Car Overheats When Idle And AC Is On? Here’s Why And How To Fix

Does your car run hot when idling with AC on? Here's the ultimate guide that explains why your car overheats and how to fix it.

Idling – otherwise known as tick-over – is the minimum speed of the engine. When your foot’s off the gas pedal, and the transmission is in neutral, the car is “idling.”

Air conditioning (AC) is a complicated system driven by a pulley connected to the engine’s crankshaft. A refrigerant circulates throughout a series of pipes, constantly changing states. This removes heat and moisture from air being sucked into the cabin.

Note: When you say “overheating,” a mechanic will assume that the engine is getting too hot. However, some use this term to mean that the AC vents blow out warm air.

So, your car overheats when idling, and the AC is on. This is an unusual issue, but if it’s happening, why?

The most common cause of an overheating engine when idling with the AC on is a broken (in some way) compressor unit.

If your AC is blowing hot air when idle, the compressor or pulley is likely to be failing but not yet completely broken. There are numerous other fault possibilities within the AC system.

Let’s get started with the basics!

Table of ContentsShow

How Does The AC Work In A Car?

how does car air conditioning work

The AC isn’t quite as simple as a “cold air blower.” It’s a pressurized network of tubes filled with a refrigerant specifically created to power these types of systems. You may have heard of them before: R-22, R-12, R-410A, and R-32 have all been used and phased out (or are being phased in).

Open your car’s hood. Do you see the belt attached to several pulleys? This is known as the drive belt or serpentine belt. It’s powered by the engine’s camshaft, and one of these pulleys drives the AC.

As a brief summary, the pulley creates the “compression” force that the compressor uses to force the refrigerant through the system. The refrigerant absorbs heat from the incoming air by switching between high-pressure and low-pressure sections and liquid and gas states.

It also passes through a condenser and an evaporator. These act a little like radiators. The evaporator removes heat and moisture from the air, while the condenser takes that heat and releases it back into the atmosphere.

All this means that the air coming through the vents and blasting onto your face on a hot day is crisp, cool, and fresh.

To keep your car’s AC running correctly, it’s essential to run it as much as possible. This lubricates the rubber seals, preventing most leaks. You’ll also need new refrigerant (a “re-gas”) every few years.

Car Runs Hot When Idling With AC On – Is It The AC?

Girl driver has problem with non working conditioner hand checking flow cold air suffering from heat

Keep an eye on your dashboard engine temperature gauge. If it shows higher-than-usual heat levels, perhaps reaching into the red zone, your car’s overheating.

Check that the AC is directly influencing this by switching it off. This disconnects the pulley from the compressor. Even though the engine’s still running, the AC system is inactive.

If the engine temperature stops rising and resets to normal, the problem is caused by the AC. The opposite would also be true.

If the AC system is causing the car to overheat, it might not be putting out any cold air. Well, not as much as usual, at least.

Thankfully, there are very few likely possible causes. The problem is almost certainly a damaged compressor, compressor pulley, or compressor clutch. If these aren’t working or they’re seizing up, the engine must put out much more power just to turn it.

This may cause it to overheat, although it’s likely to be coupled with more problems, such as low oil or coolant levels.

How To Fix An Overheating, Idling Car With The AC On

Car refilling air condition

You’ll need to take your car to an air conditioning licensed mechanic. Unfortunately, the AC is far too specialist to work on yourself at home.

You would need a wealth of professional equipment, UV lights, mechanical knowledge, and a professional certification allowing you to work with automotive refrigerants.

Refrigerants are incredibly damaging to the atmosphere and contribute significantly to climate change if accidentally released. It’s actually illegal.

The overall message is: Take your car to a professional.

They’ll likely need to swap out the entire compressor unit, including the clutch and pulley. This could come to between $500 and $1,000.

Breaking the cost down, expect to pay $200 to $400 for the part – more for rarer models. Installation labor charges will then be between $100 and $200. Finally, the AC will need re-gassing and a leak test (another $100 to $150).

The serpentine belt should also be inspected for wear. It might have been damaged by the friction from the seized or seizing pulley. It may need replacing, so expect to add up to $50 to $75. There shouldn’t be any extra labor charges since the old belt will already be off.

Car Blows Hot Air When AC Is On And Idling

Driver hand tuning air ventilation grille

When the AC is blowing hot air, it means the system isn’t working correctly. Obviously.

First, when you increase the revs, does the vent start blowing cold air? Take them up to 3,000 or 4,000, either while driving or in neutral, to feel whether the temperature lowers.

If so, the problem is probably a failing compressor (or pulley). As the engine speed increases, it forces much more power through the system, increasing its effectiveness.

If there’s no temperature change, there’s a deeper problem. This could be a broken compressor, refrigerant leak, or a failing component (such as the evaporator). 50% of the time, a simple re-gas will fix the issue for you.

How To Fix AC Blowing Hot Air At Idle

Servicing car air conditioner. Service station.

Again, this is something that’ll need to be done by a professional, air conditioning licensed mechanic. You shouldn’t work on it at home.

The cheapest possible solution could be an AC re-gas, coming in at around $100 (depending on your car’s age, refrigerant type, and where you live).

Refrigerant leaks can be found using a UV light. Rubber seals are often the culprit here, although components – including the compressor – can get damaged, causing the refrigerant to escape. Diagnosis work will be quoted on an individual basis.

A new AC compressor – likely to be necessary if you only feel hot air at idle – will most likely come to between $500 and $1,000, as discussed in the previous section.

One of these should solve the problem for you.

Car Overheats When Idle And AC Is On – Conclusion

Hand adjust way air vent cooling air flow in car dashboard

If your car’s engine is overheating, the AC isn’t likely to be the root cause. There are many more possibilities – most notably, coolant and oil levels, fan failure, or head gasket failure.

Taking your car to a mechanic is the best and surest way of solving the problem. If it’s determined that the AC is resisting the turning force of the crankshaft, it could be contributing to an overheating engine. However, as mentioned, there are most likely other factors in play.

If it’s blowing hot air through the vents into the cabin, then yes – the AC is the most likely culprit. Take it to a specialist for diagnosis and fixing.

Your car should be perfectly safe to drive. The air conditioning isn’t an integral system – it’s a comfortable optional extra.

However, please do have a quick visual check of the serpentine belt. If it’s obviously badly frayed, it could snap on your journey, doing further damage under the hood.

The takeaway message? If you’re having any problems with your AC, don’t try any DIY! Take your car straight to a licensed, professional mechanic.

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Benjamin Kitchen

Ben is an IMI-qualified light vehicle technician from England with experience in a fast-fit garage. He aims to help drivers worldwide with common automotive problems. You’ll often find him working with his 1.2 Vauxhall Corsa. It may have a tiny engine, but in eight years it's never once let him down!