GM S-Series Pick-ups and SUV's 1994-1999 Repair Guide

Exhaust Gas Recirculation (EGR)

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OPERATION



See Figures 1, 2 and 3



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Fig. Fig. 1: EGR valve identification



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Fig. Fig. 2: Linear EGR valve identification



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Fig. Fig. 3: Linear EGR valve part identification label

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. 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 parameters:



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 4 through 10

Refer to the appropriate chart for diagnosis the EGR system. On linear EGR systems, an OBD-II compliant scan tool will be needed.

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Fig. Fig. 4: Some EGR valves may be tested using a vacuum pump by watching for diaphragm movement



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Fig. Fig. 5: Common ported EGR system wiring diagram



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Fig. Fig. 6: Typical negative EGR system wiring diagram



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Fig. Fig. 7: Common linear EGR system wiring diagram



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Fig. Fig. 8: Ported EGR system check



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Fig. Fig. 9: Negative backpressure EGR system test



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Fig. Fig. 10: Linear EGR system test

REMOVAL & INSTALLATION



Except Linear EGR Valve

See Figures 11 and 12



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Fig. Fig. 11: Unfasten the EGR valve retaining bolts...



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Fig. Fig. 12: ... then remove the EGR valve from the vehicle

  1. Disconnect the negative battery cable.
  2.  
  3. Remove the air cleaner assembly from the engine.
  4.  
  5. If equipped, unplug the valve electrical connection.
  6.  
  7. Remove the EGR valve vacuum tube from the valve.
  8.  
  9. Remove the EGR bolts and/or nuts and remove the EGR valve and gasket.
  10.  

To install:
  1. Install a new gasket to the EGR valve and install the EGR valve to the manifold.
  2.  
  3. Install the nuts and/or bolts. Tighten the bolts to 17 ft. lbs. (24 Nm) and the nuts to 15 ft. lbs. (20 Nm).
  4.  
  5. Connect the vacuum tube to the EGR valve.
  6.  
  7. If equipped, attach the valve electrical connection.
  8.  
  9. Install the air cleaner and connect the negative battery cable.
  10.  

Linear EGR Valve
  1. Disconnect the negative battery cable.
  2.  

Note the position of the EGR valve prior to removal. Do not rotate the valve 180º.

  1. Unplug the valve electrical connection..
  2.  
  3. Unfasten the valve retainers, then remove the valve and the gasket.
  4.  

To install:
  1. Clean all gasket residue from the gasket mating surfaces.
  2.  
  3. Install a new gasket and the valve.
  4.  
  5. Install the valve retainers.
  6.  
  7. On 1994-97 2.2L engines and 1994-95 4.3L engines, tighten the valve retainers to 17 ft. lbs. (24 Nm). On 1998-99 2.2L models, tighten the retainers to 19 ft. lbs. (26 Nm).
  8.  
  9. On 1996-99 4.3L models, tighten the retainers on the first pass to 89 inch lbs. (10 Nm) and on the final pass to 18 ft. lbs. (25 Nm).
  10.  
  11. Attach the valve electrical connections and connect the negative battery cable.
  12.  

EGR Solenoid
  1. Disconnect the negative battery cable.
  2.  
  3. Remove the air cleaner, as required.
  4.  
  5. Unplug the electrical connector at the solenoid.
  6.  
  7. Disconnect the vacuum hoses.
  8.  
  9. Remove the retaining bolts and the solenoid.
  10.  
  11. Remove the filter, as required.
  12.  

To install:
  1. If removed, install the filter.
  2.  
  3. Install the solenoid and retaining bolts.
  4.  
  5. Connect the vacuum hoses.
  6.  
  7. Engage the electrical connector.
  8.  
  9. If removed, install the air cleaner.
  10.  
  11. Connect the negative battery cable.
  12.  

 
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