See Figures 1 through 26
Oxides of nitrogen (NOX) in engine exhausts are reduced by lowering the combustion temperature inside the cylinders. This is accomplished by allowing a predetermined amount of exhaust gas to recirculate into and dilute the incoming air/fuel mixture. However, a problem arises with this system. When an engine is cold, the combustion temperature is not high enough to create NOX emissions, and if you recirculate exhaust gasses into a cold engine, it usually causes the engine to stumble or stall. So additional systems were designed to correct this problem. The first addition was a simple vacuum delay valve, which stopped the opening of the EGR valve for several seconds, usually enough time to raise the engine rpm high enough to not be affected by the exhaust gasses now being introduced into the air/fuel mixture. Next, temperature controlled vacuum switches were used, which shut off the vacuum until a certain engine temperature was reached. Then, through advancements in electronics, control devices appeared first as electrical switches, and eventually became computer controlled, which can precisely control the timing of the EGR valve.Venturi Vacuum Control System
The venturi vacuum control system utilizes a vacuum tap at the throat of the carburetor venturi to provide a control signal. However, this is a very weak vacuum source which requires the use of a vacuum amplifier to increase the vacuum to a level required to properly operate the EGR valve. The vacuum amplifier is not serviceable, and must be replaced if found to be defective.Ported Vacuum EGR
The ported EGR valve is operated by engine vacuum. A vacuum signal from the carburetor or throttle bodyactivates the EGR valve diaphragm. As the vacuum signal increase it gradually opens the valve pintle allowing exhaust gases to flow. The amount of flow is directly proportional to the pintle position.
- The engine should be warmed up, at normal operating temperature, with the parking brake set.
- Allow the engine to idle in Neutral with the throttle closed, then quickly accelerate to approximately 2,000 rpm, watching the groove carefully on the EGR valve stem.
- You should notice movement of the valve stem during the acceleration period, and there should be a change in the location of the groove on the stem. If movement is noticed this means that the control system is functioning properly and the EGR flow test can be performed.
- If no movement is noticed, you may have to replace the valve or another component in the system.
- Connect a tachometer to the engine.
- Remove the vacuum hose from the EGR valve and connect a hand vacuum pump to the valve vacuum motor nipple.
- Start the engine and slowly apply vacuum to the motor.
- The engine rpm should drop as the vacuum reaches 3-5 in. Hg (10-17 kPa), and continue to drop as more vacuum is applied. Your engine may even stall out during this test (meaning that EGR gases are flowing through the system).
- If this and system tests are good, your EGR system is fully functioning.
- If the rpm does not drop, the valve may be plugged or defective. If so, remove the valve and inspect it, along with the intake manifold passages for any deposits. Clean if necessary.