Dan Ferrell writes around do-it-yourself car maintenance and also repair. He has certifications in automation and regulate modern technology.

You are watching: Why does my car idle rough when i turn on the ac


The Air Conditioning Shouldn't Generally Keep the Idle Speed Down

Does your vehicle idle stormy as soon as you revolve on the air conditioning?

A brief initial loss of rpm as the AC comes on is normal. When the AC clutch engperiods to run the compressor, it locations a second load on the engine. But the car's computer system (PCM) have to carry the idle rate earlier up.

If you lose over 200 rpm and also idle speed doesn’t climb again, then there’s a problem.

Possible Causes of Rough Idle When the Air Conditioning is On

A rough idle once the air conditioning is on may be resulted in by:

A faulty or dirty IAC valveA dirty throttle bodyA faulty cooling or condenser fanOther faulty units or components, depending on the model

The following sections let you discover the most prevalent causes. Each area has or links to a simple diagnostic or troubleshooting procedure you can execute at house. The diagnostic have the right to assist you solve your vehicle much faster and, if possible, save money in the process.


1. Faulty IAC Valve

2. Dirty Throttle Body

Video: Dirty Throttle Casupplies Rough Idle

3. Problems via the Cooling or Condenser Fan

4. Distributor and also Ignition System Problems

5. Adjusting Idle Speed

6. Additional Investigating a Rough Idle When the AC is Running


The IAC valve is a widespread source of trouble.

Author's very own image.

1. Faulty IAC Valve

The powertrain manage module (PCM) supplies the idle air manage (IAC) valve to regulate idle speed. Basically, the IAC allows a certain amount of air to bypass the throttle plate. The additional air enriches the air-fuel mixture throughout a cold engine begin. It additionally helps increase engine rate once an additional pack is placed on the engine, for instance, when you turn on the AC or defrost mechanism. However before, a faulty or minimal IAC valve may save the engine idle speed from increasing once it needs to.

The the majority of widespread IAC troubles are carbon buildup about the IAC valve and also throttle passeras, and IAC motor malfunction.

You deserve to carry out an easy IAC motor diagnostic and inspect the valve and also throttle bypass port for carbon buildup.

If you must remove buildup from the IAC valve or passages, you deserve to use the very same procedure used to clean a throttle body, as defined in the next section.


Check and rerelocate buildup from the throttle body, if important.

Picture courtesy of Musay16 on Wikimedia.

2. Dirty Throttle Body

A dirty throttle body have the right to also influence idle in the time of AC operation.

A throttle plate is calibrated to remajor nearly closed during idle, while the computer system regulates aircirculation past the throttle plate. However, if the throttle plate and bore are dirty, inadequate air will certainly circulation and also reason trouble once you turn on the AC.

Cleaning the throttle bore:

Pop the hood open.Rerelocate the air cleaner duct that connects to the throttle body or air cleaner snorkel, relying on your particular version.Remove the air cleaner snorkel, if important.Grab a clean shop rag and also spray it with carburetor cleaner.Use the damp rag to rerelocate deposits approximately the throttle bore and plate, and under the plate.Carefully swing open the throttle blade using your hand also. Don’t usage screwdrivers or various other sharp tools that may scratch the throttle bore. Scratches may cause performance problems later.

Cleaning an electronic throttle manage (ETC):

The digital throttle manage system (electronic fly-by-wire throttle body) is run by the PCM with sensor inputs.

You have to be careful when cleaning buildup in this device. Carelessly handling the throttle blade can upset the throttle base idle place programmed into the computer.

Pop the hood open.Gain accessibility to the throttle body.Ask a helper to rotate the key to the On position, but don’t start the engine.Ask your helper to gradually depress the accelerator pedal.Spray throttle body cleaner on a clean shop rag.

Use the shop rag to clean the throttle bore and also blade.

A couple of points to remember:

Be mindful not to relocate the throttle blade through your hand while cleaning. If important, only spray the throttle body and also blade with throttle body cleaner.When removing carbon deposits in an ETC system, usage only throttle body cleaner. Other solvents might reason damages to the system.

If you require the PCM to relearn the idle base place aget, consult the automobile repair hand-operated for your certain auto design.

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Sometimes, you have to remove the throttle body to clean it. If important, rearea the throttle gasket. The complying with video reflects just how a dirty throttle body and valve were resulting in a unstable idle.