Idle speed is maintained by the Powertrain Control Module (PCM). No adjustment is necessary or possible.
The Idle Air Control (IAC) valve assembly controls engine idle speed and provides a dashpot function. The IAC valve assembly meters intake air around the throttle plate through a bypass within the IAC valve assembly and throttle body. The PCM determines the desired idle speed or bypass air and signals the IAC valve assembly through a specified duty cycle. The IAC solenoid responds by positioning the IAC valve to control the amount of bypassed air. The PCM monitors engine rpm and increases or decreases the IAC duty cycle in order to achieve the desired rpm.
The IAC solenoid has an internal diode on some applications. If the internal diode is measured in crossed terminal position with a digital multimeter, there will be an incorrect or negative reading. It is important that the mating component and harness connectors be in the proper orientation.4.6L Engines
The traditional idle air adjustment procedure is not used on 4.6L engine applications.
During idle, the throttle body assembly provides a set amount of air flow to the engine through the idle air trim screw passage or throttle plate idle air orifice and the PCV valve. The IAC valve assembly provides additional air when commanded by the Powertrain Control Module (PCM) to maintain the proper engine idle speed under varying conditions. The IAC valve assembly mounts directly to the throttle body assembly in most applications. Idle speed is controlled by the PCM and cannot be adjusted.5.0L Engines
- Apply the parking brake, block the drive wheels, and place the vehicle in P .
- Start the engine and let it run until it reaches normal operating temperature, then turn the engine OFF .
- Back the throttle plate stop screw clear off the throttle lever pad.
- Place a 0.010 in. (0.25mm) feeler gauge between the throttle plate stop screw and the throttle lever pad. Turn the screw in until contact is made, then turn it and additional 1 1 / 2 turns for the 5.0L Hi Output engine, and 1 7 / 8 for the base engine. Remove the feeler gauge.
- Shut the engine off and disconnect battery for 5 minutes.
- Start the engine and let the idle stabilize for 2 minutes. Lightly depress and release the accelerator, then let the engine idle.