DESCRIPTION
The Exhaust Gas Recirculation (EGR) allows a predetermined amount of exhaust gas to recirculate to the intake manifold, diluting the incoming air/fuel mixture. This diluting of the air/fuel mixture reduces peak combustion temperatures. The lower combustion temperature reduces emissions of oxides of nitrogen (NOx). California models use on-board diagnostics for the EGR system. The diagnostic system uses a solenoid in the vacuum line to the EGR valve.
OPERATION
A backpressure type EGR system is used on all engines. The backpressure transducer measures the amount of exhaust backpressure and varies the strength of the vacuum signal to the EGR valve. The signal vacuum is bled off into the atmosphere when exhaust backpressure to the EGR drops below a calibrated value.
On California models, the on-board diagnostic system checks the entire EGR system for failures. The system is activated and monitored only during selected driving conditions to avoid misdiagnosis. When monitored, the EGR solenoid is energized and EGR function will discontinue.
The Oxygen (O2) feedback system is monitored to see if a change occurs. The mixture should go lean and the system will try to enrich the mixture. The Single Module Engine Controller (SMEC) monitors EGR system performance and registers a fault code if the system has become less effective or has failed, and turns on the "CHECK ENGINE" light.
Scheme 38
Scheme 39
BACKPRESSURE CONTROL SYSTEM
A backpressure transducer measures exhaust gas pressure on exhaust side of EGR and regulates vacuum signal to EGR valve. The transducer uses the backpressure signal to provide the correct amount of EGR under all conditions.
COOLANT TEMPERATURE SENSOR
This sensor is located in the thermostat housing or intake manifold runner. By monitoring engine temperature it signals the SMEC when to operate the air switching in the EGR and Spark Advance systems. Resistance across coolant temperature sensor terminals should read 7000-13,000 ohms at 70°F (21°C) or 700-1000 ohms at 200°F (70°C).
EGR MAINTENANCE REMINDER LIGHT
For EGR Maintenance Reminder Light information and reset procedure, see appropriate article in the COMPUTERIZED ENGINE CONTROLS section.
Scheme 40
California Models
On-board diagnostics for EGR system are part of vehicle diagnostic system. If a malfunction is indicated by the "CHECK ENGINE" light and a fault code for EGR system is stored, see appropriate article in the COMPUTERIZED ENGINE CONTROLS section.
Note. To ensure proper operation of the EGR system all passages and moving parts must be free from restrictive deposits. Clean or replace components as necessary.
- Warm engine to operating temperature. Attach a tachometer to the engine. Run engine at idle in Neutral an additional 70 seconds. Accelerate the engine abruptly to approximately 2000 RPM, but not over 3000 RPM.
- Check the EGR valve stem for movement. Visible movement of the valve stem should occur. Repeat test to confirm operation. Movement of the stem indicates that the control system is functioning correctly.
- If EGR valve stem does not move, check for cracked, leaking, disconnected or plugged hoses. Verify correct hose routing. Disconnect hose harness from EGR valve/transducer and connect a vacuum pump to harness.
- Start engine and raise engine speed to 2000 RPM and hold. Apply 10 in. Hg vacuum to EGR while checking for EGR valve movement. If no movement occurs, replace EGR valve/transducer assembly.
- If valve opens approximately 1/8", hold vacuum constant and check for valve diaphragm leakage. Valve should remain open for at least 30 seconds. If leakage occurs, replace valve/transducer assembly. If valve is okay, check control system.
- If EGR valve stem did not move in step 2), but operates normally on external vacuum source, remove throttle body and check port in throttle bore for blockage. Use a solvent to remove deposits and check flow with light air pressure. Normal operation should be restored.
- If engine will not idle correctly or stalls, check for leaking EGR valve. With engine running, remove vacuum hose from EGR valve. If removing the vacuum hose does not correct rough idle, remove EGR valve/transducer assembly. Check valve for proper seating. Replace assembly as necessary.
- Also check EGR-to-intake manifold tube for leak. On 2.5L models, loosen tube and tighten to 25 ft. lbs. (34 N.m). On 3.0L models, remove tube and inspect gasket. Tube end should be uniformly indented on gasket, with no signs of leakage.
- On all models, if there are signs of leakage, replace gaskets and tighten flange nuts to 17 ft. lbs. (23 N.m). If leak persists, replace EGR tube and gaskets.
Note. Do not use drills or wires to clean control passages for control system. Calibration of precision orifices could be altered, resulting in unsatisfactory vehicle operation.
EGR VALVE
- Inspect valve for deposits with particular attention to the poppet and seat area. If deposits exceed a thin film, valve should be cleaned. Clean by applying a liberal amount of manifold heat control solvent, or equivalent, to the poppet and seat area, allowing deposits to soften. CAUTION: Extreme care should be taken when using solvent cleaners to prevent spilling of solvent on valve diaphragm.
- Use an external vacuum source to open poppet and then scrape deposit from this area. If wear of stem or other moving components is noted, valve should be replaced.
Note. Do not push valve stem manually. Use an external vacuum source only.
