Car Cooling System Explained: How to Stop Overheating

Combustion turns most of the fuel a car burns into heat rather than motion, and the cooling system exists to get rid of the excess without letting the engine run too cold. Get it wrong in either direction and you pay for it: too hot warps cylinder heads, too cold ruins fuel economy and fills the oil with condensation.

Almost every serious cooling system failure gives warning first. Here is how the system works, what the warnings look like and what you can do about them.

What does a car cooling system do?

It keeps the engine within a narrow temperature band, usually somewhere around 90 degrees Celsius, by moving heat from the metal of the engine into the air passing through the radiator.

That job has two halves that people often forget about. Shedding heat is the obvious one. The less obvious one is getting the engine up to temperature quickly and holding it there, because a cold engine burns more fuel, produces more emissions and wears faster. That is why a thermostat exists at all.

What are the parts of a car cooling system?

Six main components do the work, and each one has a distinct failure mode worth recognising.

Coolant itself is the working fluid: a mixture of water and antifreeze concentrate, usually around half and half. It does three jobs at once. It carries heat, it stops the system freezing in winter, and its additive package stops the aluminium, iron and rubber inside the engine from corroding and perishing. Those additives are why coolant has a service life and cannot simply be topped up forever.

There is also a heater matrix, effectively a small radiator behind the dashboard. Your cabin heater is a branch of the engine cooling circuit, which is why a heater blowing cold is often the very first clue that something is wrong.

What are the signs of an overheating engine?

The temperature gauge climbing past its normal position is the obvious sign, but several others usually arrive first.

• Heater blowing cold when the engine is clearly warm, which points to low coolant or trapped air
• A sweet smell in the cabin or around the front of the car, which is coolant escaping and evaporating
• Steam from under the bonnet, which is coolant boiling. Not to be confused with normal exhaust condensation on a cold morning
• The temperature warning light on the dashboard, which on many modern cars replaces the gauge entirely
• Coolant level dropping between checks with no puddle on the drive, suggesting a pressurised leak or a head gasket problem
• A puddle of brightly coloured fluid under the front of the car
• Loss of power, because many engines deliberately cut performance to protect themselves when hot
• Bubbling or gurgling from the expansion tank after switching off

What should you do if your engine overheats while driving?

Pull over safely and switch the engine off. Everything else is secondary, because continued running is what turns a cheap repair into a head gasket or a scrapped engine.

1. Turn the cabin heater to full hot with the fan on maximum. It feels unpleasant but it pulls real heat out of the engine while you find somewhere to stop.
2. Pull over as soon as it is safe and switch off the engine.
3. Open the bonnet if you can do so without reaching over anything steaming, and stand back.
4. Wait at least thirty minutes. The system is pressurised and hot coolant will spray out if you open the cap.
5. Once cold, check the expansion tank level and look for the obvious: a split hose, a soaked engine bay, a puddle underneath.
6. If the level is low and you have coolant with you, top it up and drive gently to a garage, watching the gauge the whole way.
7. If the level is fine, or if it drops again quickly, do not carry on. Call for recovery.

Never pour cold water into a hot engine. The thermal shock can crack a cylinder head, and you will have converted an inconvenience into an engine rebuild.

Which coolant should you use?

Use the specification listed in your handbook, not the one that matches the colour already in the car. Coolant colour is a dye choice made by the manufacturer and tells you almost nothing reliable about the chemistry.

Broadly there are older inorganic (IAT) coolants, organic acid (OAT) types, and hybrids of the two. Mixing incompatible types can cause the additives to drop out of suspension and form a gel that blocks the radiator and heater matrix. If you genuinely do not know what is in the car, the safe answer is a full drain and refill with the correct specification rather than a top up.

Buy concentrate and mix it with deionised water, or buy premixed if you would rather not measure. Tap water in a cooling system deposits scale, particularly in hard water areas.

How do you maintain a car cooling system?

Four habits cover almost everything: check the level, check the strength, change the fluid on schedule, and look at the rubber.

Check the level monthly. Engine cold, level between the minimum and maximum marks on the expansion tank. Do it more often on an older car.

Check antifreeze strength before winter. Level and strength are different things. A system topped up repeatedly with water can be full and still freeze at minus 5 degrees. Any garage can test the concentration with a refractometer in seconds, and it costs almost nothing.

Change the coolant on schedule. Corrosion inhibitors deplete over time, and once they are gone the coolant starts eating the system from the inside. Follow the interval in the service book. A full flush and refill also clears the scale and debris that gradually reduce heat transfer, and it is a job worth giving to a garage because old coolant is toxic and must be disposed of properly rather than poured down a drain.

Inspect belts and hoses. With the engine cold, squeeze each main hose. It should feel firm and springy. Soft, sticky, spongy or rock hard rubber is perished and should be replaced before it splits. Check the clips at each end are tight and check the auxiliary belt that drives the water pump for cracking, glazing or fraying.

Keep the radiator clear. The front of the radiator collects leaves, insects and road debris that block airflow without being obvious. Rinse it gently from the engine side outwards with a low pressure hose. A jet wash will flatten the fins and make the problem permanently worse.

What causes a car cooling system to fail?

Most failures trace back to one of five root causes, and four of them are preventable.

That last one catches people out. The radiator, air conditioning condenser and cooling fans all live directly behind the front bumper, which is the part of the car that meets bollards, kerbs and shopping trolleys. A knock that looks purely cosmetic can crack a plastic radiator end tank or crush a section of fins, and the symptoms may not appear until the first hot day in traffic. If your car has taken a hit at the front, it is worth having the cooling pack checked at the same time as the bumper repair.

When should you call a professional?

Anything involving pressure testing, head gasket diagnosis, or a leak you cannot see is garage work. So is a full flush, because of the disposal requirements.

Get help promptly if you notice any of the following:

• Coolant level dropping with no visible external leak, which can mean a head gasket
• White smoke from the exhaust that smells sweet rather than the usual cold morning steam
• Mayonnaise coloured residue on the oil filler cap or dipstick
• The temperature gauge swinging up and down rather than sitting steady
• A whining noise from the front of the engine that changes with revs, which suggests a water pump bearing
• Repeated overheating after a top up

Simple checks are yours to do, and doing them monthly prevents most of the expensive outcomes. The judgement call, if the gauge goes into the red, is not about mechanical knowledge. It is about whether you stop straight away or try to reach your destination. Stopping is always the cheaper decision.

If a knock at the front of the car has damaged panels as well as the cooling pack, we can handle the bodywork side from our Radcliffe workshop or on a mobile basis across Greater Manchester, Lancashire, Cheshire, Merseyside and West Yorkshire. Details are on our vehicle body repairs page.