Idle Air Control (IAC) Valve Assembly
IAC valve assembly is used to control idle speed and provide a dashpot function. IAC valve assembly meters inlet air around the throttle plate through a by-pass within the IAC valve assembly and throttle body. See Fig. 6 or Fig. 7.
The PCM determines desired idle speed or air by-pass and signals the IAC valve assembly through specified duty cycle. The IAC solenoid is built into IAC valve assembly. IAC solenoid responds by positioning the IAC valve to control amount of air by-passed. PCM monitors engine speed and adjusts IAC duty cycle to achieve desired RPM.
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 TESTS W/CODES article.
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/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.
Friday, November 14th, 2008 AT 8:52 PM