Rough Idle When AC Is On? Why It Happens and Possible Fixes

Your car rough idle when AC is on for several reasons, including a faulty condenser fan, disconnected hoses, or bad serpentine belts. Rough idling, coupled with other issues, can lead to serious technical issues and affect your car’s performance. If your car idles normally when the AC is turned off but starts to get noisy when the AC is on, we can help you diagnose the problem.

Rough Idle When AC Is On

In this guide, our automobile team will help you understand why your car idles roughly when AC is on and how to resolve the issue once and for all.

Why Does My Car Rough Idle When AC Is On?

Your car rough idles when AC is on because of a faulty IAC valve, debris buildup on the body of the throttle, a damaged AC compressor, faulty cooling or condenser fans, or a dirty or clogged air filter, among other reasons.

One or more easily detectable problems can cause rough idling. We will discuss some of the possible causes below:

– Faulty IAC Valve

One of the more common reasons cars idle roughly when the AC is turned on is a faulty IAC valve. This valve regulates the extra air that goes into the air-fuel mixture and the AC unit.

It also regulates the engine’s idle speed, especially when an extra load is detected, such as when the AC is turned on. When you have a faulty idle air control valve, your engine will find it hard to increase speed when the AC is on, hence the roughness detected when idle.

– Debris Buildup on Throttle Body

An unclean throttle body can also affect the idle during AC operation. Every throttle body comes with a throttle blade or plate that requires calibration to stay closed when the car is idle.

That way, the car’s onboard computer regulates the amount of airflow that passes through the throttle plate. Insufficient air will be released if this component becomes dirty or blocked, leading to rough idling when the AC is on.

– Damaged AC Compressor

If your AC system lacks enough refrigerant to keep it functioning normally, there is bound to be a problem. This is because when there’s a lack of refrigerant, the compressor and engine will be forced to work overtime. When this happens, the compressor is bound to get faulty, which will cause a carbon buildup.

The compressor immediately shuts off to prevent further damage to the AC unit. A faulty compressor is easily one cause of a car idling rough when the AC is turned on.

– Faulty Cooling or Condenser Fan

The cooling or condenser fan works hand in hand with the compressor. Since the compressor is known for producing high-pressure gas, these components transform this gas into cool air. This cooling air helps the car engine regulate its idle speed.

If the condenser or cooling fan becomes faulty, a lot of high-pressure gas will be released into the car AC unit. When this occurs, the issue of rough idle when the AC is on becomes evident.

– Dirty/Clogged Air Filter

Another common yet easily overlooked reason why your car idles rough when the AC is on is a dirty or clogged air filter. Air filters ensure that particles or contaminants do not disrupt the normal combustion process.

Over time, these filters are bound to be dirty or clogged with residual matter, leading to insufficient air entering the engine and the AC system. When these filters are not cleaned or replaced, it can cause rough idling when the AC is on.

– Faulty Distributor and Ignition System

Although not common, the distributor and ignition system might be why your automotive idles rough when the AC is on. Using an older car model with a distributor system, the cap and rotor could become worn out or covered with carbon buildup.

When this happens, the rotor tip cannot produce enough voltage to reach the cap’s center and outer terminals. This buildup makes it harder for the car engine to ignite properly since the cap can’t get enough current from the spark plugs.

– Loose or Disconnected Hoses

Another uncommon yet possible reason why your car idles roughly when the AC is on is loose or disconnected hoses. Vacuum hoses are important in maintaining pressure in the car’s engine. When these hoses become loose or disconnected, they can result in leaks, which causes your car to idle rough. Your hoses might become loose or disconnected if pulled loose or weren’t reconnected after a mechanical operation.

– Serpentine Belt

A serpentine belt is a vital part of a car that provides power to most components of your car, from the compressor to the power steering pump and alternator. When the serpentine belt becomes faulty, less power is transferred to these components, causing the engine to get overloaded. A faulty serpentine belt is responsible for the stuttering or whining noise you hear when the AC is turned on while the car is idle.

– Failed Spark Plugs

Spark plugs are also a trivial reason your car roughly idles when the AC is turned on. These are responsible for the ignition process; many issues are bound to occur when they become faulty. Since a lower current is being transmitted, the car can’t run smoothly, thus increasing the load on the engine. One result of this is a rough idle when the AC is turned on, as there will surely be an increase in RPM.

– Worn or Dirty Fuel Pumps

Another rare cause of rough idle when the AC is on is a faulty fuel pump, which can spell trouble for your car’s overall performance. A worn-out or dirty fuel pump will affect the proper supply of fuel.

When this happens, only a little fuel goes to the engine, which is insufficient to match the car’s operational demands. This will cause your car to idle rough when the AC is turned on as there’s an increase in RPM and the engine is struggling to keep up.

How To Resolve Rough Idling When AC Is On

To resolve rough idling when AC is on, you can clean or replace the IAC valve, clean the dity throttle body, replace the AC’s compressor, fix the cooling or condenser fan, and clean or replace the air filter, among other fixes.

Once you’ve identified why your car idles roughly when AC is turned on, finding a quick and lasting solution becomes easy. Here are some ways you can fix rough idling when AC is on.

– Clean or Replace IAC Valve

If your IAC valve is the problem, cleaning or replacing it is a great way to stop rough idling when AC is on. Before cleaning or replacing the valve, ensure the engine is completely cool. Once you’re sure, it’s safe to run, open your hood and check your car’s manual guide for where your IAC valve is located. You should see a harness surrounding the valve.

Clean it using a carburetor cleaner and a clean rag. If you decide to replace the valve instead, separate the harness from the valve. Once separated, unscrew the bolts holding the valve in place and then clean the mount to remove residual dirt or carbon buildup. Position the new valve in place of the old one, screw the bolts back on, and attach the harness to the valve.

– Clean Dirty Throttle Body

Cleaning the throttle body is another way to stop the engine’s rough idle when AC is on. This is similar to cleaning the IAC valve, as you need to locate it under your hood and clean it with a throttle cleaner. Ensure you clean up any dirt or debris in and around the throttle blade, bore, and plate.

You must be careful when cleaning the throttle, especially the throttle bore. If you can, avoid using sharp tools or screwdrivers because any issue with the throttle bore can eventually cause your car to underperform.

– Replace the AC’s Compressor

To stop your car from idling rough when the AC is turned off, inspect your AC’s compressor and fix it immediately. Before doing this, ensure that your AC system still works as it should. Check if the system has power by switching it on and off. Once this is confirmed, locate your AC unit’s fuse box and remove all residual refrigerants before removing the old compressor.

Also, remove the old expansion valve, orifice tube, and dryer while ensuring that the area where the compressor has been taken from is cleaned of debris and dirt. You can now install the new compressor alongside its accompanying components.

– Fix Cooling or Condenser Fan

If you’re sure your cooling fan or condenser is the cause of your car’s idling roughly, you should immediately fix them. Although repairing these can be slightly expensive, it saves you the trouble of dealing with stuttering engine sounds when your car is idle. Contact a professional mechanic who will repair or replace them.

– Clean or Replace Air Filter

Cleaning or replacing your air filters is a simple way to eliminate rough idle. To clean or replace these filters, you first need to unscrew or unclasp the housing where the filter is located. Once you can take off the filter housing, remove the old filter and clean it up thoroughly. At this point, you can also place a new one in place of the old filter and screw the air filter housing back on.

– Inspect Your Distributor System

For owners of older car models, regular inspection of your distributor cap and rotor will save you the trouble of dealing with rough idle. Check your cap outer and center terminals to ensure they’re not cracked open or covered with carbon buildup.

If you need to fix the distributor, ensure that the replacement or repaired distributor looks exactly like the old one. Make sure that they both have the same number of gear teeth and are identical in length.

If you notice any disparity between old and new distributors, return to where you got it and request another one. If you use the new distributor knowing it isn’t the same as the old one, there’s a high chance you’d still experience rough idle when AC is on. Aside from these, your car would develop other performance issues.

– Inspect Vacuum Hoses

We advise that you inspect your vacuum hoses as regularly as possible to avoid leaks that can cause the engine to idle roughly. Make sure you inspect your hoses to ensure that they’re tightly connected at each end.

Also, ensure that you reconnect hoses after every mechanical operation. Once you’re sure that they are tightly and properly reconnected, run your fingers through each hose body for tear or damage.

If you can, also listen closely to ensure there are no hissing sounds that indicate leakage. Once you’re sure, everything is properly compiled, start your engine and your AC unit, and check if your car still idles rough.

– Fix Serpentine Belt

Fixing a serpentine belt is quite technical, so we recommend contacting a mechanic to help you with this. If needed, replace this belt with a new one as soon as possible. Once fixed, your car should immediately stop idling rough when you turn the AC on.

– Replace Bad Spark Plugs

Sometimes, replacing your spark plug and its wires is all you need to get rid of rough idling when the AC is turned on. To replace faulty spark plugs, ensure your engine is completely cool because it could burn your hands if you’re not careful.

Once this is ascertained, remove the high-tension wires or negative battery terminal from your spark plugs. Many spark plugs come with boots, so you’d need to remove these boots very carefully, one at a time.

Once that is done, remove the spark plugs in the same process. Install the new spark plugs and screw them tightly, although they don’t need to be overly tight to avoid breakage. After you have screwed the new spark plugs in place, place the wires or negative battery terminal back as you removed them.

– Clean or Replace Fuel Pump

We recommend cleaning your fuel pump as often as possible. There are two common ways to clean dirty fuel pumps. One way is to use a fuel cleaner to remove debris or buildup by spraying it directly into the fuel pump.

Another way is to locate the fuel pump, open it and clean out debris or carbon buildup with a clean rag sprayed with a cleaner. If your fuel pump needs replacement, contact a professional automobile expert who can do it for you efficiently.

Frequently Asked Questions

1. Why Doesn’t My Car Move When I Turn the AC On?

Your car doesn’t move when you turn on the air conditioner because your IAC valve is faulty. When this happens, your engine finds it hard to compensate because it is overloaded. At this point, you’d notice that your engine stalls or idles roughly.

Other reasons your engine stalls when you turn on the AC is a spoiled serpentine belt, a faulty throttle body, and a damaged compressor.

2. Is It Harmful To Let Your Car Idle When AC Is On?

No, letting your car idle with the AC on isn’t bad or harmful. In fact, letting your car idle with the AC on doesn’t cause any damage to your engine as long as the IAC or cooling system is working perfectly well.

You can let your car idle with the AC on for as long as you want, although you’d run out of gas faster.

3. Can Rough Idle When AC Is on Affect My Engine?

No, but although a rough idle should ideally not damage your engine, the reason for rough idling could have devastating effects on your engine. If you notice that you’re experiencing continuous rough idling when AC is turned on, identify and fix the component causing this to happen.

If you don’t fix it on time, a rough idle can damage the engine, leading to decreased fuel efficiency, engine stalling and shuddering, and several performance issues.

4. Will Bad AC Compressor Damage My Engine?

Yes, a bad or failed compressor can affect the engine, especially if left unattended for a long time. Over time, the load becomes too much for the engine to bear, and this could result in engine problems that might become more harmful over time.

You would also experience other problems like low fuel economy, jerking, and engine stalling, which means more money spent at the mechanic.

5. Is It Okay To Idle My Car for One Hour?

Yes, you can leave your car idle for an hour or more. If you idle your car for an hour or more, no harm will happen as long as the IAC valve functions normally. You only need sufficient gas to let your car run idly for any amount of time.

Conclusion

The best way to fix the problem of rough idle when AC is on is to identify the possible cause. Here’s a summary of the main points discussed in this article:

  • Your car will likely rough idle when AC is on if its IAC valve is damaged. An unclean throttle body, faulty condenser or cooling fan, or dirty air filters can also cause it.
  • One way to fix the problem of rough idling when the AC is on is to clean or replace the throttle body and IAC valve. Also, clean your air filters, replace failed spark plugs, and fix the broken air compressor or cooling fan to help fix this issue.
  • Letting your car idle with your air conditioning system on won’t cause any damage to your engine as long as all components are working as they should.

You no longer need to get worked up when you notice that your car idles roughly when the AC is turned on. With several causes and solutions, you can identify the specific problem and attend to it immediately!

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