GM Full-Size Trucks 1988-1998 Repair Information Exhaust Gas Recirculation (EGR) OPERATION TESTING REMOVAL & INSTALLATION OPERATION See Figures 1 and 2 The EGR system's purpose is to control oxides of nitrogen which are formed during the peak combustion temperatures. The end products of combustion are relatively inert gases derived from the exhaust gases which are directed into the EGR valve to help lower peak combustion temperatures. Fig. Fig. 1: Negative backpressure and positive backpressure EGR valve identification Fig. Fig. 2: Linear EGR valve identification The port EGR 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 which allows exhaust gas to be pulled into the intake manifold and enter the engine cylinders. The negative backpressure EGR valve has bleed valve spring below the diaphragm, and the valve is normally closed. The valve varies the amount of exhaust flow into the manifold depending on manifold vacuum and variations in exhaust backpressure. 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. 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. The linear EGR valve is operated exclusively by the control module command. The control module monitors various engine parame ters: Throttle Position Sensor (TPS) Manifold Absolute Pressure (MAP) Engine Coolant Temperature (ECT) sensor Pintle position sensor Output messages are then sent to the EGR system indicating the proper amount of exhaust gas recirculation necessary to lower combustion temperatures. Refer to the accompanying illustrations to identify the EGR valve used in your vehicle. TESTING See Figures 3, 4, 5, 6, 7 and 8 Refer to the appropriate chart for diagnosis the EGR system. On linear EGR systems, an OBD-II compliant scan tool will be needed. Fig. Fig. 3: Ported EGR system wiring diagram Fig. Fig. 4: Ported EGR system check Fig. Fig. 5: Negative backpressure EGR system test Fig. Fig. 6: Negative EGR system wiring diagram Fig. Fig. 7: Linear EGR system test Fig. Fig. 8: Linear EGR system test (continued) REMOVAL & INSTALLATION EGR Valve except 1996-98 models See Figures 9, 10 and 11 1. Disconnect the negative battery cable. 2. Remove the air cleaner assembly from the engine. 3. Remove the EGR valve vacuum tube from the valve. Fig. Fig. 9: EGR valve mounting-4.3L engines, except 1996-98 models Fig. Fig. 10: EGR valve mounting-5.0L and 5.7L engines, except 1996-98 models Fig. Fig. 11: EGR valve mounting-7.4L engines, except 1996-98 models 4. Remove the EGR bolts and/or nuts and remove the EGR valve and gasket. To install: 5. Install a new gasket to the EGR valve and install the EGR valve to the manifold. 6. Install the nuts and/or bolts. Tighten the bolts to 17 ft. lbs. (24 Nm) and the nuts to 15 ft. lbs. (20 Nm). 7. Connect the vacuum tube to the EGR valve. 8. Install the air cleaner and connect the negative battery cable. 1996-98 models See Figures 12 and 13 Do not try to disassemble the EGR valve. 1. Matchmark the location of the position of the EGR valve in location to its mounting. Do not rotate the EGR valve 180°. 2. Unplug the EGR valve electrical connection. 3. Unfasten the valve retaining bolts and remove the valve. 4. Remove EGR valve gasket and clean the gasket mating surfaces. To install: 5. Install a new EGR flange gasket. 6. Install the EGR valve and its retaining bolts. Make sure the marks made prior to valve removal are aligned. 7. Tighten the EGR valve bolts in two steps, fist tighten the bolts to 89 inch lbs. (10 Nm), then tighten the bolts to 18 ft. lbs. (25 Nm). 8. Attach the valve electrical connection. Fig. Fig. 12: Linear EGR valve mounting-1996-98 4.3L, 5.0L and 5.7L engines Fig. Fig. 13: Linear EGR valve mounting (1)-1996-98 7.4L engines EGR Solenoid See Figures 14 and 15 1. Disconnect the negative battery cable. 2. Remove the air cleaner, as required. 3. Unplug the electrical connector at the solenoid. 4. Disconnect the vacuum hoses. Fig. Fig. 14: EGR valve and solenoid mounting-7.4L engine shown, others similar Fig. Fig. 15: EGR control solenoid 5. Remove the retaining bolts and the solenoid. 6. Remove the filter, as required. To install: 7. If removed, install the filter. 8. Install the solenoid and retaining bolts. 9. Connect the vacuum hoses. 10. Engage the electrical connector. 11. If removed, install the air cleaner. 12. Connect the negative battery cable.