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.

Fig 1: Idle Control System Operation Diagram

Courtesy of AMERICAN HONDA MOTOR CO., INC.

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.
   Fig 2: Engine Coolant Temperature Graph

   Courtesy of AMERICAN HONDA MOTOR CO., INC.
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 (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  )
   • A/T gear position signal (see A/T GEAR POSITION SWITCH )
   • 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 (see IDLE SPEED SETTING  ).

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