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How to Spot a Failing Idle Air Control (IAC) Valve

By Mia Bevacqua, March 21, 2018

The idle air control (IAC) valve is used to control your engine’s idle speed and prevent it from stalling. It accomplishes this by controlling the amount of air that gets around the throttle plate and enters the engine. Typically, the IAC valve is mounted on the throttle body, which is seen in the diagram.

The engine’s computer controls the valve by sending voltage pulses to it. This causes the motor inside the IAC valve to move and direct airflow around the throttle plate to control idle speed. The computer uses input from various sensors, including the coolant temperature sensor and throttle position sensor, to determine how open the valve should be.

It should be noted that that most cars made within the past decade or so don’t have an IAC valve. Instead, they use an electronic throttle actuator for the same purpose.

Get it diagnosed by a professional
 

Signs of a failing idle air control valve

A faulty IAC valve can cause a number of problems:

  • Incorrect idle speed: An IAC valve that sticks in the closed position will create an idle speed that’s too low. Your RPMs will drop way down the car may feel like it’s about to stall. On the other hand, an IAC valve that sticks in the open position will create an idle speed that’s too high. The engine will run fast even though you’re not moving. A bad IAC valve can also cause a surging condition at idle, where the RPMs will increase and then drop back to normal. 
  • Your check engine light is on: The computer monitors the IAC valve to make sure it’s working correctly. If something’s wrong, it will turn on the check engine light.
  • The car stalls: A faulty IAC valve can prevent the engine from getting enough air to idle properly. As a result, the car may stall.
  • The car won’t start: In extreme cases, the IAC valve may cause too much or too little air from entering the engine when you turn the ignition, resulting in engine that cranks but doesn’t start.      

How to fix the problem

If you suspect your IAC valve is malfunctioning, have your mechanic inspect your car. Some of the above symptoms can be caused by a variety of issues.

Sometimes, the IAC valve or its passages may get clogged with carbon. Simply cleaning these parts may get the engine back to working normally. If that doesn’t work, the IAC valve’s circuit should be checked before condemning the valve itself. If neither of these fixes work, the valve is faulty and should be replaced.

It’s not worth it to replace an IAC valve with a used part — these parts fail commonly, and a used part may already have enough carbon buildup to cause the same issues.

 

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