Exhaust Gas Recirculation: Description and Operation

Exhaust Gas Recirculation:

Negative Backpressure EGR Valve:

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.

NEGATIVE BACKPRESSURE VALVE
The EGR valve used on this engine is a negative backpressure valve. The amount of exhaust gas flow into the manifold depends on ported vacuum and variations of exhaust backpressure.

The diaphragm on this valve has an internal air bleed hole which is held closed by a small spring when there is no exhaust backpressure.

Ported vacuum opens the EGR valve against the presure of a large spring. When ported vacuum combines with negative exhaust backpressure, the vacuum bleed hole opens and the EGR valve closes.

This valve will open if vacuum is applied with the engine not running and the exhaust evacuation system connected at the exhaust pipe.

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.