GM Full-Size Trucks 1988-1998 Repair Information

Exhaust Gas Recirculation (EGR)

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OPERATION



To lower the formation of nitrogen oxides (NOx) in the exhaust, it is necessary to reduce combustion temperatures. This is done in the diesel, as in the gasoline engine, by introducing exhaust gases into the cylinders through the EGR valve.

The Exhaust Pressure Regulator (EPR) valve and solenoid operate in conjunction with the EGR valve. The EPR valve's job is to increase exhaust backpressure in order to increase EGR flow. The EPR valve is usually open, and the solenoid is normally closed. When energized by the B+ wire from the Throttle Position Switch (TPS), the solenoid opens, allowing vacuum to the EPR valve, closing it. This occurs at idle. As the throttle is opened, at a calibrated throttle angle, the TPS de-energizes the EPR solenoid, cutting off vacuum to the EPR valve, closing the valve. Two other solenoids are used for EGR valve control. The EGR solenoid allows vacuum to reach the EGR vent solenoid under certain conditions. The vent solenoid then controls the EGR valve to regulate the flow of gasses into the intake manifold.

TESTING



Exhaust Gas Recirculation (EGR) Valve

See Figure 1

Apply vacuum to the EGR valve with a hand vacuum pump. The valve should be fully open at 11 in. Hg (75 kPa) and closed below 6 in. Hg (41 kPa).



Click image to see an enlarged view

Fig. Fig. 1: Some EGR valves may be tested using a vacuum pump by watching for diaphragm movement

EPR Valve
  1. Apply 11 in. Hg (75 kPa) vacuum to the EPR valve tube with a hand vacuum pump. Observe the valve actuator lever for movement.
  2.  
  3. If it does not move, spray a penetrating lubricant on the lever and try to free the valve.
  4.  


CAUTION
Make sure the valve is not hot.

  1. If the lubricant will not free the valve, it must be replaced.
  2.  

REMOVAL & INSTALLATION



EGR Valve
Non-turbo models

See Figure 2

  1. Remove the air cleaner assembly and air intake tube.
  2.  
  3. Unplug the vacuum hose from the valve.
  4.  
  5. Remove the studs securing the valve to the intake manifold.
  6.  
  7. Installation is the reverse of removal. Tighten the studs until snug.
  8.  



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Fig. Fig. 2: EGR valve mounting-non-turbo models

turbo models

See Figure 3

  1. Remove the upper intake manifold cover.
  2.  
  3. Unplug the vacuum hose from the valve.
  4.  
  5. Remove the bolts securing the valve to the intake manifold.
  6.  
  7. Installation is the reverse of removal. Tighten the bolts to 18 ft. lbs. (25 Nm).
  8.  



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Fig. Fig. 3: Remove the upper intake manifold cover to access the EGR valve-turbo models

EPR Valve

See Figure 4

  1. Raise and safely support the vehicle.
  2.  
  3. Unplug the vacuum hose from the actuator.
  4.  
  5. Disconnect the exhaust pipe from the valve.
  6.  
  7. Remove the studs securing the valve to the exhaust manifold.
  8.  
  9. Installation is the reverse of removal.
  10.  



Click image to see an enlarged view

Fig. Fig. 4: EPR valve

EGR/EPR Solenoid Assembly

See Figures 5 and 6

  1. Disconnect the negative battery cable.
  2.  
  3. Label and disconnect the vacuum hoses from the assembly.
  4.  
  5. Unplug the solenoid electrical connectors.
  6.  
  7. Remove the retainers securing the solenoid assembly.
  8.  
  9. Installation is the reverse of removal.
  10.  



Click image to see an enlarged view

Fig. Fig. 5: EGR/EPR solenoid assembly-non-turbo models



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Fig. Fig. 6: Typical EGR solenoid assembly-turbo models

 
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