Exhaust Gas Recirculation: Description and Operation

Fig. 1 EGR System:

PURPOSE
The EGR system reduces oxides of nitrogen (NOX) in engine exhaust and helps prevent spark knock. The system allows a predetermined amount of hot exhaust gas to recirculate and dilute the incoming air/fuel mixture. The diluted air/fuel mixture reduces peak flame temperature during combustion.

OPERATION
The EGR system consists of:
- EGR tube (connects a passage in the intake manifold to the exhaust manifold)
- EGR valve
- Electronic EGR Transducer
- Connecting hoses

The electronic EGR transducer contains an electrically operated solenoid and a back-pressure transducer (Fig. 1). The Powertrain Control Module (PCM) operates the solenoid. The PCM determines when to energize the solenoid. Exhaust system back-pressure controls the transducer.

When the PCM energizes the solenoid, vacuum does not reach the transducer. Vacuum flows to the transducer when the PCM de-energizes the solenoid.

When exhaust system back-pressure becomes high enough, it fully closes a bleed valve in the transducer. When the PCM de-energizes the solenoid and back-pressure closes the transducer bleed valve vacuum flows through the transducer to operate the EGR valve.

De-energizing the solenoid but not fully closing the transducer bleed hole (because of low back-pressure), varies the strength of vacuum applied to the EGR valve. Varying the strength of the vacuum changes the amount of EGR supplied to the engine. This provides the correct amount of exhaust gas recirculation for different operating conditions.

This system does not allow EGR at idle. The EGR systems can operate at all coolant temperatures above 5°C (40°F).

EGR System On-Board Diagnostics
The PCM performs an on-board diagnostic check of the EGR system. The diagnostic system uses the electronic EGR transducer for the system tests.

The diagnostic check activates only during selected engine driving conditions When the conditions are met, the PCM energizes the transducer solenoid to disable the EGR. The PCM checks for a change in the heated oxygen sensor signal. If the air-fuel mixture goes lean, the PCM will attempt to enrich the mixture. The PCM registers a Diagnostic Trouble Code (DTC) if the EGR system has failed or degraded. After registering a DTC, the PCM turns on the malfunction indicator Check Engine lamp. The Malfunction Indicator Lamp (MIL) indicates the need for service.

If a problem is indicated by the MIL and a DTC for the EGR system is set, check for proper operation of the EGR system. Use the System Test, EGR Gas Flow Test and EGR Diagnosis Chart. If the EGR system tests properly, check the system using the DRB or scan tool. Refer to the DRB or scan tool and the appropriate Diagnostic Procedure.