To reduce emissions, a portion of the exhaust gases are recirculated through the EGR valve to the intake manifold to lower the maximum combustion temperature. In all cases, the EGR valve is controlled by the same computer and vacuum switching valve which is used to operate other emission control system components. The EGR valve is operated by vacuum supplied from a port above the throttle blades and fed through the vacuum switching valve.
There are several conditions, determined by the computer and vacuum switching valve, which permits exhaust gas recirculation to take place:
Check the EGR vacuum modulator filters for contamination and clogging. If only moderate accumulations exist, clean the filters with compressed air, otherwise replace them.Bimetal Vacuum Switching Valve
WITH COLD ENGINE
Using a three-way "T" connector, connect a vacuum gauge to the hose between the EGR valve and the vacuum pipe. Start the engine and check that the engine starts and runs at idle speed to check that the EGR valve is seated. With the engine cool (113° F), verify the vacuum gauge reads 0 in. Hg at 2,500 rpm.WITH HOT ENGINE
Using a 3-way "T" connector, connect a vacuum gauge to the hose between the EGR valve and the vacuum pipe. Start the engine and allow it to warm up to normal operating temperature. The gauge should read low vacuum at 2,500 rpm. Disconnect the vacuum hose from port R of the EGR vacuum modulator and connect port R directly to the intake manifold with another hose. Check that the vacuum gauge reads a high vacuum at 2,500 rpm. As large amounts of EGR gases enter the engine will misfire slightly, this indicates the EGR valve is functioning properly. Remove the vacuum gauge and reconnect the hoses to their original locations.EGR Valve
With the engine idling, apply a vacuum directly to the EGR valve. The engine should run rough or die. Reconnect the vacuum hoses to their original locations.
If the engine continues to run rough after the vacuum is ceased, remove the EGR valve and check for sticking and sludge and heavy carbon deposits. Clean or replace the EGR valve and install with a new gasket.
If, after having completed the above tests, the EGR system still doesn't work correctly and everything else checks out okay, the fault probably lies in the vacuum switching valve systems.EGR Vacuum Modulator
- Disconnect the vacuum hoses from ports P , Q and R of the vacuum modulator.
- Block ports P and R with your finger.
- Blow air through port Q and check that the air passes through to the air filter side freely.
- Start the engine and maintain the engine speed at 2,500 rpm.
- Repeat Steps 2 and 3 and check that there is a strong air flow to ports P and R .
- Reconnect the vacuum hose to their proper locations.
This check applies to all-trac California vehicles. Disconnect and remove the temperature sensor from the EGR valve. Place the sensor into a container of water with a thermometer, without letting the terminals to get wet. Connect the probes of an ohmmeter across the 2 terminals of the sensor and slowly heat the water. At 122°F (50°C) the resistance should be 69.40-88.50kohm. At 212°F (100°C) the resistance should drop to 11.89-14.37kohm. If the resistance is not as specified, replace the sensor.