Engine Management

The Powertrain Control Module (PCM) performs an on-board diagnostic check of the EGR system.

THEORY
The EGR system consists of two main components: a vacuum solenoid and a vacuum operated valve with a back pressure transducer. The EGR monitor is used to test whether the EGR system is operating within specifications. The diagnostic check activates only during selected engine/driving conditions. When the conditions are met, the EGR is turned off (solenoid energized) and the O2 compensation control is monitored. Turning off the EGR shifts the air/fuel (A/F) ratio in the lean direction. The O2 sensor data should indicate an increase in the O2 concentration in the combustion chamber when the exhaust gases are no longer recirculated. While this test does not directly measure the operation of the EGR system, it can be inferred from the shift in the O2 sensor data whether the EGR system is operating correctly

MALFUNCTION CAUSES
As the exhaust gas enters the EGR valve, carbon deposits can form on the valve and valve seat due to the lower temperature of the intake manifold side of the valve. The accumulation of these deposits can reduce the effective orifice of the valve. This condition can cause a malfunction due to low EGR flow.

The rupture of either the EGR valve or transducer diaphragm will disable the EGR valve. This condition will cause a malfunction due to no or low EGR flow.

If an EGR valve solenoid should mechanically fail closed, there will be no vacuum supply to the control valve which will prevent recirculation of exhaust gas. A solenoid that is energized all the time will be closed all the time and also prevent EGR flow.

Excessive backpressure or no backpressure (vacuum line missing) will also cause the system to not operate properly

MALFUNCTION EFFECTS
A complete shut off or a low flow of the EGR valve will eliminate or reduce the temperature lowering effect of the EGR system. This in turn will result in an increase of NOx emissions.

A stuck open or a high flow of the EGR valve will result in a rich air /fuel mixture. This can affect hydrocarbon (HC) emissions, fuel consumption, and can also increase engine roughness.

TEST CONDITIONS
The EGR system test will run when the following conditions have been met:
- Engine coolant temperature above 41°F
- The fuel system is operating in closed loop
- Driven for more than 5 minutes
- TPS voltage between 0.137 and 0.549 volts
- Vehicle speed above 3 mph
- Engine rpm is between 1,400 and 2,500
- Engine vacuum of at least 3.4 inches of Hg.

- MAP sensor voltage is between 1.44 and 3.11 volts.

This test is performed 8 times each trip. If the measured change in the A/F ratio during the test is less than 4.5% the system test fails. If the measured change is above 4.5% the system passes and testing is stopped. If the system fails the test the PCM registers a DTC and turns on the Malfunction Indicator Lamp (MIL) (Check Engine).

If the MIL turns on, check for proper operation of the EGR system.

If the EGR system tests properly, check the system using the DRB or equivalent scan tool.