The purpose of the EGR system is to control oxides of nitrogen which are formed during the peak combustion temperatures. The system feeds small amounts of exhaust gas back into the combustion chamber. Feeding back the exhaust gas lowers the combustion temperature which in turn lowers the oxides of nitrogen. If to much exhaust was reintroduced into the intake stream, combustion would not occur or would be inhibited. Symptoms of a open valve would be stalling after a cold start, engine stalls after deceleration, engine surging during a steady speed and rough idle. If the valve was always closed spark knock, engine overheating and possible emission test failure might occur.
The EGR valve is operated by engine vacuum and generally is open only when the engine is warm and is operating above the idle speed. There are three types of valve control systems used to control EGR valve operation in General Motors trucks; the positive exhaust backpressure type, the negative exhaust backpressure type and the ECM vacuum solenoid controlled type. The ECM controlled EGR system is generally found only on trucks built for use in California.
Both the negative and positive backpressure type valves use a thermal vacuum switch to prevent operation before the engine has warmed. The switch prevents vacuum from being applied to the valve when the engine coolant is cold. Both the negative and positive valves are designed to insure that the valve only operates above the engine idle speed and the amount the valve opens is not excessive. The choice of either a positive or negative backpressure valve is determined by measurement of the engine's normal backpressure output. Negative valves are used on engines with relatively low backpressure; positive valves are used on engines with relatively high backpressure. If the valve is to be tested or replaced, it is important to know the type of installed. To identify the type of valve used, look on the top side of the valve. A "P" will be stamped on the valve after the part number if the positive type is used or an "N" for a negative type valve.
The positive backpressure type valve has an internal vacuum bleed hole that is open when exhaust pressure is low. As exhaust backpressure builds, the bleed valve closes allowing engine vacuum to be applied to open the EGR valve. The negative backpressure type EGR valve has an internal bleed hole that is closed when there is low exhaust backpressure. Engine vacuum opens the EGR valve. When engine vacuum increases combined with negative backpressure the vacuum bleed hole opens and the EGR valve will close.
The ECM controlled EGR valve uses a solenoid valve to control the operation of the EGR valve. The ECM monitors the coolant temperature sensor signal and the throttle position sensor to determine when to operate the valve. Based on the coolant temperature and throttle position, the ECM will pulse the EGR vacuum solenoid on and off many times a second and varies the amount of "on" time to vary the amount of gas recirculated. Since the ECM determines when the system operates, the level of exhaust backpressure is not important and therefore the system can use a simple standard ported EGR valve.
On 6-cylinder engines, the EGR valve is located on the intake manifold adjacent to the carburetor. On small block V8 engines, the valve is located on the right rear side of the intake manifold adjacent to the rocker arm cover. Mark IV V8 EGR valves are located in the left front corner of the intake manifold in front of the carburetor.
Fig. 1: Non–ECM controlled EGR valve system
Fig. 2: ECM controlled EGR valve system