Exhaust gas recirculation system diagnostic

Internal combustion engine exhaust gas recirculation (EGR) system diagnostics provides for intrusive analysis of the performance of the EGR system in two phases which are alternately activated while the engine is in a stable, hot idle operating region. A first phase approximates EGR system performance through an abbreviated opening of an EGR valve to permit a limited amount of EGR pass to an engine intake air passage so that limited measurement of the effect of the EGR on intake air passage pressure may be made with minimum disruption of engine performance and emissions control performance. If the first phase indicates a potential EGR system problem, the second phase is activated and the first phase is deactivated. The second phase confirms or refutes the results of the phase one analysis through a prolonged opening of the EGR valve so that sufficient exhaust gas may pass into the engine intake air passage to permit analysis of the disruption of the intake air passage pressure to reliably diagnose EGR system performance.

FIELD OF THE INVENTION 
This invention relates to automotive vehicle exhaust gas emission controls 
and, more particularly, to automotive internal combustion engine exhaust 
gas recirculation system diagnostics. 
BACKGROUND OF THE INVENTION 
Recirculation of a portion of internal combustion engine exhaust gas to the 
engine fresh air intake, generally termed exhaust gas recirculation EGR, 
reduces engine production and emission of oxides of nitrogen NOx by 
decreasing the level of oxygen in the engine combustion process, and by 
reducing the capacity of the engine intake air charge to absorb heat, 
thereby lowering combustion temperature and frustrating NOx production. 
The amount of EGR must be closely controlled as too much EGR can 
significantly reduce engine performance and can actually increase the 
level of undesirable engine emissions. Accordingly, sophisticated EGR 
control systems have been developed, for example including precision EGR 
valves for varying a degree of opening of an exhaust gas conduit 
positioned between the engine exhaust gas path and the engine fresh air 
intake path. 
The precision EGR valve necessarily must operate in a harsh environment 
characterized by temperature extremes, vibration, and various 
contaminants. Despite such harsh operating conditions, the EGR valve is 
required to maintain a high degree of control precision so that engine 
emissions may be minimized under many varying engine operating conditions. 
Likewise, other EGR system components such as the EGR conduit through 
which the exhaust gas flow and an EGR valve position sensor must remain 
"healthy" to maintain the integrity of the EGR system. In the event an EGR 
system component fails to operate as expected, corrective action must be 
taken as soon as possible, as engine performance and emissions may be 
negatively affected until the failure is remedied. Any significant EGR 
system failure that may impact the effectiveness of the system must be 
diagnosed in a reasonable amount of time and reported so that a remedy may 
be rapidly applied. 
Misdiagnosis of an EGR system fault. condition can result in inconvenient 
and wasteful fault treatment procedures including unnecessary replacement 
of EGR system components. Any EGR system or component diagnostic approach 
must therefore be highly accurate, wherein any failure reported by the 
approach is associated with a very low potential for misdiagnosis. 
EGR diagnostic approaches have been proposed which consume significant 
engine controller processing time and which add significant engine 
controller throughput burden. Further, proposed diagnostic approaches are 
prone to misdiagnosis. Still further, proposed diagnostic approaches only 
return reliable diagnosis under certain specific operating conditions. If 
the operating conditions are not present, no diagnostic is available. 
Still further, proposed intrusive diagnostic approaches may appreciably 
reduce engine performance or significantly increase engine emissions, or 
may cause sudden perceptible disturbances that may reflect poorly on 
engine or vehicle stability. 
Accordingly, it would be desirable to provide an EGR system diagnostic that 
requires minimum processor time and adds minimal additional processor 
throughput burden yet accurately diagnoses the EGR system under a variety 
of commonly occurring engine operating conditions with minimum disruption 
to vehicle operations. 
SUMMARY OF THE INVENTION 
The present invention provides a significant improvement in EGR system 
diagnostics. The present diagnostic minimizes processing time and 
minimizes processor throughput burden through a two tier diagnostic scheme 
in which a first diagnostic phase which may be quickly executed by the 
processor is executed at least once for each vehicle operating cycle to 
quickly diagnose potential fault conditions. If a potential fault 
condition is diagnosed, a second phase is activated and the first phase 
deactivated. The second phase includes a more detailed analysis of the 
health of the EGR system, requiring more processing time and adding more 
throughput burden than the first phase, but reliably and accurately 
diagnosing the fault condition to avoid misdiagnosis. The second phase is 
executed once when activated to confirm or refute the potential fault 
condition. The first phase is characterized by minimum disruption of 
ongoing vehicle control processes, as it is limited to only those 
diagnostic operations necessary to generate a suspicion of a fault 
condition. Once a suspicion is generated, phase two provides for a single 
test cycle of slightly more intrusive diagnostic activity, such as just 
enough activity to confirm or refute the suspicion. As such, engine 
performance and emissions are preserved and are only affected when a 
potential fault condition is suspected. 
In yet a further aspect of this invention, neither diagnostic phase is 
executed until the engine is operating within a predetermined, frequently 
occurring vehicle operating region which provides for stable engine 
operating conditions in-which engine intake-manifold absolute air pressure 
changes may be closely correlated to changes in delivered EGR volume, such 
as stable hot idle operating conditions. Such operating conditions are 
preselected to ensure that up-to-date diagnostic information is available 
and that the diagnostic information is an accurate reflection of the 
performance of the EGR system.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to FIG. 1, an internal combustion engine 10 receives intake air 
through intake air bore 12 in which is disposed intake air valve 16, such 
as a conventional butterfly or rotary valve manually or automatically 
rotatable to vary the degree of restriction of intake air passing through 
the bore 12. Rotational position sensor 18, such as a conventional rotary 
potentiometer, transduces the rotational position of the valve 16 and 
outputs signal TPS indicative thereof. The intake fresh air mass passing 
through bore 12 is sensed by mass airflow sensor 14, which may take the 
form of any conventional automotive mass airflow sensor, such as a thin 
film or hot wire sensor, outputting a signal MAF indicating the intake 
fresh air mass. Intake air passing the valve 16 is received in intake 
manifold 20 for distribution to engine cylinders (not shown). Absolute air 
pressure transducer 22 of any conventional automotive design is disposed 
in the intake manifold 20 for transducing absolute air pressurge in the 
manifold and for outputting signal MAP indicative thereof. Intake air 
bypass conduit 24 provides for passage of intake air past valve 16. Bypass 
valve 26, such as a conventional linear solenoid valve, is disposed in the 
conduit 24 and is positioned to vary restriction in the conduit 24 to flow 
of intake air to the engine intake manifold 20, for example to provide 
engine operation while intake air valve 16 is substantially closed. The 
position of bypass valve 26 is transduced by conventional position sensor 
28, such as a potentiometric position sensor, and is output as signal IAC. 
The intake air is combined with an injected fuel quantity and delivered for 
combustion to the engine cylinders (not shown), wherein the combustion 
produces exhaust gasses which are guided out of the cylinders through 
exhaust gas conduit 34, for example to a catalytic treatment device, for 
emissions reduction operations. A portion of the exhaust gas is guided 
through EGR conduit 42 for combination with fresh air passing through bore 
12 in an exhaust gas recirculation EGR process. The low intake manifold 
pressure relative to the pressure in exhaust gas conduit 34 supports the 
EGR process. To maintain a proper EGR flow, EGR valve 38, such as a 
conventional linear solenoid valve, is positioned in the EGR conduit 42 to 
restrict the exhaust gas flow therethrough, for example taking into 
account the relative pressure across the valve, so that the engine intake 
air charge is sufficiently diluted by the EGR to reduce NOx emissions, as 
is generally understood in the art, but not so diluted to significantly 
reduce engine performance or to increase other engine emission elements. 
The position of the EGR valve 38 is transduced by conventional linear 
potentiometer or other conventional position sensor 40, such as may be 
suited for use in detecting the absolute valve position or displacement, 
and for outputting signal EGRPOS indicative thereof. The combustion of the 
air/fuel mixture in the engine cylinders drives engine output shaft to 
rotate, and the rate of rotation is sensed by conventional Hall effect or 
variable reluctance sensor 32 which outputs signal RPM having a frequency 
proportional to engine speed and containing information that may be 
translated into engine relative engine angular position. A conventional 
microcontroller 36, such as a generally available Motorola eight-bit or 
sixteen bit microcontroller is provided including such standard elements 
as a central processing unit including arithmetic logic circuitry, a read 
only memory ROM, a random access memory RAM, and input-output circuitry. 
The controller 36 receives the described input signals and further receives 
input signals generally understood to be available in conventional 
commercial engine control applications, including signal ACC indicating in 
a binary manner whether the air conditioner clutch (not shown) is engaged 
or disengaged, signal Vbat indicating the battery voltage level, signal 
BARO indicating barometric pressure external to the vehicle, signal VSS 
indicating the speed of motion of the vehicle, and signal ECT indicating 
engine coolant temperature. It should be noted that various 
generally-known sensing approaches may be used to generate the input 
signals applied to the controller 36. For example, the signal BARO may 
come from a dedicated barometric pressure sensor, or may come from air 
pressure transducer 22 under engine operating conditions in which there is 
substantially no pressure drop across the intake air valve 16. As a 
further example, signal VSS may be generated by a conventional vehicle 
wheel speed sensor (not shown) or may be generated by the controller 36, 
for example using sensed engine speed RPM and information on current 
transmission gear ratio. 
The controller 36, through execution of a series of control, diagnostic and 
maintenance operations, generates control and diagnostic signals and 
outputs the signals to various conventional actuators and indicators to 
provide for vehicle control and diagnostic operations including, in this 
embodiment, EGR system diagnostic operations for diagnosing the EGR system 
performance, wherein the EGR system may be defined as including the EGR 
valve 38, the EGR conduit 42, and the EGR valve position sensor 40. 
Such EGR diagnostic operations are generally described in a step by step 
manner by the flow of operations of FIGS. 2a-2c. Such operations may be 
periodically executed while the engine 10 is running under authority of 
the controller 36. In this embodiment, the routine of FIGS. 2a-2c is 
executed once for each vehicle operating cycle to carry out the two tier 
(two phase) EGR system diagnostic in accord with the present invention. In 
other words, between each engine power-on operation and the following 
engine power-off operation, the diagnostic of the present embodiment as 
illustrated in FIGS. 2a-2c will be executed once, to ensure an up-to-date 
diagnosis of the EGR system without adding significantly to the controller 
throughput burden. Generally, this diagnostic provides for a first 
diagnostic phase that is quickly executed with minimum disruption to other 
vehicle control and diagnostics operations and with minimum additional 
controller 36 throughput burden and processing time. The first phase is 
simply designed to quickly estimate EGR system performance and to activate 
more detailed diagnostic operations, called phase two operations, when the 
performance is suspect. When phase two is activated, phase one operations 
are deactivated. Phase two operations are more burdensome to the 
controller 36, and have a greater potential to disrupt ongoing control or 
diagnostic operations. However, phase two operations can, with a high 
degree of confidence, affirm or refute the suspicions raised by the phase 
one operations. Once the phase two operations have confirmed that indeed a 
fault condition is present in the EGR system, or have cleared the EGR 
system as substantially fault free, phase two operations are deactivated. 
No further diagnostic analysis of the EGR system is then provided until 
the next vehicle operating cycle, so that the controller 36 may be freed 
up to carry out its other control, diagnostic and maintenance tasks with 
minimum disruption. 
Returning to the routine of FIGS. 2a-2c, the operations of such routine are 
initiated about every 100 milliseconds while the controller 36 is 
operating until a complete diagnostic of the EGR system has been provided. 
Initiation of the routine may be provided through a timer interrupt 
configured to direct controller attention to, among other conventional 
control or diagnostic routines, the operations of FIGS. 2a-2c, beginning 
at a step 60 and moving next to a step 62 to determine if any phase of the 
diagnostic test is active. The current diagnostic test comprises two 
phases. The first phase may be characterized as a brief diagnostic phase 
to generally estimate EGR system performance, as described. The first 
phase is active automatically when power is first applied to the 
controller following a period in which the engine 10 is turned off. If the 
first phase indicates a potential performance problem with the EGR system, 
the second phase is activated and the first phase is deactivated, wherein 
the second phase may be described as a more throughput-intensive analysis 
of EGR system performance to precisely determine whether a performance 
problem must be indicated and acted on. Once the second phase has 
completed analysis of the EGR system performance, it is disabled to 
prevent further throughput burden until the engine is turned off and then 
turned back on, at which time phase one is automatically activated to pass 
through another test, etc. 
Returning to step 62, if neither phase 1 nor phase 2 are currently active, 
the diagnostic operations proceed to a step 126 to return to any 
operations that were ongoing at the time the routine of FIGS. 2a-2c was 
initiated. However, if phase one is determined to be active at the step 
62, the time since the last diagnostic data sampling is determined at a 
next step 64. The time of the last diagnostic data sampling corresponds to 
the most recent time that engine intake manifold pressure was sampled 
during a prior iteration of the routines of FIGS. 2a-2c. If the determined 
time exceeds 30 seconds at a next step 66, then prior test conditions may 
affect the veracity of the current diagnostic operations, and the 
diagnostic of the current iteration of FIG. 2a is aborted by proceeding to 
the described step 126. Generally, a plurality of test conditions must be 
sustained over a test period to ensure the most accurate test results in 
the current embodiment. If any test conditions are not sustained, the test 
is aborted and a period of time is required for the effects of the test to 
substantially diminish so that future testing is not affected thereby. In 
this embodiment, the amount of delay between test attempts is calibrated 
as about 30 seconds, as described at the step 66. 
However, if the time between samples exceeds 30 seconds at the step 66, the 
testing may continue by proceeding to sample a plurality of input signals 
at a next step 68 to estimate the current engine operating condition. More 
specifically, current values of ECT, VSS, BARO, IAC, TPS, MAF, RPM, ACC, 
and Vbat are sampled at the step 68, wherein such input signals are as 
described in FIG. 1. After sampling the input signals, a plurality of test 
conditions are examined at a next step 70, to determine generally whether 
the engine and vehicle are operating in a stable hot idle condition for a 
period of time. Such a condition occurs commonly during typical automotive 
vehicle operation and as such is well-suited to the current diagnostic so 
that up to date diagnostic information is available for more complete 
fault coverage in accord with a described aspect of this invention. Such a 
condition is characterized by a close correlation between intake manifold 
pressure and EGR valve position. If such conditions are present during the 
analysis of the step 70, the diagnostic operations of the present 
embodiment may be carried out. Such entry conditions are as follows: 
include ECT above a calibrated threshold temperature of about 86 degrees 
Celsius, zero VSS, BARO above a calibrated threshold pressure of about 85 
kPa, very little recent movement of the bypass valve 26 of FIG. 1, such as 
less than a one percent change in IAC valve position over a time period of 
about one hundred milliseconds, zero TPS, change in MAF of less than 0.1 
grams over a time period of about 100 milliseconds, engine speed between 
700 r.p.m. and 800 r.p.m., ACC not currently in transition, and Vbat above 
about twelve volts. If any of these conditions are not met as determined 
at the step 70, the engine is assumed to not be in the stable hot idle 
operating region, and the test is aborted at a next step 72, by proceeding 
to the described step 126. If all conditions are satisfied, the diagnostic 
test is continued by proceeding to check for any current sensor fault 
conditions at a next step 74. For example, if the EGR valve position 
sensor 40 (FIG. 1) or the MAP sensor 30 (FIG. 1), both of which provide 
essential information for the current diagnostic, or the sensors providing 
the signals sampled at the described step 68 are faulty, then the veracity 
of the current diagnostic operation may be reduced to an unacceptably low 
level. 
If any fault condition, such as may be diagnosed through any conventional 
diagnostic approach generally understood in the art, is determined to be 
present at the step 74, the current diagnostic test is aborted by 
proceeding to the described step 126. If no fault conditions are present, 
a next step 76 is executed to determine if any system fault conditions are 
present, such as in the engine coolant circulation system which may be any 
conventional coolant circulation system known in the automotive art, or in 
the idle air control system of FIG. 1. If fault conditions are present in 
either system, the integrity. of the current diagnostic may be reduced to 
an unacceptably low level, and the current diagnostic is aborted by 
proceeding to the described step 126. 
If no system fault conditions are determined to be present at the step 76, 
a data storage position in random access memory, labeled MAPSUM is cleared 
at a next step 78, and-manifold absolute pressure from signal MAP of FIG. 
1 is sampled at a next step 80. The MAP sample is added to MAPSUM at a 
next step 82, and the number of MAP samples is next compared to a 
predetermined value n at a next step 84. In this embodiment, n is 
calibrated as 3. If the number of samples is not greater than 3 at the 
step 84, then more MAP samples are required and a delay of about 12.5 
milliseconds is provided at a next step 86, before proceeding back to 
repeat the steps 80 and 82. The delay ensures that MAP samples used to 
form MAPSUM are spaced by at least 12.5 milliseconds to provide for a 
gathering of MAP information over a longer time period, so that the MAPSUM 
includes a general MAP signal characteristic, for example so as to not be 
polluted by any single engine event. The steps 80 and 82 are repeated 
along with the delay of step 86 until n samples of MAP have been applied 
to form MAPSUM. When the number of samples equals n at the step 84, a 
simple average of the MAP samples is formed at a next step 88, by dividing 
MAPSUM by n. 
The current active test phase is next determined at a step 90. If in phase 
one of the current diagnostic test, the EGR valve 38 (FIG. 1) is commanded 
to open to a test position P at a next step 92. The test position P is a 
predetermined position that provides for a significant change in engine 
intake manifold absolute pressure MAP that is to be used in diagnosing the 
EGR system including the EGR valve 38, the valve position sensor 40, and 
the conduit 42. In this embodiment, due to the short duration of the 
diagnostic test, position P is the maximum valve open position 
corresponding to a minimum restriction of the conduit 42 for maximum 
exhaust gas flow therethrough. The commanding of the valve to open to the 
position P may be provided by setting command signal EGR to a maximum 
calibrated drive current or drive voltage, for example as is generally 
understood in the art. Signal 204 of FIG. 4a illustrates the step change 
in command signal EGR applied to the EGR valve 38 (FIG. 1) at the step 92. 
The actual, sensed EGR valve position EGRPOS and the current manifold 
absolute pressure MAP are next sampled at a step 94, to determine not only 
how the EGR valve 38 is moving toward the commanded position, but how that 
change in position is affecting manifold pressure, in accord with the 
diagnostic of the present invention. Signal 206 of FIG. 4b illustrates a 
typical change in EGR valve position over time, such as sampled via sensor 
output signal EGRPOS of FIG. 1, in response to the corresponding change in 
command signal EGR of signal 204 of FIG. 4a. 
The amount of time that the EGR valve 38 has been allowed to open is next 
compared to a constant t1 at a step 96. To minimize the intrusiveness of 
the current diagnostic on other control processes, the amount of Valve 
opening time is limited to the minimum amount of time necessary for the 
described phase one diagnostic to estimate generally the EGR system 
performance. In this embodiment, this time is t1 of about 40 milliseconds. 
If the valve opening time exceeds time t1 at the step 96, the EGR valve 38 
is commanded to move to its closed position corresponding to no exhaust 
gas flow through the Conduit 42 (FIG. 1), at a next step 102, such as 
illustrated by the falling edge of signal 204 of FIG. 4a. The number of 
samples taken during the current test is next compared to a calibration 
constant n1 at a step 104. 
Generally, the present diagnostic requires periodic sampling of the EGR 
valve position and of the manifold pressure to measure the maximum 
pressure disruption in the engine intake manifold as a result of a change 
in EGR valve 38 position. To ensure that the maximum disruption is 
detected, sampling during and after the opening of the EGR valve 38 is 
required, for example to account for transportation delays in the system. 
As illustrated in the typical MAP signal 208 of FIG. 4c in which MAP is 
responding to commanded change in position of the EGR valve as illustrated 
in the corresponding signal 204 of FIG. 4a in accord with this diagnostic, 
the MAP signal increases significantly with change in commanded EGR valve 
position, as the actual EGR valve position EGRPOS of FIG. 4b moves to 
position P in a "healthy" EGR system. Signal 210 of FIG. 4d. illustrates 
the same MAP response curve for an EGR system experiencing a fault 
condition, such as an EGR valve failure, a EGR valve position sensor 
failure, or an EGR conduit failure, such as due to blockage therein. The 
MAP change in signal 210 resulting from the command change in EGR valve 
position of FIG. 4d is of much lower amplitude than that of signal 208 of 
FIG. 4c, indicating in an essential aspect of this invention, that 
controlled variation in EGR valve position does not impact MAP in a manner 
characteristic of calibrated "healthy" EGR systems. The significant change 
in amplitude is diagnosed by comparing the peak MAP amplitude during and 
shortly after the EGR valve 38 is repositioned to an average of MAP value 
with no such EGR valve 38 repositioning. 
AS illustrated in both response signals 208 and 210, due to the short EGR 
valve 38 opening time of 40 milliseconds of the present phase one, the 
maximum change in MAP resulting from the EGR valve 38 opening may not 
occur until after the EGR valve 38 is commanded closed, such as after the 
falling edge of signal 204 of FIG. 4a. To measure this maximum change, 
sampling may have to continue after the time of closing of the EGR valve 
38. Returning to step 104, if the-number of MAP and EGRPOS samples equals 
n1, calibrated as six in this embodiment, then sampling for phase one is 
complete, and the routine moves to analyze the sampled information 
beginning at a step 110. If the number of samples taken does not equal n1 
at the step 104, a delay of about 12.5 milliseconds is processed at a next 
step 106, and then another sample of MAP and EGRPOS is taken at a next 
step 108. Steps 104, 106 and 108 are continuously repeated in this manner 
until the number of samples equals n1, as determined at the step 104, at 
which time the step 110 is executed. 
Returning to step 96, if the valve opening time is not greater than or 
equal to t1, the number of samples is compared to n1 at a next step 98. If 
the number of samples does not equal n1, more are required, and are taken 
at the step described step 94 following a 12.5 millisecond delay at a next 
step 100. If the number of samples do equal n1, the EGR valve 38 is closed 
at the described step 104, to minimize the intrusiveness of the test, as 
there is no further incentive to intrusively leave the EGR valve 38 open 
following the completion of sampling. 
Continuing with the phase one test operations, the sampled MAP and EGRPOS 
information is processed and analyzed at steps 110-124. First, the EGRPOS 
samples are integrated over the period of the test at a step 110, to form 
value .intg.EGRPOS as an indirect measurement of total EGR flow during the 
test. Next, the maximum sampled MAP value during sampling operations of 
the phase one diagnostic is identified and labeled as MAXMAP at a step 
112. The difference between MAXMAP and the determined AVGMAP from step 88 
is next calculated at a step 114 and is labeled .DELTA.MAP. 
If the test is in phase one as determined at a next step 154, a .DELTA.MAP 
threshold value is referenced at a next step 116 as a function f1 of 
.intg.EGRPOS. Curve 200 of FIG. 3 illustrates the calibrated relationship 
f1 between .intg.EGRPOS and the .DELTA.MAP threshold value for the 
hardware of FIG. 1. The function f1 is determined through a conventional 
calibration process for each of a range of .intg.EGRPOS values likely to 
be encountered in the current diagnostic analysis as the minimum MAP 
change that is normally caused by a "healthy" EGR system in which the EGR 
valve 38 (FIG. 1) moves to the position P for the period of time t1. A 
"healthy" EGR system is generally characterized in this embodiment as an 
EGR system capable of meeting generally understood or publicly promulgated 
performance standards, such as pursuant to a significant reduction in NOx 
levels in the engine emissions of FIG. 1. The function f1 of curve 200 may 
be stored as a series of paired lookup values in a conventional lookup 
table format in controller read only memory, wherein a .DELTA.MAP 
threshold value is returned from the table when the corresponding when the 
current .intg.EGRPOS value is applied as a lookup index or pointer into 
the table. 
After referencing the threshold at the step 116, the determined .DELTA.MAP 
value is compared to the threshold at a next step 118. If .DELTA.MAP is 
less than the threshold, then the change in commanded EGR did not have the 
expected impact on engine intake manifold absolute pressure, indicating a 
potential EGR system fault condition. To avoid misdiagnosis of the 
condition, further testing of a more accurate albeit intrusive nature will 
then be required under phase two of the present diagnostic, to confirm or 
refute the estimated conditions made under phase one analysis. 
Accordingly, if MAP is less than the threshold at the step 118, the phase 
two analysis is activated and the phase one analysis is deactivated at a 
next step 120, and all test variables are cleared at a next step 122 to 
prepare for a completely new analysis of the EGR system under phase two. 
Returning to step 118, if .DELTA.MAP is greater than or equal to the 
threshold, the phase one diagnostic did not diagnose any potential fault 
condition in the EGR system, as the intake manifold pressure under the 
test conditions was adequately responsive to the change in EGR to indicate 
a "healthy" EGR system, with a minimum of intrusiveness and added burden 
to the controller 36 of FIG. 1, as. described. Accordingly, the intrusive 
testing of phase two is not required, and the phase one diagnostic 
analysis is deactivated at a next step 124, until the next time the engine 
10 is turned on, as described. Additionally, a "pass" flag may be set at 
the step 124 to indicate that the diagnostic test of this embodiment has 
been passed. After deactivating phase one analysis and, if necessary, 
activating phase two, the described Step 126 is executed to return to any 
prior temporarily suspended controller operations. 
Phase two diagnostic analysis provides for a more detailed albeit more 
intrusive investigation of the responsiveness of engine intake manifold 
absolute pressure to EGR flow changes to confirm or refute the potential 
fault condition diagnosed through the quick analysis of phase one. 
Returning to step 62 of FIG. 2a, if phase two is active, such as provided 
at the described step 120 of FIG. 2c, then the time since the phase one 
test operations is determined at a next step 130. If the time exceeds a 
calibrated time, such as 30 seconds, as determined at a next step 132, 
then sufficient time has elapsed since the described phase one operations 
that the effects of the intrusive testing thereof have settled so as to 
not significantly impact the phase two analysis, and the testing is 
allowed to continue by proceeding to a next step 64. However, if the 
elapsed time since phase one does not exceed the calibrated time as 
determined at the step 132, the phase two analysis is not provided for by 
proceeding to the described step 126. 
If the phase two operations are allowed to continue from the step 132, the 
described operations of step 66-88 are executed in the manner described 
for phase one, wherein a plurality of entry conditions must be met to 
continue the test and wherein an average MAP value AVGMAP is determined at 
the step 88. After determining the AVGMAP value, the step 90 is executed 
to determine the active test phase. If phase two is active, a next step 
134 is executed at which the EGR valve 38 (FIG. 1) is commanded to open to 
a predetermined calibration position, such as the described position P of 
the step 92 of the phase one analysis. The position is selected during a 
conventional calibration process as a sufficient position to precisely 
establish a relationship between the recirculated engine exhaust gas and 
the corresponding change in intake manifold absolute pressure MAP. After 
commanding the EGR valve 38 to open to the position P, samples of MAP and 
EGRPOS are taken at a next step 136. The valve opening time is next 
compared to a predetermined time t2 at a step 138. If the opening time 
exceeds or is equal to t2, which is set to about 60 milliseconds in this 
embodiment, to provide for more intrusive albeit more precise diagnostics 
of the EGR system, then it is assumed that the EGR system has had ample 
time to measurably impact intake manifold pressure so that a reliable 
diagnosis of the EGR system may be made. The time t2 is calibrated to 
allow for reliable diagnostic analysis of the EGR system despite having a 
potential to temporarily reduce powertrain performance or increase 
emissions. Nonetheless, by avoiding the phase two analysis until the 
substantially unintrusive phase one analysis indicates a potential fault 
condition, the intrusiveness of the overall diagnostic of this embodiment 
is minimized. The time t2 of the present embodiment is but one example of 
the time required for the detailed analysis of phase two. Other test times 
may be required in accord with the hardware to which they are applied in 
order to produce. reliable confirmation or refutation of the potential 
fault condition indicated through the phase one analysis. 
Returning to step 138, if the valve opening time exceeds or is equal to t2, 
the EGR valve 38 is commanded to return to a closed position corresponding 
to no exhaust flow through conduit 42 at a next step 144. After commanding 
the EGR valve 38 to close, such as by dropping the voltage or current 
level of signal EGR (FIG. 4a) to zero, the number of EGRPOS and MAP 
samples taken during the current test is compared to a calibration 
constant n2 at a next step 146. The constant n2 is established as the 
number of samples, at the determined sampling rate, needed to ensure that 
the maximum change in MAP occurring as a result of the intrusive admission 
of EGR into the engine intake manifold is substantially captured through 
the sampling process of step 136 or step 150. In this embodiment, n2 is 
set to nine, to provide for sampling during and after the EGR open time, 
as the maximum intake pressure deviation may come well after the valve is 
being commanded closed, as illustrated in FIGS. 4c and 4d, wherein the 
important maximum MAP amplitude signal may not occur until after the 
falling edge of signal 204 of FIG. 4a, as described. 
If the number of samples equals n2 at the step 146, the described step 110 
is executed to begin analysis of the sampled MAP information. If the 
number of samples does not equal n2 at the step 146, further EGRPOS and 
MAP samples are taken at a step 150 following a delay period of about 12.5 
milliseconds at a step 148. The steps 146-150 are repeated in this manner 
until nine sets of samples have been taken, at which time the described 
step 110 is executed. 
Returning to step 138, if the valve opening time is less than t2, a next 
step 140 is executed to determine if the number of samples equals n2. If 
so, the EGR valve 38 is closed at the described step 144, as no further 
intrusive EGR valve 38 control activity is required if the sampling of MAP 
is completed. However, if the number of samples does not equal n2 at step 
140, then further samples are taken at the step 136 following a delay of 
about 12.5 milliseconds imposed at a step 142. The steps 136-142 are 
repeated in this manner until the valve open time exceeds or is equal to 
t2, as described, or until the number of samples taken equals n2. 
Following collection of the MAP and EGRPOS samples during the phase two 
test, and after the EGR valve 38 of FIG. 1 is returned to its closed 
position, the nine EGRPOS samples are integrated to form value 
.intg.EGRPOS at the described step 110, a maximum MAP sample over the nine 
taken samples is identified and labeled as MAXMAP at the step 112, and a 
.DELTA.MAP is generated as a difference between MAXMAP and AVGMAP at the 
step 114. The step 154 then determines the active test phase. If phase two 
is active, a .DELTA.MAP threshold value us referenced at a next step 152 
as a function of the .intg.EGRPOS value, such as through conventional 
lookup table operations applied to the calibrated function f1 illustrated 
as curve 200 of FIG. 3, wherein .intg.EGRPOS is the lookup index or 
pointer into the lookup table, and the threshold is the reference value. 
After referencing the threshold, the .DELTA.MAP value is compared to the 
threshold at a next Step 156. If .DELTA.MAP is less than the threshold, 
the commanded amount of EGR under the test conditions did not produce the 
expected increase in manifold pressure as would be characteristic of a 
"healthy" EGR system under the more accurate test analysis of the phase 
two diagnostic, and a fault condition is therefore assumed to be present 
in the EGR system of FIG. 1. Such fault condition is next indicated at a 
step 158, such as by storing a fault code in controller non-volatile 
memory or by illuminating a display to notify the vehicle operator of the 
condition so that appropriate action may be taken to alleviate the fault 
condition. Next, or if .DELTA.MAP was greater than or equal to the 
threshold at the step 156, the phase two diagnostic analysis is disabled 
at a next step 160, to prevent further controller throughput burden or 
further intrusion on control performance through the present diagnostic. 
Further diagnostic analysis through the operations of FIGS. 2a-2c will not 
occur in this embodiment until the next time the engine 10 is turned on, 
as described. Following deactivation of the diagnostic phase two, 
execution of any suspended controller operations is resumed through 
execution of the described step 126. 
The preferred embodiment for the purpose of explaining this invention is 
not to be taken as limiting or restricting this invention since man 
modifications may be made through the exercise of ordinary skill in the 
art without departing from the scope of the invention.