GM Full-Size Trucks 1988-1998 Repair Information

Idle Speed and Mixture Adjustments

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CARBURETED ENGINES



Mixture screws are concealed under staked-in plugs. Idle mixture is adjustable only during carburetor overhaul, and requires the addition of propane as an artificial mixture enricher. For these reasons, mixture adjustments are not considered part of routine maintenance. Refer to Fuel Systems for these procedures.

4.8L Engines
CURB IDLE SPEED

See Figure 1



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Fig. Fig. 1: Idle speed adjustment-4.8L engines

With the idle speed solenoid energized, turn the solenoid body to establish the curb idle speed shown on your underhood sticker.

BASE IDLE SPEED

With the solenoid wire disconnected, turn the 1 / 8 in. hex head solenoid plunger adjusting screw to establish the base idle speed shown on your underhood sticker.

5.7L Engines

See Figures 2 and 3



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Fig. Fig. 2: Idle speed adjustment-5.7L engines without solenoid



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Fig. Fig. 3: Idle speed adjustment-5.7L engines with solenoid

  1. All adjustments should be made with the engine at normal operating temperature, air cleaner on, choke open, and air conditioning off, unless otherwise noted. Set the parking brake and block the rear wheels. Automatic transmissions should be set in Drive, manuals in Neutral, unless otherwise noted in the procedures or on the emission control label.
  2.  
  3. Refer to the underhood emission sticker and prepare the vehicle for adjustment as specified on the sticker. On models without a solenoid, turn the idle speed screw to obtain the idle speed listed in the underhood emission control label. On models with a solenoid, turn the solenoid screw to obtain the idle speed listed in the underhood emission control label. Disconnect the wire at the air conditioning compressor and turn the air conditioning on. Rev the engine momentarily to fully extend the solenoid plunger. Turn the solenoid screw to obtain the solenoid idle speed listed on the underhood emission sticker. Reconnect the air conditioning wire at the compressor.
  4.  

GASOLINE FUEL INJECTED ENGINES



The fuel injected vehicles are controlled by a computer which supplies the correct amount of fuel during all engine operating conditions and controls idle speed; no adjustment is necessary or possible.

DIESEL FUEL INJECTION



Idle Speed Adjustment

See Figure 4



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Fig. Fig. 4: Idle speed adjustment-diesel engines

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 the specification listed on the underhood label in Neutral or P 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:
    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 (check the underhood label) in Neutral.
    6.  
    7. Remove the jumper wire and reinstall the connector to the fast idle solenoid.
    8.  

  2.  
  3. Disconnect and remove the tachometer.
  4.  

 
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