Exhaust Gas Recirculation (EGR), is used to reduce oxides of nitrogen (NOx) exhaust emissions. NOx formation occurs at very high combustion temperatures so that the EGR system reduces combustion temperature slightly by introducing small amounts of inert exhaust gas into the intake manifold. The result is reduced formation of NOx.
The EGR valve is mounted on the intake manifold and contains a vacuum diaphragm. The unit is operated by intake manifold vacuum and controls the flow of exhaust gases.
A vacuum signal supply port is located in the throttle body of the carburetor above the throttle plate. Vacuum is supplied to the EGR valve (causing recirculation), at part-throttle conditions. EGR does not occur at idle or at wide open throttle.
Some models also use a Thermal Vacuum Switch (TVS), mounted in the outlet water housing to block vacuum to the EGR valve until engine coolant temperature is approximately 100°F (38°C).
An engine that idles roughly may be caused by a bad EGR valve. Push on the diaphragm plate to check for freedom of movement. If it sticks, replace the valve. Hook up a vacuum gauge between the signal tube and the vacuum hose. With the engine running and warmed up, increase the engine speed to obtain 5 in. Hg of vacuum. Remove the vacuum hose downward. This should be accompanied by increased engine speed. Replace the vacuum hose and check to see that the plate moves upward. The engine speed should drop. If the diaphragm is not moving check for vacuum at the EGR hose. If there is none, check for leaking, plugged, or misplaced hoses. If the diaphragm moves but there is no change in rpm, check for blocked EGR manifold passages.
REMOVAL & INSTALLATION