Engine Idle Speed Control Operation

The engine idle speed is controlled by the powertrain control module (PCM) through the idle air control (IAC) valve which allows more or less air to bypass the throttle valve. There are several reasons for idle speed control:

1. Maintain the engine idle speed at the specified RPM at all times. The engine idle speed can vary due to any of the following reasons:
   • A change in atmospheric pressure.
   • A change in the engines condition over time.
   • Other factors causing the idle speed to change.
2. Improve the driveability of the engine during warm up.
3. Compensate for the change in the air/fuel mixture ratio when decelerating (dash-pot effect).

The IAC valve opens the idle air bypass passage when it is energized by the PCM and closes the idle air bypass passage when de-energized.

The PCM detects the engine condition by using inputs and signals from various sensors and switches. While repeating the ON and OFF cycle of the IAC valve at a certain rate, 200 times a second, the PCM controls bypass air flow, IAC valve opening, by increasing and decreasing the ON time portion of the cycle.