Chrysler Full-Size Trucks 1997-2000 Repair Guide

Idle Speed and Mixture Adjustments



Unlike carbureted vehicles, the fuel injected engines covered in this guide Do NOT require idle speed adjustment checks as a part of a regular tune-up. In fact normal idle speed is NOT adjustable and should not be attempted. Base idle speed is controlled by the engine controller (computer).

This adjustment is the minimum idle speed with the Automatic Idle Speed (AIS) closed.

  1. Before adjusting the idle on an electronic fuel injected vehicle, the following items must be checked (and corrected, if necessary):

    AIS motor operation
    Engine vacuum and/or EGR leaks
    Engine timing
    Coolant temperature sensor operation

  3. Connect a tachometer and timing light to the engine, according to the manufacturer's instructions.
  5. Close the AIS by using ATM tester C-4805 or equivalent, and ATM test code #03.
  7. Connect a jumper to the radiator fan, so that it will run continuously.
  9. Start and run the engine for three minutes to allow the idle speed to stabilize.
  11. Check engine rpm and compare the result with the specifications listed on the underhood emission control sticker.
  13. If idle rpm is not within specifications, use tool C-4804 or equivalent to turn the idle speed adjusting screw to obtain 790-810 rpm. If the underhood emission sticker specifications are different, use those values for adjustment.

If idle will not adjust down check for binding linkage, speed control servo cable adjustment or throttle shaft binding.

  1. Turn the engine OFF , then disconnect the tachometer, reattach the AIS wire, and remove the jumper wire from the fan motor.


The low speed idle is the only adjustment that can be made to the injection pump. All other injection pump service requires a qualified diesel engine technician.

Use an optical diesel engine tachometer to read engine rpm. If necessary, adjust low idle speed at the low idle speed screw. If the vehicle is equipped with an automatic transmission, with the A/C on, set the low speed idle to 700 rpm. If equipped with a manual transmission, with the A/C ON , set the idle to 750 rpm. A special Cummins tool No. 3823750 is required to loosen the idle speed locknut for adjustment.