Idle Adjustment Precautions

Idle speed on all engines is controlled by the Powertrain Control Module (PCM) and Idle Air Control (IAC) valve. If control system is operating properly, engine idle RPM is fixed and cannot be changed by standard adjustments. Some engines do not have an idle RPM specification. A scan tester must be used on these systems because control strategy and idle speed are controlled by PCM. Before performing idle speed adjustment procedure, complete a thorough basic inspection and self-test (KOEO, KOER, Continuous Memory self-tests) to verify proper operation of systems which may contribute to idle speed control problems. See appropriate TESTS W/CODES article in the ENGINE PERFORMANCE Section.

Before performing idle speed check and adjustment procedures, ensure throttle bore and both sides of throttle plate(s) are clean. Ensure throttle plate does not stick in bore and/or linkage is not limiting throttle plate operation. Throttle lever MUST rest against throttle stop screw. When idle speed control device is disconnected to set minimum air rate, engine may stall. This is acceptable only if throttle plate and linkage are not stuck.

Some throttle bodies use sludge tolerant throttle plate designs; they are identified with a Yellow and Black label. These units must not be cleaned, because a protective coating is applied to the lip of throttle plate(s).

Turn off all accessories before checking and adjusting idle speed. Ensure cooling system is filled to correct level. Ensure there are no vacuum leaks or unmetered air entering the throttle chamber on mass airflow design systems. If vehicle is equipped with electric cooling fan, check and adjust idle speed when fan is off.