Why Is My Car Overheating?

Ellie Dyer-Brown, 9 months ago

5 min read

  • Diagnostic
Mechanics investigating an overheating car.

FixMyCar discusses what can cause a car to overheat.

Seeing smoke pouring out of your car bonnet is enough to fill even the most relaxed drivers with dread. When the temperature gauge on your dashboard pushes ever higher, it spells trouble for your engine

This guide discusses what to do - and, just as importantly, what to avoid doing - when your car overheats.

Contents:

Signs your car is overheating

What to do when your car overheats

What not to do when your car overheats

What causes a car to overheat

How to check your car's coolant level

How to prevent your car from overheating

What damage can overheating do to your engine?

Book a cooling system repair

FAQs


Signs your car is overheating

It's easy to tell when your car is overheating. There are three signs to look out for:

  • Temperature gauge in the red/spiking to the “H” symbol.

  • Steam coming from under your bonnet.

  • An unusual smell coming from the engine bay indicating a leak (a sweet odour suggests coolant, while a burnt smell indicates the presence of oil).

A car temperature gauge at 130 degrees C.

What to do when your car overheats

Follow these steps if you spot steam coming from your engine or the temperature warning light comes on.

Man looking at the engine of an overheating car by the roadside.
  • Pull over in a safe place, switch off the engine, and evacuate passengers from the vehicle.

  • Open the bonnet if you can, taking care not to burn your hands on any hot components.

  • Leave the engine to cool down for at least half an hour with the bonnet open.

  • Do not open the radiator or expansion tank cap of an overheated engine. It is pressurised and can cause severe burns.

  • If you open the cap when the engine has cooled down, do so slowly and ensure your hand is covered.

  • Check the coolant tank once the engine is cool. If the coolant is low, there may be a leak, so you should look for puddles around the car. If it is still full, the problem will likely be mechanical, so you need to contact a mechanic. 

  • Top up your coolant tank if needed using either coolant or a mix of warm water and coolant. Avoid using water that isn’t mixed with coolant, and never pour cold water into a hot radiator; doing so may damage the engine.

If you don’t feel confident assessing the problem, contact a mechanic who can diagnose and fix the problem.


What not to do when your car overheats

When your engine is too hot, you should never:

  • Keep driving

  • Panic

  • Immediately open the bonnet

  • Ignore the problem


What causes a car to overheat?

It’s usually the fault of the cooling system if your car is overheating, but there can be a range of other causes that are worth familiarising yourself with.

1. Leaking coolant

The cooling system circulates coolant from hot areas around the engine to the radiator, keeping it at the right temperature. A leak in the system is sure to make your car overheat once the coolant falls below the required level. 

Some components are more likely to leak than others, such as the gasket, hoses and radiator. Regular servicing and vehicle health checks can spot early signs of corrosion before it turns into a leak. 

While prevention is better than cure, it’s also possible to have leaking components repaired or replaced. The price varies depending on the severity and location of the leak; you can expect to pay anywhere between £100-500.

2. Low engine oil

Engine oil is essential for lubrication, which reduces heat-generating friction, and it also plays a similar role to coolant by transferring heat away from the engine. It’s important to make sure that you not only have enough engine oil but that you also have the correct type of oil, discussed here. 

Mechanic changing engine oil and fluids

When there isn’t enough engine oil in your car, components may rub together and become hot, causing your engine to overheat. 

It’s easy to top up your engine oil at home, as we explain in this guide. 

3. Faulty thermostat

There is a thermostat located near your car’s engine, which is responsible for regulating its temperature. When the thermostat stops working correctly and can’t detect high temperatures, it will withhold the coolant flow, making your engine overheat.

A thermostat replacement usually costs between £100-200

4. Defective radiator

The radiator constantly circulates coolant to and from the engine to absorb excess heat. The liquid is then cooled using fans in the radiator. Common causes of radiator problems include blocked passages, which stop the flow of coolant, bent or damaged radiator fins, and leaks.

5. Damaged water pump

For a car’s cooling system to work, it needs more than just a functional radiator - it also requires a water pump to push coolant around the engine. It is often the pump’s impeller that causes problems when it gets blocked or stops rotating. When this happens, it can’t be repaired and must be replaced.


How to check your car’s coolant level

Firstly, locate your car’s coolant tank. It should be in an easy-to-access place under the bonnet, but if you’re unsure, consult your vehicle handbook.

Inspect the coolant level, which should be between the maximum and minimum marks. After you’ve done this, see if you can spot any damp patches or leaking hoses

If the coolant level is too low, top it up when the engine is cool.

Engine coolant

Learn more about how to check your car's fluid levels.


How to prevent your car from overheating

You have no control over whether your cooling system suddenly springs a leak, but there are a few preventative measures you can take to stop your car from overheating. These are:

  • Regularly check your car’s coolant level

  • Check the temperature gauge while driving

  • Replace your coolant as per the manufacturer’s instructions

  • Follow a regular servicing schedule to keep your vehicle and its engine in the best possible shape

Your coolant will be topped up in your annual service. Find out what else is included in a service here.


What damage can overheating do to your engine?

When your car overheats, it can cause damage to all sorts of engine components, including the bearings, cylinder walls, top piston rings, head gasket, hoses and (of course) the radiator itself. 

Driving with an overheating engine will almost certainly result in an expensive repair job, so it’s sensible to stick to a regular car servicing schedule and keep up to date with maintenance tasks like checking and topping up your coolant. Doing so helps to prevent your engine from overheating.


Book a cooling system repair

If your car is overheating and you have enough coolant in the system, or you’ve spotted a leak, it’s time to book a repair.


Frequently asked questions

How long will it take my car to cool down after overheating?

You should pull over when your engine is overheating and wait at least 30 minutes before opening the bonnet. However, it’s best to leave it for an hour or two if you can to ensure it’s completely cool.

What is engine coolant?

Coolant is added to a car’s engine cooling system to help regulate the temperature. It has already been diluted with water so it can be poured straight into the system, whereas antifreeze has to be mixed with water before being added.

What does the temperature warning light mean?

The temperature warning light is a small thermometer on your dashboard. When illuminated, it signals your engine is overheating, or your coolant level is too low. 

Find out what other dashboard warning lights mean.

Why does my car overheat when idling?

The same problems could be at play if your car overheats when idling as if it overheats when driving - there is likely a fault with the cooling system, such as a leak or broken component, or you may have run out of coolant.


Looking for affordable car repairs or maintenance? FixMyCar can help you find the right garage at the right price.

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