Only the 1987-90 vehicles covered by this guide are equipped with an Exhaust Gas Recirculation (EGR) system.
NOx (oxides of nitrogen) is a tailpipe emission caused by the oxidation of nitrogen in the combustion chamber. When the peak combustion temperatures go over 2500ºF (1371ºC) NOx is formed in excessive amounts. Exhaust gas is recirculated in order to dilute air/fuel mixtures entering the combustion chamber, thereby lowering the combustion temperatures.
Recirculation of the exhaust gases is accomplished with a movable valve between the exhaust and intake manifolds. Upon a predetermined demand, engine vacuum is routed to the valve, opening the connecting port and allowing exhaust gases to enter the intake tract.
See Figure 1, 2, 3, 4 and 5
- Start engine and bring to normal operating temperature. Allow engine to idle while performing tests.
- Check vacuum at solenoid vacuum source hose. Disconnect the hose and attach a vacuum gauge.
- Vacuum should be 15 in. Hg. If low, check for leaks, loose fittings or kinks in the line.
- Check vacuum at solenoid port. Disconnect the line and attach a vacuum gauge.
- If vacuum reading is zero, go to Step 6. If vacuum is present, check solenoid operation with the Diagnostic Readout Box (DRB II) service tester and repair as necessary.
- Disconnect electrical connector at solenoid. If vacuum is present, proceed to EGR valve test. If not, replace the solenoid.
- Leave solenoid electrical connector disengaged. Bypass the vacuum transducer, if equipped, and connect the EGR valve solenoid output hose directly to the nipple on the EGR valve.
- The engine should run roughly or stall. If this occurs, the valve is good. Proceed to the Transducer test. If engine rpm does not change, disconnect hose from EGR and connect a hand vacuum pump.
- Apply 12 in. Hg (41 kPa) of vacuum. If engine runs rough or stalls, inspect vacuum lines in EGR system for leaks and repair as necessary. If no leaks are found, go to to Step 4.
- If engine idle still does not change, remove the EGR valve and inspect for a blockage in the intake manifold passage. Repair as necessary. If no blockage is found, replace the EGR valve.
4.2L EngineCTO SWITCH
- Check vacuum lines for leaks and proper routing.
- Disconnect vacuum line from EGR valve and connect a vacuum gauge.
- Start engine and ensure that coolant is below 100ºF (38ºC).
- Operate engine at 1,500 rpm. There should be no vacuum. If vacuum is present, replace the CTO switch.
- Allow engine to idle until coolant temperature exceeds 115ºF (46ºC).
- Operate the engine at 1,500 rpm. Vacuum should be present. If not, replace the CTO switch.
- With the engine at normal operating temperature and idling, rapidly open and close the throttle (allowing the engine to reach 1500 rpm each time).
There should be a distinct movement in the EGR control valve diaphragm. If there is no movement check for:
- Faulty vacuum hose to EGR valve.
- Defective EGR diaphragm. Use a hand vacuum pump and test see if the EGR will hold vacuum.
- Defective backpressure sensor.
- Vacuum hose leaks.
- With the engine idling at normal operating temperature, use a protective glove and manually depress the EGR valve diaphragm. This should cause an immediate drop in engine rpm and indicate the EGR passages are clear,
- If there is no drop in rpm and the engine is idling properly, check for a restricted passage between the EGR valve and the intake manifold. Clean as necessary.
- If the engine idles improperly and the rpm is not greatly affected, check for a carbon-fouled EGR valve. Clean or replace as necessary.
- With the engine cold and ambient air temperature in the air cleaner below 40ºF (4ºC), disconnect the vacuum hoses from the TVS (located on the air cleaner).
- Connect a hand vacuum pump to the inner port and apply vacuum.
- Vacuum should be maintained at air cleaner intake temperatures below 40ºF (4ºC). If vacuum is not held, check to see that temperature is below 40ºF (4ºC). If so, replace the TVS.
- Start the engine and warm to normal operating temperature. With an air cleaner intake temperature above 55ºF (13ºC), the switch should not hold vacuum. If vacuum is held, check to see that temperatures are above 55ºF (13ºC). If so, replace the TVS.
Temperatures are nominal values and the actual switching temperature may vary slightly.