C 2500 Truck 2WD V8-350 5.7L (1988)
EGR Valve: Description and Operation
Negative Backpressure EGR Valve (Under 8500 GVW)
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 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 closed solenoid vacuum valve which is opened when
the ECM completes the ground. The ECM will energize the EGR solenoid (EGR "ON") when the engine is warm and above idle. There is little EGR
flow at wide open throttle due to lack of vacuum.
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 air bleed hole which is held closed by a small spring when
there is no exhaust backpressure. The 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. The valve will open if vacuum is applied to it with the engine not running.
EGR Valve Identification
