter against a predefined (preferably calibratable) count value. If the error counter is less than the predefined count value, algorithm execution loops back to step  258  for calculation of another variance value. If, at step  266 , control computer  58  determines that the error counter is greater than or equal to the predefined count value, algorithm execution continues at step  268  where control computer  58  logs a fault code therein indicative of an erratic pressure sensor failure. In one embodiment, the predefined count value is set at 36 counts, although the present invention contemplates utilizing other count values. Algorithm execution continues from step  268  at step  270  where control computer  58  is operable to execute a limp home fueling algorithm. Preferably, the limp home algorithm is directed to providing at least minimum fueling to sustain engine operation so that the vehicle may be driven out of danger and/or to a service/repair facility. One example of such a limp home algorithm is detailed in pending U.S. patent application Ser. No. 09/033,338, filed by Olson et al., entitled APPARATUS FOR CONTROLLING A FUEL SYSTEM OF AN INTERNAL COMBUSTION ENGINE and assigned to the assignee of the present invention, the contents of which have been incorporated herein by reference. Algorithm execution continues from step  270  at step  272  where algorithm execution is returned to its calling routine. Alternatively, step  270  may loop back to step  254  for continuous execution of algorithm  250 . 
     Referring to FIG. 9, example waveforms of the variance in the sampled reference pressure values  160 , the variance in the sampled accumulator pressure values  162  and the variance in the computed pressure error values  164  are shown for a normally operating fuel system  10 . By contrast, FIG. 10 shows example waveforms of the variance in the sampled reference pressure values  170 , the variance in the sampled accumulator pressure values  172  and the variance in the computed pressure error values  174  for a pressure sensor  72  exhibiting erratic sensor behavior. FIG. 11 shows a comparison of the variance in the computed pressure error values for a normally  180  operating fuel system  10  and for a pressure sensor  72  exhibiting erratic sensor behavior  182 . 
     While the invention has been illustrated and described in detail in the foregoing drawings and description, the same is to be considered as illustrative and not restrictive in character, it being understood that only one preferred embodiment thereof has been shown and described and that all changes and modifications that come within the spirit of the invention are desired to be protected.