Why Does My Car Rough Idle With AC On?

Are you facing a rough idle while the air conditioner is on? When you stop your car at a traffic light or gas station suddenly your car idle rough, isn’t this really irritating. Your whole mood may get ruined because of rough idle with AC on. So the question is why this happens and how to fix it.

When your ac is on, a drop in idle speed is normal but if it drops below 200 rpm and doesn’t rise again, then it means there is some problem.

Common Causes Of Rough Idle:

As the air conditioner place extra load on the engine, your car’s idle speed drops, and a faulty component make it even more miserable. Here are some common causes why your car idle rough when AC is on:

1. Dirty Throttle Body:

A dirty throttle body may cause stalling when you stop or at a very low idle. The throttle body is connected to the gas pedal by some cable and is responsible for controlling the amount of air allowed to enter the engine.

Throttle body, over time, gets dirty and although it isn’t a part of scheduled maintenance, it’s still better to clean it. A lot of dirt, carbon, or gum gets stuck under the throttle body. This can lead a car to rough idle and stalling.

2. Faulty Idle Air Control Valve:

If you notice an irregular idle speed, rough engine idle, stalling, or check engine light issue you need to check the IAC valve. The idle air control valve controls the engine speed and allows air to pass. If the IAC valve or throttle passage has too much carbon stuck around it, it may also cause rough idle with ac on.

3. Faulty Air Compressor:

If hot air is emerging out rather than cold air this might be a sign of a bad air compressor. If the compressor is not turning on or there is a lack of cooling power or If you hear a loud noise when turning on AC, this can also be a sign of air compressor failure. Your car may stall or idle rough with ac on if your compressor belt is worn or damaged.

4. Faulty Fuel Pump:

If you hear whining noises or having difficulty starting the car, this can be a sign of a faulty fuel pump which results in staling. The fuel pump is responsible for transporting fuel to the engine. So if there is a blocked fuel filter, the flow of fuel will get slow and will cause problems.

5. Low Battery Or Bad Alternator:

The air conditioner actually puts an extra load on your battery which results in fast drainage of the battery. This means that the alternator isn’t working how it was supposed to. Your car may stall due to a low battery. If you have a bad alternator, Ac will put more load on the bad alternator and battery which may result in their failure.

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If you notice dim or over bright lights or whining noises, this may mean that the alternator’s failing. A burning smell may also indicate a bad alternator.

6. Bad Or Worn Out Belts:

A worn-out or damaged belt can also cause rough idle with ac on as it can slip during operation. A compressor belt older than 5 years or those who have cracked surfaces should be replaced immediately.

A bad compressor belt puts strain on the engine because of which, the engine has to compensate for air conditioning load.

What To Do If Your Car Idle Rough?

Here are some checks you can do that may help you identify the issue:

1. Check The Belts:

Checking your compressor belts should be the first thing you do if you face rough idle with ac on as this can fix your problem in one go. You can check if the compressor belt has any crack on its surface and if there’s any crack, replace it or have them replaced.

2. Check The Throttle Body:

The throttle body gets dirty over time, so check if there’s any dirt or carbon accumulated around the throttle valve or throttle body’s edges. If a high amount of carbon gathers around the throttle body it will not let the throttle body work properly. It’s better to check the throttle body in few years and clean it if needed.

3. Check The Idle Air Control Valve:

Some cars don’t have an IAC valve. So you can check its presence and location in the manual. If there is a faulty IAC valve, you will get a signal by Check Engine Light. Or if your car was manufactured before 1996, you will have to check it yourself. Sometimes cleaning the moving parts inside the IAC valve can solve the problem, but mostly it gets blocked and need to be replaced. If you have an appropriate scanner you can check this by yourself.

4. Check The Battery Or Alternator:

It is quite easy to check the battery and can be done at home. Open your hood and start your car, as your engine is running remove the negative cables. If your car stalls or dies, this means the alternator is probably failing. The alternator is supposed to keep your battery charged so if the alternator’s gone bad, your car will start but will die again.

5. Check The Fuel Pump:

A faulty fuel pump may cause you great trouble so if you notice any signs of the faulty fuel pump, take your vehicle to a mechanic and get a full check-up. Get it replaced if needed. If you want to check it yourself, disconnect the hose from the pump and block the hose passage with your finger tightly. If you feel the pressure that means it’s alright or else you need to contact a mechanic.

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How To Change Air Compressor Belt?

  1. Locate the air compressor belt.
  2. Remove the belt guard by removing pressure snaps or screws.
  3. Check if there are any cracks on the belt.
  4. If there is any sign of damage, it’s time to change it.
  5. Loosen the screws and shift the motor, as these screws hold the motor in its place.
  6. Now remove the screws carefully.
  7. Put the motor to its original place and tighten the bolts.
  8. Adjust the belt tension before attaching it.
  9. Replace the belts with new belts of proper length.
  10. Reattach the belt guard.
  11. Fasten the screws and bolts properly.

How To Clean Throttle Body?

  1. Locate the throttle body.
  2. Disconnect the negative cables of your car’s battery.
  3. There are air ducts that you have to remove in order to access the throttle body.
  4. Locate small hoses that connect to the throttle body.
  5. Mark those hoses so that you can remember which hose is connecting where.
  6. Now carefully remove air ducts.
  7. Do not mess with any other electrical wires.
  8. If you are not able to remove the air duct, please leave it right there and hand it to a professional mechanic.
  9. If you manage to remove the air duct, you can now see the throttle body. (Note-Remove enough air ducts so that you can see the throttle body).
  10. Now that you can see the throttle body, spray a decent amount of throttle body cleaner and clean it with the help of a brush.
  11. Remove the dirt very gently and be careful to not let bristles or spray nozzle or anything fall into it.
  12. Once it’s clean put a decent amount of oil into the throttle shaft using a cotton ball so that the throttle blade can keep rotating smoothly.
  13. Now put the throttle ducts and reattach everything and the battery.
  14. Start your car and let it idle for few minutes.
  15. Test drive your car.

How Much Cost Does It Take?

  • The throttle body cleaning may cost you around $250 and If you clean it by yourself, an average throttle body cleaner may cost you around $10-$15.
  • If you want to replace the throttle body, parts may cost you around $110-$600. But if you are getting it replaced, it may cost you around $400-$1500 depending on the model and service. Also, labor costs may be around $100-$600.
  • Depending on your car’s model, the replacement of the IAC valve may cost you around $120-$500. If you want to buy parts alone, it may cost you around $70-$400.
  • The compressor belt may cost you around $25-$80 if you buy it alone. If you get it replaced, it may cost you around $100-$200 excluding labor costs, which may cost you around $75-$120.


Your car may suffer from rough idle with ac on due to some reasons. It doesn’t sound like a big problem but it can be very irritating. Get this problem fix as soon as you can because if the problem lies within the battery or throttle body or any other component, it can cause a big problem. If your battery has a problem it can be very dangerous if it suddenly dies. Also, your engine may get damaged due to much pressure on it.

So it’s recommended to not ignore this problem and consult to a professional mechanic. You can do some easy checks beforehand and also clean the throttle body every 5-6 years so that it won’t become a problem. If you are changing anything by yourself please be sure to have safety measures and be careful and ask for help if needed.

Stay safe. Thank you.

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