Idle Control System

System Description




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/PCM. When the IAC Valve is activated, the valve opens to maintain the proper idle speed.

1. After the engine starts, the AC 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 or P1508, check the following items:
- Adjust the idle speed
- Air conditioning signal
- ALT FR signal
- Starter switch signal
- A/T gear position signal
- PSP switch signal
- Brake switch signal
- Fast idle thermo valve
- Hoses and connections
- IAC 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 or P1508) after AC valve replacement, substitute a known-good ECM/PCM and recheck. If symptom goes away, replace the original ECM/PCM.

System Description:

Idle Air Control

Idle Air Control Valve (IAC Valve)
When the engine is cold, the A/C compressor is on, the transmission is in gear, the brake pedal is pressed, the P/S load is high, or the alternator is charging, the ECM/PCM controls current to the IAC Valve to maintain the correct idle speed.





Description
To prevent erratic running when the engine is warming up, it is necessary to raise the idle speed. The IAC thermal valve is controlled by a thermowax plunger. When the engine is cold, the engine coolant surrounding the thermowax contracts the plunger, allowing additional air to be bypassed into the intake manifold so that the engine idles faster. When the engine reaches operating temperature, the valve closes, reducing the amount of air bypassing into the intake manifold.