System Description: Notes

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 PCM. When the IAC 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.

1. When the idle speed is out of specification and the scan tool does not indicate Diagnostic Trouble Code (DTC) P0505, check the following items:
   • Adjust the idle speed (see Idle Speed Setting  )
   • Starter switch signal (see Starter Switch Signal  )
   • Air conditioning signal (see Air Conditioning Signal  )
   • ALT FR signal (see Alternator (ALT) FR Signal  )
   • Brake switch signal (see Brake Switch Signal  )
   • PSP switch signal (see Power Steering Pressure (PSP) Switch Signal  )
   • A/T gear position signal (see )
   • Hoses and connections
   • IAC valve and its mounting O-rings
2. If the above items are normal, substitute a known-good IAC valve and readjust the idle speed (see Idle Speed Setting  ).

If the idle speed still cannot be adjusted to specification (and the scan tool does not indicate DTC P0505) after IAC valve replacement, substitute a known-good PCM and recheck. If symptom goes away, replace the original PCM.

DTC	Description
DTC P0505	Idle control system malfunction
DTC P1509	A problem in the idle Air Control (IAC) valve circuit