5 Major Reasons For a Car to Rough Idle When You Turn On the AC

Sometimes your car idles rough when the AC is on. While you can keep the AC off to avoid the problem, it’s not really a solution. So, you have to start by asking yourself, “Why does my car idle rough when I turn on the AC?”

Fortunately, we have the answer to that question. Typically, the reasons include a dirty throttle body, damages or clogs in the IAC valve, a bad cooling fan, and a faulty AC compressor. A vacuum leak can also cause this problem in some manner.

Below, we will discuss these reasons in detail and share some fixes you can try to solve the problem. Let’s get going.

How does the AC relate to a car’s rough idle?

An AC unit uses power from the engine to run smoothly. And honestly, it requires a lot of power to do it. 

At the same time, your engine requires power to idle at an ideal speed. So, both functions need power from the engine. 

The AC unit in your car can relate to a rough idle in two primary ways. The first is when the AC uses too much of the vehicle’s engine. 

Second, a car’s air flow can be hampered by an AC unit’s excess carbon buildup, which can harm the engine.

For the first issue, whenever the AC compressor cycles, it will draw a bit of extra power from the car engine. 

Idling your car also requires power from the engine. So, when the engine fails to deliver that kind of power due to parts failing to work in the AC unit, it will cause a rough idle.

The second issue is due to the fact that AC units produce a lot of excessive carbon in your car. 

And the engine idles too low when there is carbon buildup in components like the throttle body or IAC valve. This issue results in a rough idle when you turn the AC on.

In either case, it all boils down to a component related to the AC unit and car idling failing to perform.

5 Reasons for a car to idle rough when the AC is on

We will now go into more detail about the causes of rough idle when the air conditioner is turned on. 

These are general reasons and not specific to any one car model. So, when you have a problem, look out for these first.

1. The throttle body is too dirty

A clog in the throttle body is the number one cause for a car to idle rough with AC. The throttle body is the component that supervises the airflow throughout your car’s engine.

When the engine is idle, the throttle body is supposed to adjust the opening to maintain an ideal idle speed. 

As you turn the AC on, the throttle opening should adjust itself to maintain a proper idle speed even with the added load.

However, a clogged-up throttle body fails to do that and causes insufficient air to pass through, putting more pressure on the engine. Your car starts to idle poorly as a result.

2. IAC valve is damaged or clogged

Another major cause for a car to idle rough with AC on is the clog or damage in the IAC valve. 

The IAC valve is a key component of controlling the speed when you let your car idle. It’s also responsible for maintaining the airflow of your engine and AC unit.

The IAC valve’s function helps the engine compensate for the extra pressure it gets from turning the AC on. It helps gain the required idle speed to keep things normal when you put the car idle.

But it will not function normally when the IAC valve is clogged with carbon buildup or malfunctioning. 

As a result, your car’s engine can idle at a low speed and fail to compensate for the extra power requirement. It causes the car to idle badly.

3. The cooling fan has gone bad

A cooling fan for any car AC unit greatly regulates the flow of air in your car engine. It makes sure the engine won’t overheat from using the AC at a high enough power level.

If your car’s cooling fan has any defects, it will not maintain the airflow properly. 

As a result, there will be added pressure on the car engine, eventually leading to a rough idle at some point.

4. Faulty AC compressor

An AC compressor is one of the most important parts of an AC unit. The function of an AC compressor is to compress the refrigerant you use in your car to provide the unit with cooler air.

The gas compression makes sure the engine doesn’t get overloaded with pressure. 

So, whenever the AC compressor fails to perform, it will put a lot of pressure on the car engine as you turn the AC on. Later, this causes the car to idle.

5. Vacuum leak

A vacuum leak can be responsible for a rough idle if you are running a manual AC unit. 

When there is a vacuum leak in your car’s system, it will insert unwarranted air into the car. It will mess up the measurement of air quality on the computer.

The computer won’t provide the true air volume or temperature that your car needs to compensate for using an AC unit while idle. As a result, the car goes through a rough idle instead of a regular one.

How to fix a car with rough idle while the AC stays on 

After the reasons, now it’s time for the solutions. Here are some ideas for fixes you can try out for the problem.

Clean the throttle body

All you have to do to get rid of a dirty throttle body is clean it. It’s something you can do in the comfort of your home. 

You will need a throttle body cleaner for the job. As for the process, you can follow the video below to clean the throttle body.

How To Clean a Throttle Body ~ The RIGHT Way

Clean or replace the IAC valve

When the issue with your IAC valve arises, you have two fixes available. First, you have to check the IAC valve. 

If it’s clogged, you go to clean it, but if it’s damaged in any way, you have to replace the valve.

Here’s a video showing you how to clean the IAC valve of your car.

How to Fix a Car that Idles Poorly (Rough Idle)

Replacing the cooling fan

In the case of a damaged or bad cooling fan, the only solution you have is to replace it. You can go to a mechanic or follow the video guide below to replace the fan.

Replacing your engine cooling fan | AUTODOC tips

Replacing the bad AC compressor

When you deal with a faulty AC compressor, all you have to do is replace it. 

The best solution is to go to a mechanic and let them replace the AC compressor. Besides that, here’s a video showing you how to do it.

How to Replace an AC Compressor in your Car

Replace the parts causing a vacuum leak

Usually, a vacuum leak occurs with damage to the intake hose or the tubes that go into the intake manifold.

To fix the issue, you have to replace these parts. Find out where the leak is, and then replace the damaged part. 

Check out the video to better understand how to find the leaks and replace the parts.

How to Find and Fix Vacuum Leaks – Ultimate Guide

FAQs

Until now, we have answered all the crucial inquiries about the rough idle issue on your car when the ac is on. Now, let’s check out some common questions many people have about this problem.

Conclusion

Hopefully, you don’t have to wonder, “Why does my car idle rough when I turn on the AC?” anymore. We gave you all the information you needed to find out what’s causing the problem and how you can fix it.

The whole article clears up one thing: it’s all about engine power when it comes to this problem. If you can maintain proper power on the engine, you can avoid the issue.

So, our suggestion is to follow regular maintenance for your car. It’s the best way to prevent the problem. 

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