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 ported 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
The EGR vacuum control has a vacuum solenoid that uses "Pulse Width Modulation". What this means is that the ECM turns ON and OFF the solenoid many times a second and varies the amount of ON time (Pulse Width) to change the amount of EGR. The ECM uses the information from the the following sensors to control the solenoid:
Coolant temperature
Throttle Position (TPS)
P/N Switch

Negative Backpressure EGR valve:

VALVE OPERATION
The valve used on this system is a negative backpressure valve. It varies the amount of exhaust gas flow into the manifold depending on the manifold vacuum and variations in exhaust backpressure. The diaphragm on this valve has an internal vacuum bleed hole which is held closed by a small spring when there is no exhaust backpressure. The amount of vacuum to the valve is controlled by the ECM controlling solenoid. Engine vacuum opens the EGR valve against the pressure of a large spring. When the manifold vacuum combines with negative exhaust backpressure, the vacuum bleed hole opens and the EGR valve closes.

EGR Valve Identification:

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