Idle Control System

The idle speed of the engine is controlled by the Idle Air Control (IAC) Valve. The valve changes the amount of air bypassing into the intake manifold in response to electric current controlled by the ECM. When the AC Valve is activated, the valve opens to maintain the proper idle speed.

1. After the engine starts, the IAC valve opens for a certain time. The amount of air is increased to raise the idle speed about 150 - 300 rpm.

2. When the coolant temperature is low, the IAC valve is opened to obtain the proper fast idle speed. The amount of bypassed air is thus controlled in relation to the engine coolant temperature.

3. When the idle speed is out of specification and the scan tool does not indicate Diagnostic Trouble Code (DTC) P0505, P1508 or P1509, check the following items:
- Adjust the idle speed
- Air conditioning signal
- A/T FR signal
- Starter switch signal
- A/T gear position signal
- PSP switch signal
- Brake switch signal
- Hoses and connections
- AC valve and its mounting O-rings

4. If the above items are normal,substitute a known-good IAC valve and readjust the idle speed.
- If the idle speed still cannot be adjusted to specification (and the scan tool does not indicate DTC P0505, P1508 or P1509) after IAC valve replacement, substitute a known-good ECM and recheck. If symptom goes away, replace the original ECM.