Engine Troubleshoot

What Causes A Car To Overheat When Idling?

car overheating at idle

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IF you find that your car is regularly overheating when you are stuck in traffic or after driving home on a hot day, then there may be something wrong with the cooling system in your car.

A car overheating at idle can be caused by a leak in the coolant system, a low coolant level, a faulty radiator fan, a leak in the radiator body or its cap, a loose fan belt, a broken fan belt or a water pump that has failed.

It's usually easy to figure out what the problem is by inspecting the various parts of the engine cooling system. In this article I'll explain how the cooling system works and how to figure out what is causing your car to overheat when idling.

How Does the Cooling system in your car work?

A properly functioning engine cooling system is vital when it comes to keeping your car running. Its job is to keep the engine temperature from getting too high and potentially causing irreversible damage to the engine block.

So how does it achieve this?

car engine cooling system

The cooling system in your car keeps the engine cool by pumping coolant (water and antifreeze) around the engine using a water pump. As the coolant passes around the hot engine it soaks up the heat and cools the engine. The coolant is then pumped through the engine radiator at the front of the engine, where it cools, before it heads back into the engine block again.

The radiator plays a vital role in the cooling system. It is usually located behind the front bumper of the car, where cool air from the outside can pass through the grille and through the fins of the radiator, cooling the coolant inside.

If the outside air temperature is too high, or if you are driving too slow, then the air coming through the front grille will not be enough to cool the coolant. 

To help keep the radiator cool, there is a fan located behind the radiator that help to blow cool air onto the radiator surface. Nearly all radiator fans are electric fans that are controlled by the engine ECU. If the coolant temperature sensor detects that the engine is getting too hot, then the fan will be turned on to help lower the temperature.

What Can Cause A Car To Overheat When Idling?

So why does your car overheat when it is idling but not when you are driving? 

The reason your car can overheat when idling, is because when you are driving, cool air is being sucked into the front of the engine bay and through the radiator, thus cooling the coolant in the radiator fins and also cooling the front of the engine.

If the vehicle is not moving, then it takes a lot longer for the heat to dissipate from the radiator, especially on a hot day. This will usually result in too much heat hanging around the engine, causing it to overheat.

If there is a problem with the engine cooling system that is stopping it from working properly, the problem may only become apparent when the car is not moving.

Most of the time the problem will be caused by a broken radiator fan, a broken water pump or insufficient amount of coolant in the system.

1. Broken Radiator Fan

if the radiator fan in your car does not come on when the car is sitting in traffic or on a hot day, then the engine is likely to overheat.

When inspecting the radiator fan, be careful not to put your hand into the blades of the fan, and keep any loose clothing or hair away from the fan too. Electric fans can come on at any time even after the engine has been turned off.

I usually start by running a diagnostic check to see if there are any cooling system related errors stored in the ECU. Common cooling system errors include:

​- P2181 (problem with the coolant sensor)

- P0480, P0481, P0482, P0484, P0485 (cooling fan circuit relay fault)

-To manually check the fan: use a stick or pole, to try to turn the fan blades. The blades should spin easily, if not they may be jammed. 

-Check the fan wiring connector. Ensure that it is fitted tightly and that none of the wires are broken or frayed. Sometimes removing the connector plug and cleaning the contacts can help, especially if the car is older.

-Some older cars have radiator caps. Check that the radiator cap is on tightly and that there are no signs of leaking coolant from round the cap seal.

- Try turning on the air conditioning in the car with the engine running. After a short period the fan should start to run. This is not a conclusive way of finding a fault with a radiator fan as it does not always come on with the AC. 

2. Coolant level is too low

There are lots of reasons why the coolant level can be too low. Sometimes it's due to neglect, as coolant will naturally need to be topped up over time.

Often it caused by a leak in the system. You can check for a leak by checking the various rubber hoses that are located in the cooling system. Over time, the rubber hoses will harden and can split. When the engine is cold, run your hand along each of the hoses and squeeze them gently to identify cracks.

Leaks at joints in the cooling system are also common. They are usually easy enough to spot as the coolant will dry and turn white if it leaks. It's also worth checking the coolant expansion bottle, especially the underneath if you can access it. These can develop hairline cracks that will cause a slow leak of coolant.

You may also have a leaking heater core. I go into more details here about how to identify and fix this problem.

If there are no obvious leaks in the system, then it's best to top up the coolant level (using the correct water and antifreeze mixture) and let the engine run or go for a short drive. Some coolant leaks take weeks of driving before the coolant level will drop below the minimum level again.

3. Check the water pump

The water pump is not as easy to check as it usually hidden behind lots of other parts and is not accessible. 

Depending on your vehicle, you may need to remove plastic covers, the air filter box, various hoses or pipes or even rubber belts to get to the water pump.

-Once you do locate the pump, check around the water pump gasket for coolant leaks. You may be able to see white coolant deposits around the edge of the pump or on nearby engine components. 

-Turn on the engine and listen for a grinding noise or worn bearing noise. Sometimes the inner blades of the water pump can wear down or even break and it will become noisy when it is running.

-Often the best way to check for a worn water pump is to remove it from the car and inspect it that way.

What should you do if your car overheats when you are stopped?

If you suspect that the engine in your car is overheating then pull over, turn off the engine and open the hood (bonnet) as soon as you can. The most important thing you can do is try to cool the engine as quickly as possible.

Once the engine has cooled down a bit there are a few things that you can check to help you figure out what has gone wrong.

Be careful what you touch, as it can take an engine a longtime to cool down, and they can get very hot!

1. Start by checking the coolant level. Every car has a coolant expansion tank that allows you to add coolant to the system. Some cars have an opaque tank that allows you to see the coolant level in the tank. For those that don't, you'll need to screw off the lid to check the coolant level. If you are unsure, check your vehicle handbook to see how to check the coolant level and be careful if you are removing the expansion bottle lid as it can be pressurized.

Check that the coolant level is above the minimum level and below the maximum.

If the coolant level is below the minimum level, then you can add a small amount of water to the expansion bottle to bring the level up below the minimum. Once you have done this, then restart the engine and allow it to idle for a few minutes. If there is a coolant leak in the system, adding water will buy you enough time to drive a few miles home or to a local garage. 

2. Check the engine for coolant leaks. If the coolant level is low then check the engine bay for leaking coolant. Many engines have an engine underbody cover, so it should be easy to see if coolant is dripping from the engine. Common places that coolant will leak from are the coolant expansion bottle( sometimes they crack), rubber radiator hoses (they can also crack) and the bottom of the radiator (not easy to check but coolant will pool on the subframe below it if it is leaking).

Sometimes you may get lucky, and the leak can be coming from a loose radiator hose. If you tighten the clamp on the joint, this may stop the leak.

3. Check for blown fuses. Nearly all radiator fans are fully electric these days and they'll have their own fuse in the fuse box. It's worth checking that the fuse hasn't blown, although if it has this may suggest a short the motor of the electric fan and it will probably blow the fuse again.

4. Check the fan belt (if present). If you are driving an older car, it may have a rubber drive belt that operates the radiator fan. Check that the belt is tight and is not torn or slipping. You may be able to tighten the belt by adjusting the tensioner, or reinstall it if it has come off.

If you can't locate the source of the problem then it's not advisable to drive the car as it will likely overheat again. Get it towed to your nearest mechanics and let them investigate it further. 

How much does it cost to repair a car that overheats when idling?

This all depends on what exactly is broken and if you are able to source and replace the parts yourself, or if you are paying a mechanic to fix it for you.

If you are going to get a mechanic to carry out the work for you, then you can expect to pay if anywhere from $500 to $1000. This would include diagnosing the problem and replacing one or more parts such as the water pump, electric fan or hoses. It would also include flushing and refilling the system with fresh coolant.

If you are able to diagnose the problem yourself, then there are significant savings to be made. The coolant system is fairly easy to work on and there are lots of tutorials online that can guide you if you get stuck.

Here are some examples of prices for the various cooling system parts:

1. Water pump average price of $65 to $150 (example AC Delco Original GM water pump)

2. Cooling system hoses $20 - $80 (example Continental cooling hose)

3. Electric Radiator Fan $40-$200

Of course you can also source plenty of parts second hand from breaker yards or eBay. And there are lots of electric fan repair kits available that will allow you to disassemble and rebuild a broken fan for a fraction of the cost of a new one.

Summary

A engine can overheat when idling if there is a problem with one or more of the engine cooling system components. Sometimes the problem only comes to light when you are stopped because there is no air coming in through the front grille to help cool the radiator.

A car that overheats when idling may have a faulty radiator fan, a broken water pump or water pump belt or may have a coolant leak.

It's best to pull over if you think your car is overheating and open the hood. Once the engine has cooled down, then check the coolant level and top up if necessary. If the coolant level is ok then it is probably a good idea to get your vehicle towed home or to a garage to help prevent any engine damage.

About the author

Matt Taylor

Matt Taylor

Hey, I'm Matt, founder of themotorguy.com

I've been a car fanatic all of my life. As a youngster, I loved nothing more than taking things apart and rebuilding them. I would also spend my weekends helping my dad with all sorts of car maintenance jobs.
For the past decade I've been a keen amateur mechanic with a love of classic BMW's. Over the years I've learned lots about car maintenance, car detailing and troubleshooting common engine problems.
It's my goal to share my knowledge with fellow amateur car fanatics around the world, and to help people to diagnose and fix their own cars.
If you have any questions please contact me, I'd love to hear your feedback and suggestions!