Engine Idle Air (Speed) Control

DESCRIPTION
Engine idle speed is controlled by the ECM through the Idle Air Control (IAC) valve which allows more/less air to bypass the throttle valve. The reasons for idle speed control:

1. To keep the engine idle speed as specified at all times. The engine idle speed can vary due to the following reasons:

A. Load applied to engine (when electric load is applied, A/C is turned "ON," headlights "ON," stop lights, transmission is shifted to "R," "D," "2" or "L" range, etc.).
B. Variation in atmospheric pressure.
C. Change in engine itself with passing of time.
D. Other factors causing idle speed to change.

2. To improve starting performance of engine.
3. To compensate air/fuel mixture ratio when decelerating (dash-pot effect).
4. To improve driveability while engine is warming up. The IAC valve opens the bypass air passage when it is energized by the ECM and closes it when de-energized.

COMPUTER CONTROLLED IDLE
The ECM detects the engine condition by using signals and inputs from various sensors and switches, and while repeating ON and OFF cycle of the IAC valve at a certain rate (200 times a second), it controls bypass air flow (IAC valve opening) by increasing and decreasing its ON time within a cycle.

When the vehicle is at a stop and the throttle valve is at the idle position and engine running, the engine speed is kept at a specified idle speed.