Why Does My Car Idle Rough When I Turn On The AC?

When you’re driving on a hot summer day, it’s normal to turn on the AC to keep the interior temperature at a comfortable level. Then you stop at the next intersection, and your car starts idling rough, causing concern about the state of the engine.

There are several reasons why your car idles rough when you turn on the AC, but the most common is the adjustment of the idle control actuator to the increased load on the engine, characterized by a drop in RPM while it adjusts the throttle. If the problem is persistent, there could be a fault with one of the throttle or AC parts, or a sensor. 

This guide will take you through the steps of what to do when the car idles rough. We’ll troubleshoot and eliminate parts, and talk about how to fix rough idle when ac is on, so keep reading to find the solution that will work for you!

How Does Car AC Work?

The standard principle of AC operation involves changing the refrigerant between gas and liquid state to absorb heat and humidity and passing on cooled air to the interior.

The major components of a vehicle air conditioning unit are:

Compressor

Compresses low-pressure gas and creates a pressure barrier. It’s mounted on the engine and driven by the serpentine belt.

Condenser

Cools off the high-pressure gas coming from the compressor, works in the same way as the standard engine radiator. It’s mounted near the engine radiator, usually in front, or below it.

Dryer

Removes moisture from the refrigerant, it can be found between the condenser and metering device on the high-pressure side of the AC.

Metering Device

A valve that controls refrigerant pressure, found between the firewall and dryer.

Evaporator

Reverts the refrigerant back into a gas. Air coming into the cabin passes over the evaporator, cooling it. It’s located behind the dashboard.

There are two parts that directly control the AC system – the compressor and clutch cycling switch.

1. Air Conditioning Unit

When rough idle appears when you turn on the AC, it’s completely normal to assume that the AC itself is the problem. This can often be the case, especially if the AC doesn’t have enough refrigerant due to consumption or a leak. When that happens, the compressor will automatically shut off to prevent damage to the AC unit, which can cause surges in RPM.

Fixing any problem with the air conditioning unit requires specialized and very expensive tools that only specialized mechanics have. If you suspect that the AC is what’s causing the rough idle, take your vehicle to a professional for an inspection.

2. Serpentine Belt

When you turn on the AC, a clutch connects the compressor to the belt to provide power. This puts an additional load on the crankshaft, which naturally slows it down. If the idle control actuator is good, it will quickly realize that the engine is losing power, and it will automatically raise the idle RPM to better match the current load.

A bad serpentine belt has a lot of accompanying symptoms to the rough idle. The characteristic whining sound, especially when the engine is cold is a tell-tale sign of a serpentine belt problem. As the power steering pump, AC and alternator are all powered by the belt, you may also experience battery drain, loss of power steering, and weaker AC cooling because the belt cannot properly transfer energy.

Replacing the serpentine belt isn’t a task I would recommend performing yourself. It’s not necessarily a difficult part to replace, but a skilled mechanic will notice faults in the drive system you might miss. Depending on the type of vehicle, parts, and labor range between $100 and $300, which is not a lot considering how many expensive components the serpentine belt is in charge of.

If you’re confident in your skills, here’s a quick overview to help you get started with the repair.

3. Idle Control Actuator

Idle control actuator, also known as IAC, idle control valve, or idle speed controller does exactly as the name implies – it regulates the engine speed by adjusting the air and fuel mix to create optimal idle RPM. The typical construction of an IAC consists of a closed housing with a solenoid valve servo unit or a plunger, controlled by the computer.

Symptoms of a faulty idle control actuator are:

  • Inconsistent, rough idle.
  • The engine stalls when idle.
  • The engine stalls when idle if AC is turned on.
  • The check engine warning light turns on.

The likely causes for faulty idle control actuator are the contamination and forming of resin inside the valve system, a short circuit, and no voltage supply from ECU. Because the throttle control overrides the idle control actuator, if the engine operates smoothly when gas is applied, that is a solid indicator that there might be a problem with the IAC. To be completely sure, you can get an inexpensive OBD scan tool that will help you identify problems with the engine.

 

Now we can go through the steps of replacing the idle control actuator. Make sure that the engine is completely cool before starting, or you risk burning your hands!

  1. Find the idle control actuator. It’s positioned around the intake manifold and looks like a small, cylindrical object. A quick google search will tell you the exact location of your vehicle.
  2. Disconnect the wiring harness from the actuator.
  3. Remove the actuator by unscrewing the bolts holding it in place.
  4. Clean the mount to ensure a secure fit, then place the new actuator in its place.
  5. Screw the bolts back in, then reconnect the wiring harness.

If you’re having a hard time visualizing the steps, take a look at the following video. It has alternative steps of installation, but the core process remains the same.

4. Throttle Body

Throttle body is the next suspect on the list, especially on older vehicles. The throttle control is programmed into the computer to be optimal for a brand new engine. However, over time, the throttle control will gradually accumulate carbon deposits, hindering its effectiveness.

Cleaning the throttle control is a fairly simple task. You’ll need a mechanic’s toolset and a can of throttle cleaner.

  1. Disconnect the air duct and electrical connectors getting in the way of the throttle body.
  2. The throttle control is a round plate pivoting at the center point. Take the throttle cleaner with the dispensing straw and spray a generous amount of cleaner into the throttle body.
  3. Use a non-shed microfiber cloth to clean the inside of the throttle body. Be gentle with the valve itself as it can break.
  4. Spray and clean for 5-10 minutes, removing all the deposits is going to be challenging.
  5. Leave the valve open and let the residue throttle cleaner evaporate to avoid a rough start. For the best effect, wait at least an hour.
  6. Return the valve to its original position and reconnect the air duct and electrical harnesses.
  7. Start the vehicle and let it run idle. It may stall a few times, but do not step on the gas for the first few minutes to let the cleaner fully leave the system.

My go-to youtube mechanic, Scotty Kilmer has another short and to-the-point guide on how to clean the throttle body, so make sure to watch it before you start cleaning!

5. Air Filter

The most trivial, yet likely cause of rough idle with AC turned on is a clogged air filter.

Different car manufacturers and owners recommend different replacement intervals. My rule of thumb is to change the air filter together with the oil filter and oil. Some change the oil filter with every other oil change, while others change the air filter every 3 years. However, as air filter costs just $10-20 and requires no time or skill to replace, changing it annually is a negligible expense.

 

To replace it, take off the top cover of the air filter housing. It’s usually secured with bolts, a butterfly nut or just clamps, so at most you’ll need a Phillips screwdriver. Take out the old filter, and compare it to the new one to see the obvious difference in condition. Put the new filter in and attach the top cover to complete the process.

6. Spark Plugs

Problems with spark plugs also include rough idle with AC turned on. In the early stages of spark plug failure, the problem is masked when you’re applying gas, but the shift in idle load and increase in RPM will lead to a rougher idle operation and surging.

Replacing spark plugs is a fairly simple task. You’ll need a spark plug socket and a 3/8″ drive ratchet, and of course a set of spark plugs. You can buy the same brand as the spark plugs you’ve removed, but when in doubt, Bosch is always a safe pick.

  1. Make sure the engine is completely cold before you start.
  2. Take the high tension leads (wires) off spark plugs carefully to avoid damaging them.
  3. Place the socket in the hole and gently set it on the spark plug. Once you feel the socket binding, unscrew the spark plug.
  4. Repeat the process for every spark plug in the engine. The number of spark plugs is equal to the number of cylinders, but in some cases, there can be two spark plugs per cylinder.
  5. Screw the new spark plugs in. Do not overtighten as the spark plugs can break.
  6. Place the wiring back on in the same order you’ve removed them.

 

FAQs

Why does my car stall when I turn on the air conditioner?

As explained in the serpentine belt section, the air conditioner puts additional load on the crankshaft. To compensate, the engine has to run faster. If one of several components controlling the idle throttle operation fails, it will not raise the RPM to a level required for the engine to run, and it will stall.

Is it bad to let your car idle with the AC on?

No, letting your car idle with AC on is completely fine. The idle control actuator compensates for the extra load on the engine by raising the RPM, so the only thing you’ll get from a prolonged idle operation is a slightly increased fuel consumption.

Can rough idle damage engine?

Technically speaking, rough idle is a symptom that shouldn’t damage the engine, but its cause definitely can. If there’s a problem with the serpentine belt, you may lose power steering while driving. The serpentine belt usually drives the radiator fan, but sometimes it’s also responsible for the water pump. Snapping of the serpentine belt can quickly lead to engine overheating, ultimately resulting in a total engine failure.

Will a bad AC compressor affect the engine?

When it comes to the AC, the major thing to be concerned about is the compressor clutch and pully bearing becoming stuck and not rolling with the serpentine belt, which may snap, and result in problems covered by the previous question.

Is it OK to start the car with AC on?

Yes, it’s fine to start the vehicle with AC turned on, because the engine will automatically shut down all systems that will put extra strain on the starter and car battery.

Can I idle my car for an hour?

You can leave your car idle for as long as you’d like, provided it has enough fuel to run and the coolant temperatures remain normal. Cars that are a few decades old may rely on the air stream hitting the radiator to adequately cool the engine, so keep an eye on the temperature gauge.

What are the signs of a vacuum leak?

Rough idling, engine hesitations, vacuum sounds coming from the engine bay and engine warning light turning on are some of the symptoms of a vacuum leak.

Summary

The best way to identify the cause of rough idle when AC is on is to eliminate the causes, starting with the simplest problem. 

First, check the air filter, then the throttle body. Use the OBD scanner to see what kind of error is reported by the computer. Replace the idle control actuator, and if that doesn’t help, change the spark plugs. Listen for the signs of a vacuum leak or a failing serpentine belt. If either is causing the issue, consult with a professional.