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Car Overheats When AC is On & Idle – Symptoms + Fixes

Written by Nick Steiner

When your car overheats when the AC is on, a few common problems could be the cause. We have found the issues likely to cause overheating and the solutions to fix the problem.

If you find the problem still exists after trying the remedies, speak to a vehicle specialist. Never ignore the problem. Overheating that is ignored can cause major damage to the internal components and the engine.

Car Overheats When AC Is on or Idle  – 7 Symptoms

1. Temperature gauge

Your car will either have a warning temperature gauge or a coolant temperature check.

If your gauge is getting higher and not sitting in one spot, your car is getting hot. Your car will most likely give you a warning when the gauge gets up to high.

If this happens, you need to pull over and turn the car off to allow it to cool down.

2. Car feels hot

If you can smell an odd odor and your car feels very hot inside, there is a problem.

If the smell resembles plastic or rubber burning, then the car has overheated. Quite often, the smell means the gasket and plastic internals are burning.

2. Ticking sound coming from the engine

A ticking sound isn’t good at any time but specifically coming from the engine can be troublesome. If you haven’t changed your oil for a long time, it could be a problem.

The oil cannot do its job in the engine when it is dirty and old. Turning the a/c on adds extra strain on the engine and can cause friction and lumps to occur.

3. The engine is shutting down

If the car is on and the A/C to and it’s overheating to a dangerous temperature, some cars will actually shut the engine off.

You will not be able to start the car again until the vehicle has cooled down enough.

4. Puddles underneath your car

It is normal to see water puddles under the car if you have been driving around with the a/c on. It could be coolant if the car hasn’t been driven or the puddles are differently colored, like green or blue.

Coolants help keep the vehicle and losing them with or without the aircon on will cause overheating.

5. Speed problems

Your car might maintain speed well until you turn the air conditioner on.

However, if your car develops acceleration problems shortly after turning the a/c on, then there may be a problem with your expanded pistons. These will likely need to be replaced.

6. Overloading of AC compressor

You may have noticed that when you turn the AC on, the RPM slightly increases. That’s because the AC compressor gives extra load to the engine. The cooling system of the engine is designed in a way that it should not create any running issues.

The problem arises when the compressor isn’t turned on by the accessory belt and the compressor gets seized. However, the car keeps trying to start the compressor, which puts an extra strain on the engine and leads to overheating.

Solution 

You can start by checking the AC compressor wire. Then check the voltage if it’s proper for the compressor. Also, look for any damage in it and check the clutch.

7. Engine coolant sensor problem

There is a sensor in your car that signals the engine to start the cooling system. You can find this sensor on the cylinder head located in the water passages. This sensor measures the temperature of the water that comes from the radiator and goes to the engine parts.

When this sensor gets defective, it gives wrong signals to the engine, resulting in malfunction of the control module that turns on the fans and maintains their proper speed. This way, this sensor can result in overheating of the engine.

Solution

If the sensor is the real culprit, you may need to change it to fix the problem. However, you need to make sure that you clean the sensor and the surrounding area before giving it a couple of tries.

Why Does My Car Overheat When the AC Is On?

Car Overheats When AC Is on or Idle

1. Radiator problems

If your car is overheating when you turn the air conditioner on, then look into the radiator.

The radiator is what keeps the car cool. This is done by passing radiator fluid along the fins to the engine.

If the radiator is faulty, the car will overheat when the a/c has been turned on. Servicing the radiator can help, or you might need a replacement altogether.

Solution:

Sometimes leaves and dirt can become lodged in the radiator fans. Remove these as they will stop the radiator from getting maximum airflow.  Get yourself a hose and a basic brush and clean the radiator fins.

Be gentle as they can be easily deformed. Once you have done that, you can use a hose to clean out the radiator. This should remove any trapped particles causing the fan to be faulty and cause overheating.

2. Faulty fan

The fan is another problem to look into when the car overheats, particularly when the a/c is on. If the flow of air from the fan is blocked, then you are going to get overheating problems.

If you continue to drive like this, the fan switch can break, which will cause overheating problems too.

Solution:

A faulty fan is fairly simple to replace with the right tools. First make sure the car is off and cool. Now disconnect all the connections and then remove the fan motor from its spot.

If you have trouble pulling it out, you might need to remove the brackets. Then, grab the new fan, pop it into the spot you removed the old one, and reconnect the hardware, connectors, and brackets.

If you are just installing the radiator fan blades, put the blade onto the shaft of the fan motor. If you need a hand getting it into place, you can use a mallet to tap it in gently.

3. Wrong coolant

Coolant needs to be connected with the right type of vehicle.

If you are putting the wrong coolant in for the car, it won’t do the job properly, leading to overheating. In this case, the car will gradually overheat, but once the a/c is on, it will overheat fast and cause damage to the engine.

Your car manual will tell you the right coolant to use. If this is the problem draining the coolant out and putting the correct one in for your vehicle should fix the overheating problem.

Solution:

If you have poured the wrong coolant into the system, drain it out.  Wait until your car is cool, then take off the radiator cap. Slowly turn it and let the air escape. This will sound like a hiss.

Once that sound stops, you can open it up.

Place a container under the draining valve to catch the coolant. The drain port location is different on all cars, so refer to your booklet to find it. Let the coolant drain out completely.

If you have a car that does not have a drain valve take off the lower radiator hose.

The coolant will drain out from there. Once it has completely drained, you can refill it with the correct fluid.

4. Coolant leak

If your car is overheating mainly when the a/c is on, you might have a coolant leak. If the leak is only small, you might not notice any symptoms.

Once you turn the a/c on, the engine has to work harder, thus needing more coolant.

If there is a pressure problem or a leak, it will result in overheating.

Solution:

To stop a radiator leak, you can buy a k-seal. This is a liquid that will stop the leak in your radiator or the casing. Ensure your car is turned off and pop the hood.

Give the bottle of k-seal a good shake and pour it into the radiator or the coolant reservoir. Shut the hood and let the engine run until it gets to the operating temperature.

The liquid will move around and fill in any holes or cracks permanently.

5. Trapped air

It is quite common for air to become trapped within the cooling parts of the car.

Sometimes they will fix themselves other times, they can become stuck and cause the car’s temperature to rise.

Solution:

Getting the air out of the system will bring the engine temp back down to normal. So shut off your car and let it cool completely if you have driven it.

Add coolant to the reservoir, so the level comes back up to the radiator neck bottom. Next, grasp the radiator hose to push the air out.

This should solve your problem. It is a good idea to put the coolant in, in intervals squeezing the hose to expel air after every amount is poured in.

What About the Car Overheating When the AC Is Idle?

Faulty internal fan or defective fan relay

If your car is overheating when the A/C is on and idling, then you might have a problem with the internal fan or its controls and connectors.

If the connectors are not secure properly, then the internal cooling parts don’t get activated.

Also, some vehicles will have two fans, and each is needed to cool the engine down equally. If one is weak or faulty, the engine will overheat.

Solution:

If there are wiring problems with the fan, the car will overheat. Pop your hood and inspect the wiring around the radiator fan.

If you see wiring that looks worn or melted, it may need to be replaced. Next, unplug all the wires and check the flowing electrical currents using a voltmeter.

If you have both the wires positive and negative conveying 12 volts, then they are working correctly. If you are getting a different volt reading, it indicates a problem with the wiring.

Overall bad cooling system

Once you have checked the above, go ahead and check the rest of the cooling setup.

The cooling system cools down the condenser, which heats up once you have turned the a/c on. The condenser converts the gas into a liquid, but this won’t get achieved if there is a problem.

A faulty water pump will also do the same and overheat your car once you turn it on. A bad pump creates poor heat dissipation, which causes the engine to overheat when the a/c is on and idling.

Solution:

To test this, first turn the car off. Now open the hood and, with gloves on, find the water pump pulley. Get a good grip and wiggle it around.

You should not be getting any movement. If there is, get a new water pump installed.

If your car has a timing cover over the water pump pulley, you cannot go DIY since it will require a mechanic to fix the problem.

Tips to Prevent Your Car From Overheating

  • Tint the windows to keep the sun out
  • Use dark window shades
  • Open the windows a touch
  • Park in the shade when you can
  • Ensure all the air vents are open
  • Use fresh air as much as you can instead of the a/c
  • Turn on your heat which will help to cool the engine
  • Always have your coolant topped up
  • Always get your aircon serviced before the warmer weather hits
  • Always monitor the temperature gauge

FAQs

1. Why isn’t my A/C cold anymore?

Ans. If the air blowing from your aircon is not as cold as normal, it often is not a problem with the car but the air conditioner itself. Get a service done to clean the a/c out and top up the gas. You may also have a leak in the a/c system somewhere. You will need a professional to fix this.

2. How to tell if I have a blown head gasket?

Ans. The common signs for a blown head gasket are the engine overheating, white smoke from the exhaust, bubbling in the coolant reservoir, and unexplained coolant loss where no leaks exist. This is a big job and requires professional assistance for replacement.

3. Why does the temperature gauge rise when idling?

Ans. If you notice that the temperature level is rising when you are idling, this is why. When the car is moving, it is getting air inside, which is going through the fins and radiator. This takes away the heat from the coolant inside the car’s radiator.

About the author

Nick Steiner

Nick has been a car nut for his whole life as far as he can remember. His father was a car dealer who used to change and repair his cars himself. As a result, Nick had the opportunity to get around all sorts of cars and learned to get his hands dirty repairing vehicles from an early age.

Nick is a great fan of Japanese quality and German preciosity. His deep passion lies in older models that he believes have a flair that takes him back to his childhood. He also loves their durability and reliability when compared to the modern models.

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