Exhaust Gas Recirculation: Description and Operation

The EGR system routes exhaust gases from the exhaust manifold to the intake manifold at specified engine operating conditions. The EGR valve is opened by manifold vacuum to let exhaust gas flow into the intake manifold. The exhaust gas then moves with the air/fuel mixture into the combustion chamber. Since there is less air/fuel mixture to burn in the combustion chamber during EGR operation, the temperature is reduced enough to limit NOx formation. If too much exhaust gas enters combustion will not occur. Because of this, very little exhaust gas is allowed to pass through the valve, especially at idle. The valve should only open when the engine is at operating temperature and above idle speed.



EGR CONTROL
To regulate EGR flow an ECM controlled solenoid is used in the vacuum line. This is a normally open solenoid vacuum valve which is closed when the ECM completes the ground. The ECM will de-energize the EGR solenoid allowing vacuum to pass to the EGR valve.

Positive Backpressure EGR Valve:

VALVE OPERATION
The positive backpressure valve has an air bleed, located inside the EGR valve assembly, that acts as a vacuum regulator. This bleed valve controls the amount of vacuum in the vacuum chamber by bleeding vacuum to atmosphere during the open phase of the cycle.
When the air valve receives sufficient exhaust backpressure through the hollow shaft it closes the bleed. At this point, maximum available vacuum is applied to the diaphragm and the EGR valve opens. If there is little or no vacuum in the vacuum chamber such as at idle or wide open throttle, or if there is little or no pressure in the exhaust manifold, the EGR will not open. This valve will not open if vacuum is applied to it with the engine stopped or idling.

EGR Valve Identification:

Positive backpressure EGR valves will be stamped with a "P" on the top side of the valve after the part number.