Exhaust Gas Recirculation: Description and Operation

EGR Valve Operation:

PURPOSE
The Exhaust Gas Recirculation (EGR) system is used to lower NOx (Oxides of Nitrogen) emission levels caused by high combustion temperature and excessive oxygen. It does this by decreasing combustion temperature and displacing oxygen.

The main element of the system is the EGR valve mounted on the intake manifold and operated by vacuum. The EGR valve feeds small amounts of exhaust gas back into the combustion chamber as shown in the accompanying image.

OPERATION
The EGR valve is opened by vacuum to let exhaust gas flow into the intake manifold. The exhaust gas then moves with the air/fuel mixture into the combustion chamber. If too much exhaust gas enters, combustion will not occur. For this reason, very little exhaust gas is allowed to pass through the EGR valve. The EGR valve is usually open during warm engine operation when the vehicle is above idle speed.

EGR VACUUM CONTROL
The EGR vacuum control has a vacuum solenoid valve that is controlled by pulse width modulation. This means the PCM turns the solenoid valve "ON" and "OFF" many times a second and varies the amount of "ON" time ("pulse width") to vary the amount of vacuum applied to the EGR valve.

The PCM uses RPM and information from the following sensors to regulate the EGR solenoid valve:

^ Engine Coolant Temperature (ECT) sensor.
^ Intake Air Temperature (IAT) sensor.
^ Throttle Position (TP) sensor.
^ Manifold Absolute Pressure (MAP) sensor.
^ Park/Neutral Position (PNP) switch.
^ Vehicle Speed Sensor (VSS).