Exhaust Gas Recirculation: Description and Operation

Fig 2 EGR Mounting:

PURPOSE
This engine uses an Exhaust Gas Recirculation (EGR) system (Fig. 2). 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

Fig 3 Electric EGR Transducer Assembly:

The Electronic EGR Transducer contains an electrically operated solenoid and a back-pressure transducer (Fig. 3). The 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 60°F as long as the battery ambient temperature is above 7°F.

PCM EGR MONITOR -- DTC 32
The Powertrain Control Module (PCM) performs an on-board diagnostic check of the EGR system.

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 O2S compensation control is monitored.

Turning OFF the EGR shifts the Air/Fuel (A/F) ratio in the lean direction. The O2S 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 O2S sensor data whether the EGR system is operating correctly. Because the O2S sensor is being used, the O2S sensor test must pass its test before the EGR test.