Exhaust Gas Recirculation: Description and Operation

Exhaust Gas Recirculation:

PURPOSE
The Exhaust Gas Recirculation (EGR) system is designed to decrease NOx (Oxides of Nitrogen) in exhaust emissions.

OPERATION
It does this by introducing exhaust gas, which contains very little oxygen, into the intake manifold. The exhaust gas will not support combustion, but does occupy volume, reducing the total amount of air/fuel mixture which burns in the cylinder. This reduces combustion temperatures.

The EGR valve is opened by ported vacuum to allow exhaust gas to flow into the intake manifold. The exhaust gas then flows with the air / fuel mixture into the combustion chamber. If too much exhaust gas enters, combustion may not occur. For this reason, very little exhaust gas is allowed to pass through the valve, and none at idle, or wide open throttle.

EGR CONTROL
To regulate exhaust gas flow, a Powertrain Control Module (PCM) controlled EGR control solenoid valve is used in the vacuum line. The PCM uses information from the following sensors to regulate the solenoid:

^ Engine Coolant Temperature (ECT) sensor.
^ Throttle Position (TP) sensor.
^ Manifold Absolute Pressure (MAP) sensor.
^ Vehicle Speed Sensor (VSS).

RESULTS OF INCORRECT OPERATION
If there is any EGR leakage at idle or when engine is cold or too much EGR at cruise any of the following may occur:

^ Engine stalls after cold start.
^ Engine stalls at idle after deceleration.
^ Vehicle surges during cruise.
^ Rough idle.
^ Emission test failure.

If the EGR valve should stick in the open position a no start condition may occur if engine is cold.

Too little or no EGR flow allows combustion temperatures to get too high, this could cause:

^ Spark knock (detonation).
^ Engine overheating.