GM Full-Size Trucks 1980-1987 Repair Guide

Exhaust Gas Recirculation (EGR)

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GENERAL INFORMATION



See Figures 1, 2, 3 and 4

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 EGR system is not used on vehicles with heavy duty emissions system.

Except California Models

On the 379 diesel, an Exhaust Pressure Regulator (EPR) valve and solenoid operate in conjunction with the EGR valve. The EPR valve is located between the exhaust pipe and manifold, it's job is to increase exhaust backpressure in order to increase EGR flow (to reduce nitrous oxide emissions) during idle. The EPR valve increases exhaust backpressure when it is closed. The EPR valve is usually open, and the EPR solenoid valve is normally closed. When the EPR solenoid valve is energized by the B+ wire from the Throttle Position Switch (TPS), the EPR solenoid valve opens, allowing vacuum to applied to the EPR valve, closing it. This occurs at idle. As the throttle is opened to a calibrated angle, the TPS de-energizes the EPR solenoid valve, cutting off vacuum to the EPR valve and thus closing the valve.

California models

On California models, the EGR system is controlled differently but the theory of operation is similar to the system used in the 49 states. The EGR/EPR solenoid valve assembly's operation in California models is controlled by the Engine Control Module (ECM). The ECM controls the EGR solenoid to regulate the vacuum applied to the EGR valve. The vacuum is applied in an on and off fashion in order to precisely regulate EGR operation. The ECM calculates the amount of EGR that is needed based on inputs from the engine speed sensor and the throttle position sensor. The ECM is programmed to calculate the on and off time of EGR solenoid operation based on these two sensor inputs. To insure correct EGR function, the ECM also monitors the amount of absolute pressure in the EGR vacuum line using a the manifold absolute pressure sensor. If the variation in EGR operation is beyond what the ECM can correct for, the ECM will record a diagnostic trouble code in memory and the system will go into default mode. When the ECM recognizes that no EGR operation is needed, the ECM will turn on the EGR vent solenoid to allow venting of the EGR valve vacuum. The ECM will also energize the EPR solenoid valve to close the EPR valve whenever the engine is at idle.



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Fig. Fig. 1: Non-California diesel EGR system schematic



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Fig. Fig. 2: Exhaust pressure regulator valve and solenoid, 379 diesels



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Fig. Fig. 3: Diesel EGR valve location; 379 (6.2L) shown



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Fig. Fig. 4: Diesel Vacuum Regulator Valve (VRV) is mounted to injection pump

TESTING



Generally, heavy black exhaust is an indication that the exhaust gas recirculation system is malfunctioning.

EGR Operational Test Non-California Models

  1. Start the engine and run until normal operating temperature is reached.
  2.  
  3. Carefully remove the air cleaner to observe the EGR valve operation with the engine idling. The EGR valve should be open. An open valve's head will be in the up position and noticeable exhaust intake can be heard. If the valve is not open, check vacuum hoses and all electrical connections in the system.
  4.  
  5. Disconnect the vacuum hose from the EGR valve. The valve should close and exhaust noise should be reduced.
  6.  
  7. Connect a vacuum gauge to the EGR valve vacuum hose with the engine idling. A reading of 20 inches of vacuum (6.75 kPa) should be measured. If the reading is lower, check the output of the vacuum pump. If the pump is OK, replace the hoses.
  8.  
  9. If the vacuum at the EGR valve is OK, connect and disconnect the vacuum hose to the EGR valve while allowing the engine to idle. While doing this, observe the EGR valve operation. If it appears to stick or bind, replacement of the valve is required.
  10.  
  11. Manually operate the throttle lever at the injection pump through 15 to 20 degrees of travel. The EGR valve should remain open.
  12.  
  13. Manually operate the throttle lever at the injection pump beyond 20 degrees of travel. The EGR valve should close.
  14.  
  15. Turn the engine off. Check the pink wire to the throttle position switch for 12 volts when the ignition is on. If no voltage is measured, repair the wiring to the switch.
  16.  
  17. With the ignition on, check the blue wire to the throttle position switch for 12 volts. If no voltage is measured, the throttle position switch adjustment should be checked (Refer to TPS Adjustment listed below). Replace the switch if adjustment does not correct the problem.
  18.  
  19. With the ignition on, leave the voltmeter connected to the blue wire at the throttle position switch. Operate the throttle through 20 degrees of travel. At approximately 15 degrees, the voltage should drop to 0 volts. If voltage is measured, the throttle position switch adjustment should be checked (Refer to TPS Adjustment listed below). Replace the switch if adjustment does not correct the problem.
  20.  
  21. Connect the voltmeter to the yellow wire at the throttle position switch. Operate the throttle lever beyond 20 degrees of travel. 12 volts should be measured when the throttle lever is opened beyond the 20 degrees. If no voltage is measured, the throttle position switch adjustment should be checked (Refer to TPS Adjustment listed below). Replace the switch if adjustment does not correct the problem.
  22.  
  23. Check to see that the electrical connections are made at the EGR and EPR solenoid assemblies and that the hoses are routed correctly and connected to the solenoids.
  24.  
  25. If the system checked out OK in the previous Steps, the EGR or EPR solenoid valve is inoperative and should be replaced.
  26.  

Exhaust Gas Recirculation (EGR) Valve

In order to check EGR valve operation directly, apply vacuum to vacuum port on the valve. The valve should be fully open with 10.5 inches of vacuum applied, and closed when vacuum is below 6 inches.

REMOVAL & INSTALLATION



EGR Valve
  1. Remove the air cleaner.
  2.  
  3. Disconnect the vacuum hose from the EGR valve.
  4.  
  5. Remove the two stud bolts from the EGR valve. Remove the valve from manifold.
  6.  
  7. To install the EGR valve, follow the removal procedure in reverse order.
  8.  

EGR/EPR Vacuum Solenoid
  1. Disconnect the negative battery cable.
  2.  
  3. Disconnect the electrical connectors and vacuum hoses.
  4.  
  5. Remove bolt at solenoid bracket.
  6.  
  7. To install, follow the removal procedure in the reverse order.
  8.  

ADJUSTMENT



Throttle Position Switch (TPS)

This procedure is for Non-California models only. California models do not use a throttle position switch but a throttle position sensor.

  1. Disconnect the throttle position switch connector.
  2.  
  3. Loosen the mounting screws that hold the throttle position switch.
  4.  
  5. Connect an ohmmeter test light to the "IGN" terminal (pink wire) and the "EGR" terminal (yellow wire) of the throttle position sensor.
  6.  
  7. Insert the specified gauge block for the "switch closed" position between the gage boss on the injection pump and the wide open stop screw on the throttle shaft. Refer to the emission control label on the truck for the "switch closed" gauge block specification.
  8.  
  9. Rotate the throttle lever and hold the wide open stop against the gauge block.
  10.  
  11. Rotate the throttle position switch until there is continuity between the terminals.
  12.  
  13. Hold the throttle position and tighten the mounting screws to 53 inch lbs. (6 Nm).
  14.  
  15. Return the throttle lever to the idle position and remove the gauge block.
  16.  
  17. Insert the specified gauge block for the "switch open" position between the gauge boss on the injection pump and the wide open stop screw. Refer to the emission control label on the truck for the "switch open" gauge block specification. Rotate the throttle lever against the block. There should be no continuity measured. If there is continuity, repeat Steps 1 thru 9.
  18.  
  19. Remove the gauge block and ohmmeter.
  20.  
  21. Connect the throttle position switch connector.
  22.  

 
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