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
- Disconnect the vacuum line at the EGR valve.
- Remove the bolt securing the EGR valve and remove the valve from the intake manifold.
- Use a new gasket and install the EGR valve on the intake manifold. Tighten the mounting bolt to 13-18 ft. lbs.
- Connect the vacuum line to the valve.
- Disconnect the vacuum lines.
- Remove the switch from the thermostat housing.
- Use sealer on the threads of the switch, install the switch and torque to 15 ft. lbs.
- Turn the switch head if necessary to align for hose routing and connect vacuum hoses.
THERMAL VACUUM SWITCH CHECK
This test must be performed with the engine at normal operating temperatures, permitting the vacuum signal to reach the EGR valve.
- Remove the EGR valve vacuum hose at EGR valve and attach the hose to a vacuum gauge.
- Start the engine and open the throttle partially. Do not race the engine. As the throttle is opened, the vacuum gauge should respond proportionately.
- If the vacuum gauge responds correctly, remove it and replace the hose to the EGR valve.
- If the gauge does not react correctly, remove the carburetor-to-switch hose at the switch and attach the vacuum gauge to it. Repeat Step 2. If the gauge responds to the opening of the throttle, the switch is defective and must be replaced.
- If the gauge does not respond to the opening throttle, suspect a plugged hose or a defective carburetor.