Exhaust Gas Recirculation: Description and Operation

Exhaust Gas Recirculation Flow:

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 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 and when the vehicle is above idle speed.

EGR CONTROL
The EGR vacuum control has a vacuum solenoid valve that is controlled by pulse width modulation. This means the Powertrain Control Module (PCM) turns the solenoid "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.
^ Throttle Position (TP) sensor.
^ Manifold Absolute Pressure (MAP) sensor.

Grounding the DLC diagnostic "test" terminal, with the ignition "ON" and the engine not running, will energize the solenoid valve and allow vacuum to the EGR valve.

RESULTS OF INCORRECT OPERATION
Too much EGR flow dilutes the fresh intake air/fuel mixture, causing the engine to run roughly or stall. With too much EGR flow (at idle, cruise, or cold operation) tends to weaken combustion and may result in any of the following conditions:
^ Engine stops after cold start.
^ Engine stops at idle after deceleration.
^ Vehicle surges during cruise.
^ Rough idle.

Too little or no EGR flow allows combustion temperatures to get too high during acceleration and load conditions. This could cause:
^ Spark knock (detonation).
^ Engine overheating.
^ Emission test failure.