Electric Exhaust Gas Recirculation (EEGR) System Monitor

ELECTRIC EXHAUST GAS RECIRCULATION (EEGR) SYSTEM MONITOR

The EEGR system monitor is an on-board strategy designed to test the integrity and flow characteristics of the exhaust gas recirculation (EGR) system. The monitor is activated during EGR system operation and after certain base engine conditions are satisfied. Input from the cylinder head temperature (CHT), intake air temperature (IAT), throttle position (TP), mass air flow (MAF), and manifold absolute pressure (MAP) sensors is required to activate the EGR system monitor. Once activated, the EGR system monitor carries out each of the tests described below during the engine modes and conditions indicated. Some of the EGR system monitor tests are also carried out during the on-demand self-test.

The EEGR system monitor consists of an electrical and functional test that checks the stepper motor and the EGR system for correct flow. The powertrain control module (PCM) controls the EGR valve by commanding from 0 to 52 discreet increments or steps to get the valve from fully closed to fully open. The stepper motor electrical test is a continuous check of the four electric stepper motor coils and circuits to the PCM. A concern is indicated if an open circuit, short to power, or short to ground has occurred in one or more of the stepper motor coils/circuits for a calibrated period of time. If a concern has been detected, the EGR system is disabled, setting the key ON engine running (KOER), and continuous diagnostic trouble code (DTC) P0403. Additional monitoring is suspended for the remainder of the driving cycle, or until the next engine startup.

After the engine has warmed up and normal EGR rates are being commanded by the PCM, the EGR flow check is carried out. The flow test is carried out once per drive cycle when a minimum amount of EGR is requested and the remaining entry conditions required to initiate the test are satisfied. If a concern is detected, the EGR system and the EGR system monitor are disabled until the next engine startup.

The EGR flow test is done by observing the behavior of two different values of MAP - the analog MAP sensor reading, and the inferred MAP (MAP calculated from the MAF sensor, TP, and RPM). During normal, steady-state operating conditions, the EGR is intrusively commanded ON to a specified percentage. Then, the EGR is commanded OFF. If the EGR system is working correctly, there is a significant difference in both the observed and the calculated values of MAP between the EGR ON and the EGR OFF states.

When the flow test entry conditions have been satisfied, the EGR is commanded to flow at a calibrated test rate (about 10%). At this time, the value of MAP is recorded (EGR ON MAP). The value of inferred MAP EGR ON IMAP is also recorded. Next, the EGR is commanded off (0%). Again, the value of MAP is recorded (EGR OFF MAP). The value of EGR OFF IMAP is also recorded. Typically, 7 such ON/OFF samples are taken. After all the samples have been taken, the average EGR ON MAP, EGR ON IMAP, EGR OFF MAP and EGR OFF IMAP values are stored.

Next, the differences between the EGR ON and EGR OFF values are calculated:
- MAP delta equals EGR ON MAP - EGR OFF MAP (analog MAP).
- IMAP delta equals EGR ON IMAP - EGR OFF IMAP (inferred MAP).

If the sum of MAP delta and IMAP delta exceeds a maximum threshold or falls below a minimum threshold, DTC P0400 (high or low flow concern) is registered.

As an additional check, if the EGR ON MAP exceeds a maximum threshold (BARO, a calibrated value), DTC P0400 low flow concern is registered.

BARO is inferred at engine startup using the ignition on engine off MAP sensor reading. It is updated during high, part-throttle or high RPM engine operation.

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 EGR flow test cannot be reliably done. In these conditions, the EGR flow test is suspended and a timer starts to accumulate the time in these conditions. If the vehicle leaves these extreme conditions, the timer starts to decrement, and if conditions permit, attempts to complete the EGR flow monitor. If the timer reaches 500 seconds, the EGR flow test is disabled for the remainder of the current driving cycle and the EGR monitor I/M readiness bit is set to a ready condition.

A DTC P1408, like P0400, indicates an EGR flow concern (outside the minimum or maximum limits) but is set only during the key ON engine running (KOER) self-test. The DTCs P0400 and P0403 are MIL codes, and the DTC P1408 is a non-MIL code.