How to Diagnose Engine Surging

You may find your engine surging with no check engine light and wonder what to do. Find out how to diagnose the IAC valve quick and easy yourself. All you’ll need to do is remove the air filter housing cover and check the valve with your hand using these step-by-step instructions.

What Is an Engine Surge?

Mechanic reviewing what can cause engine surging

An engine surge means the idle is surging, or the RPMs are fluctuating up and down when the engine is running.

Why Is My Engine Surging?

Bad Idle Air Control (IAC) Valve

Idle Air Control (IAC) Valve
Idle Air Control (IAC) Valve

The system in the example featured in this article’s video has an idle air control valve (IAC). This valve searches for the correct air-fuel mixture, how much air to let in, how much air there is and how hot and cold it is, and whether it needs less or more air.

The IAC valve has a connector, module, and air intake. It has a damper valve that spins, allowing the valve to open and close similar to the throttle body blade. It lets air in and out depending on the engine temperature. Carbon can build up on the valve and cause problems opening and closing, letting in too much air and causing engine surging.

How to Diagnose Engine Surging

Steps to diagnose an engine that keeps surging

  1. Check for Vacuum Leaks

    Make sure the air box is secured tightly. Check the air intake system for any signs of leaks. Check all of the vent lines and hoses, like one that travels to the brake booster.

  2. Remove the Air Box Cover and the Air Intake Tube

    Remove the airbox cover and disconnect any hoses from the air intake tube that connects to the air box and throttle body. The IAC valve on this car is located underneath the throttle body to the upper intake.

  3. Start the Engine and Cover the IAC Valve’s Port By Hand to See If the Idle Smoothens

    You can test the IAC valve with the air box cover removed. With the air box cover removed, start the engine. Then place your fingers over a port of air coming up and into the throttle body. If the engine idle smooths out with the port covered, the IAC valve is stuck open and will need to be replaced.

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Engine Surging? - Expert Tips to Diagnose and Fix - 1A Auto
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Engine Surging? - Expert Tips to Diagnose and Fix - 1A Auto
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If your car has engine surging, find out how to diagnose the IAC valve quickly and easily from our expert mechanic in this article and video
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1A Auto
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