EXHAUST GAS RECIRCULATION (EGR) SYSTEM GENERAL DESCRIPTION PURPOSE The EGR system is used to lower NOx (oxides of nitrogen) emission levels caused by high combustion temperatures. The EGR valve feeds small amounts of exhaust gas back into the combustion chamber to decrease combustion temperature. The main element of the system is an EGR valve operated by vacuum, and mounted on the intake manifold. OPERATION The EGR valve is opened by vacuum to let exhaust gas flow into the intake manifold. The exhaust gas then moves with the air/fuel mixture into the combustion chamber. If too much exhaust gas enters, combustion will not occur. For this reason, very little exhaust gas is allowed to pass through the valve, especially at idle. The EGR valve is usually open under the following conditions: • Warm engine operation • Above idle speed EGR CONTROL Solenoid 2.5L, 4.3L (except ST), 5.0L & 5.7L (Under 8500 GVW) To regulate EGR flow an ECM controlled solenoid is used in the vacuum line. This is a normally closed solenoid vacuum valve which is opened when the ECM completes the ground. The ECM will energize the EGR solenoid (EGR "ON”) when the engine is warm and above idle. There is little EGR flow at wide open throttle due to lack of vacuum. 2.5L Only On cold engine start-up the ECM receives input from the MAT sensor (manifold air temperature), and if the MAT reading is less than 12°C (57°F) the EGR solenoid is "OFF” until the coolant warms to 80°C (176°F). If there is a malfunction of the MAT sensor then the ECM receives input from the coolant sensor. EVRV 2.8L, 4.3L (ST), 7.4L & 5.7L (over 8500 GVW) To regulate EGR flow, an ECM controlled electronic vacuum regulator valve (EVRV) solenoid is used in the vacuum line. The ECM uses information from the following sensors to regulate the solenoid: • Coolant Temperature • Throttle Position (TPS) • P/N Switch • Distributor (rpm Signal) The EGR vacuum control has an EVRV solenoid that uses "pulse width modulation”. This means the ECM turns the solenoid "ON” and "OFF” many times a second and varies the amount of "ON” time ("pulse width”) to vary the amount of EGR. TYPES OF EGR VALVES Two types of EGR valves are used on these engines: • Port (2.8L, 4.3L (ST), 7.4L & 5.7L over 8500 GVW) • Negative backpressure (2.5L, 4.3L (except ST), 5.0L & 5.7L under 8500 GVW) Port EGR Valve This valve is controlled by a flexible diaphragm which is spring loaded to hold the valve closed. Vacuum applied to the top side of the diaphragm overcomes the spring pressure and opens the valve in the exhaust gas port. This allows exhaust gas to be pulled into the intake manifold and enter the engine cylinders. NEGATIVE BACKPRESSURE EGR VALVE The negative backpressure EGR valve has the bleed valve spring below the diaphragm, and the valve is normally closed. The negative backpressure valve varies the amount of exhaust gas flow into the manifold depending on manifold vacuum and variations in exhaust back pressure. The diaphragm on this valve has an internal air bleed hole which is held closed by a small spring when there is no exhaust backpressure. Engine vacuum opens the EGR valve against the pressure of a large spring. When manifold vacuum combines with negative exhaust backpressure, the vacuum bleed hole opens and the EGR valve closes. This valve will open if vacuum is applied with the engine not running. EGR VALVE IDENTIFICATION • Positive backpressure EGR valves will have a "P” stamped on the top side of the valve after the part number. • Negative backpressure EGR valves will have a "N” stamped on the top side of the valve after the part number. • Port EGR valves have no identification stamped after the part number. DIAGNOSIS RESULTS OF INCORRECT OPERATION With too much EGR flow at idle, cruise, or cold operation, any of the following conditions may occur: • Engine stops after cold start. • Engine stops at idle after deceleration. • Vehicle surges during cruise. • Rough idle. If the EGR valve should stay open all of the time, the engine may not idle. Too little or no EGR flow allows combustion temperatures to get too high during acceleration and load conditions. This could cause: • Spark knock (detonation). • Engine overheating. SYSTEM CHECK Diagnosis of the EGR system is covered in the following charts. These charts begin on page 9-4. • EGR System Check on a 2.5L, 4.3L (except ST), 5.0L & 5.7L (under 8500 GVW) engine. • EGR System Check on a 2.8L, 4.3L (ST), 7.4L & 5.7L (over 8500 GVW) engine. ON-VEHICLE SERVICE EGR VALVE Remove or Disconnect: 1. Air cleaner. 2. EGR valve vacuum tube at valve. (Figure 9-5 to 9-10). 3. Bolts or nuts. 4. EGR valve and g a sk e t from manifold (Figures 9-11 to 9-15). Discard gasket. ! Important !/B] Do Not wash EGR valve in solvents or degreaser - permanent damage to valve diaphragm may result. Also, sand blasting of the valve is not recommended since this can affect the operation of the valve. EGR Manifold Passage Inspect If EGR passage indicates excessive build-up of deposits, the passage should be cleaned. Care should be taken to ensure that all loose particles are completely removed to prevent them from clogging the EGR valve or from being ingested into the engine. EGR SYSTEM CHECK 2.5L, 4.3L (EXCEPT ST), 5.0L & 5.7L (UNDER 8500 GVW) Circuit Description: The ECM operates a solenoid to control the exhaust gas recirculation (EGR) valve. This solenoid is normally closed. By providing a ground path, the ECM energizes the solenoid which then allows vacuum to pass to the EGR valve. The ECM control of the EGR is based on the following inputs: • Engine coolant temperature - above 25°C. • TPS - "OFF” idle • MAP If Code 24 is stored, use that chart first. Code 32 will detect a faulty solenoid, vacuum supply, EGR valve or plugged passage. This chart checks for plugged EGR passages, a sticking EGR valve, or a stuck open or inoperative solenoid. Test Description: Numbers below refer to circled numbers on the diagnostic chart. 1. Checks for solenoid stuck open. 2. Checks for solenoid always being energized. 3. Grounding test terminal should energize solenoid and vacuum should drop. 4. Negative backpressure valve should hold vacuum with engine "OFF”. 5. When engine is started, exhaust backpressure should cause vacuum to bleed off and valve to fully close. Diagnostic Aids: • Before replacing ECM, use an ohmmeter and check the resistance of each ECM controlled relay and solenoid coil. Refer to "ECM QDR Check” procedure in Section "3”. See ECM wiring diagram for coil terminal I.D. of solenoid(s) and relay(s) to be checked. Replace any solenoid where resistance measures less than 20 ohms. EGR SYSTEM CHECK 2.8L, 7.4L, 4.3L (ST), & 5.7L (OVER 8500 GVW) Circuit Description: The EGR valve is controlled by a normally closed solenoid (allows a vacuum to pass when energized). The ECM pulses the solenoid to turn "ON” and regulate the EGR. The ECM diagnoses the system using an internal EGR test procedure. The ECM control of the EGR is based on the following inputs: • Engine coolant temperature - above 25°C. • TPS "OFF” idle • MAP If Code 24 is stored, use that chart first. Code 32 will detect a faulty solenoid, vacuum supply, EGR valve or plugged passage. This chart checks for plugged EGR passages, a sticking EGR valve, or a stuck open or inoperative solenoid. Test Description: Numbers below refer to circled numbers on the diagnostic chart. 1. With the ignition "ON”, engine stopped, the solenoid should not be energized and vacuum should not pass to the EGR valve. 2. Grounding the diagnostic terminal will energize the solenoid and allow vacuum to pass to valve. 3. Checks for plugged EGR passages. If passages are plugged, the engine may have severe detonation on acceleration. 4. The vehicle must be driven during this test in order to produce sufficient engine load to operate the EGR. Lightly accelerating (approximately 1/4 throttle) will produce a large and stable enough reading to determine if the ECM is commanding the system "ON.” Diagnostic Aids: • Before replacing ECM, use ohmmeter and checkresistance of each ECM controlled relay and solenoid coil. Refer to "ECM QDR Check” in Section "3”. See ECM wiring diagram for coil terminal identification of solenoid(s) and relay(s) to be checked. Replace any relay or solenoid if the coil resistance measures less than 20 ohms. Clean 1. With a wire wheel, buff the exhaust deposits fromthe mounting surface and around the valve. 2. Look for exhaust deposits in the valve outlet. Remove deposit build-up with a screwdriver. 3. Clean mounting surfaces of intake manifold and valve assembly. Install or Connect 1. New EGR gasket. 2. EGR valve to manifold. 3. Bolts or nuts. 4. Vacuum tube to valve. 5. Air cleaner. SYSTEM HOSES Refer to Vehicle Emission Control Information label for routing of system hoses. When replacing hoses, use hose identified with the word "Fluoroelastomer”. EGR SOLENOID 2.8L, 4.3L (ST), 7.4L & 5.7L (over 8500 GVW) (Figure 9-16) Remove or Disconnect 1. Negative battery cable. 2. Air cleaner, if necessary. 3. Electrical connector at solenoid. 4. Vacuum hoses. 5. Bolt(s) and solenoid. 6. Filter, if required. Install or Connect 1. Filter, if required. 2. Solenoid. Tighten bolts to 24 N-m (17 ft. lbs). 3. Vacuum hoses. 4. Electrical connector. 5. Air cleaner, if removed. EGR FILTER CLEANING / REPLACEMENT 1. Grasp and pull filter off with a rocking motion. 2. Push new filter on making sure cut-out for wires is properly aligned. 3. Negative battery cable. EGR VACUUM SOLENOID 2.5L, 4.3L, 5.0L & 5.7L (under 8500 GVW) (Figure 9-17) Remove or Disconnect 1. Negative battery cable. 2. Electrical connector at solenoid. 3. Vacuum hoses. 4. Bolt(s) and solenoid. Install or Connect 1. Solenoid. See Figures 9-5 thru 9-10 for torque specification. 2. Vacuum hoses. 3. Electrical connector. 4. Negative battery cable.