All engines are equipped with this system, which consists of a metering valve, a vacuum line to the carburetor or intake manifold, and cast-in exhaust passages in the intake manifold. The EGR valve is controlled by vacuum, and opens and closes in response to the vacuum signals to admit exhaust gases into the air/fuel mixture. The exhaust gases lower peak combustion temperatures, reducing the formation of NO x . The valve is closed at idle and wide open throttle, but is open between the two extreme positions.
There are actually two types of EGR systems: Vacuum Modulated and Exhaust Backpressure Modulated. The principle of both systems is the same; the only difference is in the method used to control how far the EGR valve opens.
In the Vacuum Modulated system, the amount of exhaust gas admitted into the intake manifold depends on a ported vacuum signal. A ported vacuum signal is one taken from the carburetor above the throttle plates; thus, the vacuum signal (amount of vacuum) is dependent on how far the throttle plates are opened. When the throttle is closed (idle or deceleration) there is no vacuum signal. Thus, the EGR valve is closed, and no exhaust gas enters the intake manifold. As the throttle is opened, a vacuum is produced, which opens the EGR valve, admitting exhaust gas into the intake manifold.
In the Exhaust Backpressure Modulated system, a transducer is installed in the EGR valve body. The vacuum is still ported vacuum, but the transducer uses exhaust gas pressure to control an air bleed within the valve to modify this vacuum signal.
- Check to see if the EGR valve diaphragm moves freely. Use your finger to reach up under the valve and push on the diaphragm. If it doesn't move freely, the valve should be replaced. The use of a mirror will aid the inspection process.
- Install a vacuum gauge into the vacuum line between the EGR valve and the carburetor. Start the engine and allow it to reach operating temperature.
- With the car in either Park or Neutral, increase the engine speed until at least 5 in. Hg (7 in. Hg for 1981) is showing on the gauge.
- Remove the vacuum hose from the EGR valve. The diaphragm should move downward (valve closed). The engine speed should increase.
- Install the vacuum hose and watch for the EGR valve to open (diaphragm moving upward). The engine speed should decrease to its former level, indicating exhaust recirculation.
If the diaphragm doesn't move:
- Check engine vacuum; it should be at least 5 in. Hg (7 in. Hg for 1981) with the throttle open and engine running.
- Check to see that the engine is at normal operating temperature.
- Check for vacuum at the EGR hose. If no vacuum is present, check the hose for leaks, breaks, kinks, improper connections, etc., and replace as necessary.
If the diaphragm moves, but the engine speed doesn't change, check the EGR passages in the intake manifold for blockage.
REMOVAL & INSTALLATION
- Disconnect the vacuum hose.
- Remove the bolts or nuts holding the EGR valve to the engine.
- Remove the valve.
- Clean the mounting surfaces before replacing the valve. Install the valve onto the manifold, using a new gasket. Be sure to install the spacer, if used. Connect the vacuum hose and check the valve operation.