GM Full-Size Trucks 1980-1987 Repair Guide

Diesel Engine

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IDLE SPEED ADJUSTMENT



350 Engine

See Figure 1

A special tachometer with an RPM counter suitable for the 8-350 V8 diesel is necessary for this adjustment; a standard tach suitable for gasoline engines will not work.

  1. Place the transmission in Park, block the rear wheels and firmly set the parking brake.
  2.  
  3. If necessary, adjust the throttle linkage as described in Chassis Electrical .
  4.  
  5. Start the engine and allow it to warm up for 10-15 minutes.
  6.  
  7. Shut off the engine and remove the air cleaner.
  8.  
  9. Clean off any grime from the timing probe holder on the front cover; also clean off the crankshaft balancer rim.
  10.  
  11. Install the magnetic probe end of the tachometer fully into the timing probe holder. Complete the remaining tachometer connections according to the tach manufacturer's instructions.
  12.  
  13. Disconnect the two-lead connector from the generator.
  14.  
  15. Make sure all electrical accessories are OFF.
  16.  

At no time should either the steering wheel or the brake pedal be touched.

  1. Start the engine and place the transmission in Drive (after first making sure the parking brake is firmly applied).
  2.  
  3. Check the slow idle speed reading against the one printed on the underhood emissions sticker. Reset if necessary.
  4.  
  5. Unplug the connector from the fast idle cold advance (engine temperature) switch, and install a jumper wire between the connector terminals.
  6.  

DO NOT allow the jumper to ground.

  1. Check the fast idle speed and reset if necessary according to the specification printed on the underhood emissions sticker.
  2.  
  3. Remove the jumper wire and reconnect it to the temperature switch.
  4.  
  5. Recheck the slow idle speed and reset if necessary.
  6.  
  7. Shut off the engine.
  8.  
  9. Reconnect the leads at the generator and A/C compressor.
  10.  
  11. Disconnect and remove the tachometer.
  12.  
  13. If the car is equipped with cruise control, adjust the servo throttle rod to minimum slack, then put the clip in the first free hole closest to the bell rank or throttle lever.
  14.  
  15. Install the air cleaner.
  16.  



Click image to see an enlarged view

Fig. Fig. 1: 350 diesel injection pump showing idle speed screws

379 V8 Engine

See Figure 2

A special tachometer suitable for diesel engines must be used. A gasoline engine type tach will not work with the diesel engine.

  1. Set the parking brake and block the drive wheels.
  2.  
  3. Run the engine up to normal operating temperature. The air cleaner must be mounted and all accessories turned off.
  4.  
  5. Install the diesel tachometer as per the manufacturer's instructions.
  6.  
  7. Adjust the low idle speed screw on the fuel injection pump to 650 rpm in Neutral or Park for both manual and automatic transmissions.
  8.  

All idle speeds are to be set within 25 rpm of the specified values.

  1. Adjust the fast idle speed as follows:
  2.  



Click image to see an enlarged view

Fig. Fig. 2: 379(6.2L) diesel injection pump showing idle adjustments

    1. Remove the connector from the fast idle solenoid. Use an insulated jumper wire from the battery positive terminal to the solenoid terminal to energize the solenoid.
    2.  
    3. Open the throttle momentarily to ensure that the fast idle solenoid plunger is energized and fully extended.
    4.  
    5. Adjust the extended plunger by turning the hex-head screw to an engine speed of 800 rpm in neutral.
    6.  
    7. Remove the jumper wire and reinstall the connector to the fast idle solenoid.
    8.  

  1. Disconnect and remove the tachometer.
  2.  

 
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