Exhaust Gas Recirculation: Description and Operation: Overview

Fig. 25 - EGR Control Valve:

This system is used to reduce oxides of nitrogen emissions at the engines exhaust. This is accomplished by introducing exhaust gases into the intake manifold at throttle positions other than idle. The system on 1974-82 models, uses an EGR valve. The exhaust gas intake port of the EGR valve is connected to the intake manifold exhaust crossover channels where it can pick-up exhaust gases. As the throttle valves are opened and the engine accelerates, vacuum is applied to a vacuum diaphragm in the EGR valve through a connecting tube. As the diaphragm moves up, it opens the valve in the exhaust port which allows exhaust gas to be pulled into the intake manifold and enter the cylinders.


Two different type systems are used to control EGR operation on 1974-82 models, a Ported Vacuum Control System and Venturi Vacuum Control System. Both systems use the same type exhaust gas recirculation (EGR) control valve, only the method of controlling the valve is different. The valve is a vacuum actuated, poppet type unit used to modulate exhaust gas flow from the exhaust gas crossover into the incoming air fuel mixture.

Ported Vacuum Control System
The ported vacuum control system utilizes a slow type port in the carburetor throttle body which is exposed to an increasing ratio of manifold vacuum as the throttle blade opens. This throttle bore port is connected through an external nipple directly to the EGR valve. The flow rate is dependent on three variables, 1) manifold vacuum, 2) throttle position, and 3) exhaust gas back pressure. Recycle at wide open throttle is eliminated by calibrating the valve opening point above manifold vacuums available at wide open throttle as port vacuum cannot exceed manifold vacuum. Elimination of wide open throttle recycle provides maximum performance.

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. This vacuum signal is amplified to the level required to operate the EGR control valve. Elimination of recycle at wide open throttle is accomplished by a dump diaphragm which compares venturi and manifold vacuum to determine when wide open throttle is achieved. At wide open throttle, the internal reservoir is "dumped", limiting output to the EGR valve to manifold vacuum. The valve opening point is set above the manifold vacuums available at wide open throttle.


NOTE: This system is dependent primarily on engine intake airflow as indicated by the venturi signal, and is also affected by intake vacuum and exhaust gas back pressure.