Idle Air Control Valve

Most modern engines use the ‘idle air control valve’ (IACV) to stabilize the engine speed when the engine is at idle (when the gas pedal is not being pressed). IACV’s are opened and closed by request from the engine control module, and they allow enough air to flow into the engine to prevent stalling.

Idle air control valves are subject to failure, as they often clog with carbon. Also, they can fail electronically, which will likely illuminate the check engine light.

Symptoms of Wear or Failure of the Idle Air Control Valve

  • Engine will not start in cold weather.
    The colder the engine is, the more apparent IACV problems become, so when the engine and weather are cold, the engine may not be able to start due to the low amount of air flowing into the engine.

  • Engine stalls when the vehicle stops.
    The idle air control valve already has a little bit of latency, meaning, it does not action instantaneously on request. A slight pause between the engine speed slowing to idle, and the IACV opening is already responsible for a very slight dip in engine speed when the car stops, however, when the IACV is clogged or damaged, that dip in engine speed can actually stop the engine.

  • Engine runs well except when idling.
    This results because the only time the IACV is needed is at an idle. All other times, the gas pedal regulates the amount of air needed in the engine, and the idle air control valve is closed.

  • Engine stalls immediately after starting.
    When the engine is first started, there is plenty of air in the intake manifold to burn the fuel being pumped in, but that air is used quickly, and the throttle body in the closed position may not allow the engine enough air to idle. The result is an engine that will run only a few short seconds before stalling. Restarting may be difficult.

  • Idle speed is erratic, too high, or too low.
    When the engine computer signals the IACV to open, it adjusts other engine management systems to allow the engine to run at idle. When the IACV does not allow enough air or has a leak allowing too much air to enter the engine, the computer will constantly try to correct the idle speed, causing the engine to speed up and slow down rapidly. Also, a leak or clog could cause the idle speed to raise or drop, respectively.

Related Repair Advice for the Idle Air Control Valve

  • Before replacing the idle air control valve, note that these symptoms can be caused by other components, and testing is the only way to ensure time and funds are not wasted on replacing good parts.

  • Many vehicles use a relay for the idle air control valve, and failure of the relay will cause the IACV to cease functioning.

  • Replacing an idle air control valve with a used part is never recommended because these parts fail commonly, and a used part may already have enough carbon build-up to cause the same issues. 


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