GM Chevy Mid-Size Cars 1964-1988 Repair Guide

Minimum Idle Speed


On fuel injected engines, idle speed is controlled by the ECM using the IAC valve. No periodic adjustments are necessary. However, if a throttle body has been replaced and no other causes can be found for an incorrect idle speed, then base or minimum idle speed may be adjusted. ONLY use this adjustment as a last resort. If this adjustment is improperly altered, the ECM will be unable to properly control engine idle.


Only if parts of the throttle body have been replaced should this procedure be performed; the engine should be at operating temperature. The idle stop screw is set at the factory to allow a minimum amount of air when the IAC valve pintle is fully extended into the throttle body. This amount of air will determine the minimum idle speed the ECM can order by fully extending the pintle.

  1. Remove the air cleaner assembly from the engine for access. Be sure to plug any vacuum ports which were disconnected in order to remove the air cleaner assembly.
  3. Remove the plug over the idle stop screw plug by piercing it with an awl and carefully prying the plug free of the throttle body.
  5. With the IAC valve connected, ground the diagnostic test terminal of the underdash ECM diagnostic connector.
  7. Turn the ignition ON , but DO NOT start the engine and wait for 30 seconds. This allows time for the ECM to fully extend the IAC valve into the throttle body.
  9. With the ignition ON , disengage the electrical connector from the IAC valve.
  11. Remove the ground from the diagnostic lead, then start the engine.
  13. Adjust the idle stop screw to obtain 400-450 rpm with the transmission in Drive.
  15. Once the minimum idle speed has been set, turn the ignition OFF and engage the electrical to the IAC valve.
  17. Check the TPS voltage output and, if necessary, replace the sensor.
  19. Install the air cleaner assembly and reconnect any vacuum hoses.
  21. Start the engine and inspect for proper idle operation.