EGR Valve: Description and Operation

PURPOSE:
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 vacuum and exhaust backpressure, letting 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. Variations in vacuum and exhaust backpressure, control the amount of exhaust gas recirculation.

Negative Backpressure EGR Valve:

OPERATION:
The negative backpressure EGR valve has the bleed valve spring below the diaphragm, and the valve is normally in the closed position. The negative backpressure EGR valve varies the amount of exhaust gas flow into the manifold depending on manifold vacuum and variations in exhaust backpressure.

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

Engine vacuum opens the EGR valve against the pressure of a large spring. When manifold vacuum combines with negative exhaust backpressure, the vacuum bleed hole opens and the EGR valve closes.

This valve will open if vacuum is applied and the engine is not operating.

EGR Valve Identification:

IDENTIFICATION:
The negative backpressure EGR valve can be identified by the letter "N" stamped on the top side of the valve immediately following the part number. See illustration.