Non-Intrusive EEGR System Monitor
The non-intrusive EEGR system monitor is activated during EGR system operation and after certain base engine conditions are satisfied. Inputs from the engine coolant temperature (ECT) or cylinder head temperature (CHT), intake air temperature (IAT), throttle position (TP), crankshaft position (CKP), MAF, and MAP sensors are required to activate the EEGR system monitor. Once activated, the EEGR system monitor carries out each of the tests described below during the engine modes and conditions indicated. Some of the EEGR system monitor tests are also carried out during a key ON engine OFF (KOEO) or key ON engine running (KOER) self-test.
After the vehicle has warmed up and normal EEGR flow rates are being commanded by the PCM, the EEGR flow check is carried out. The flow test is carried out once per drive cycle when a minimum amount of exhaust gas is requested and the remaining entry conditions required to initiate the test are satisfied. If a concern is detected, the EEGR system, as well as the EEGR system monitor, is disabled until the next engine startup.
An EGR flow concern is indicated by either a no flow condition or a low flow condition prior to exceeding 1.5 times the applicable emission standard. The criteria used to determine which flow concern threshold applies is based upon whether or not the applicable emission standards are exceeded on the federal test procedure test cycle without EGR delivery.
The EGR flow test is done by observing the behavior of 2 different values of manifold absolute pressure: the analog MAP sensor reading, and inferred manifold absolute pressure, (manifold absolute pressure calculated from the MAF, throttle position, RPM, barometric pressure [BARO] and other sensors). Due to the location of the MAF sensor, the calculation of inferred manifold absolute pressure is not compensated for EGR flow. Therefore, it does not account for the effects of EGR flow whereas measured manifold absolute pressure does respond to the effects of EGR flow. The amount of EGR flow can therefore be calculated by looking at the difference between measured manifold absolute pressure and inferred manifold absolute pressure under the correct engine operating conditions.
Some differences always exist between measured manifold absolute pressure and inferred manifold absolute pressure due to hardware variations. These variations are learned during steady engine operating conditions without EGR flow and the estimated EGR flow is compensated for these differences. The result of this compensation is values of measured manifold absolute pressure and inferred manifold absolute pressure that are equal under conditions where no EGR is flowing. Hence, when EGR is flowing the increased pressure in measured manifold absolute pressure over inferred manifold absolute pressure represents the pressure change due to EGR flow. This pressure change is normalized to a value between 0 and 1 representing the ratio of measured EGR flow to the scheduled EGR flow and is referred as the EGR flow degradation index. A value near 1 indicates the system is functioning correctly whereas a value near 0 reflects EGR severe flow degradation.
The EGR flow degradation index is compared to a calibrated threshold to determine if a low flow concern has occurred. If an EGR flow concern has occurred, DTC P0400 flow concern is registered.
If the inferred ambient temperature is less than -7°C (20°F), greater than 54°C (130°F), or the altitude is greater than 8,000 feet (BARO less than 22.5 in-Hg), the EEGR system monitor cannot be reliably done. In these conditions, the EEGR system monitor is suspended and a timer starts to accumulate the time in these conditions. When the vehicle leaves these extreme conditions, the timer starts to decrement, and if conditions permit, attempts to complete the EEGR system monitor. If the timer reaches 800 seconds, the EEGR system monitor is disabled for the remainder of the current driving cycle and the EEGR system monitor is set to a ready condition.
The DTC P1408, like P0400, indicates an EGR flow concern (outside the minimum or maximum limits) but is only set during the KOER self-test. The DTCs P0400 and P0403 are malfunction indicator lamp (MIL) codes. The DTC P1408 is a non-MIL DTC.