How Does the Cooling System Work in a Car?
FixMyCar explains how a car's cooling system works.
Book a car repair nowEllie Dyer-Brown
Wednesday February 26 2025
5 min read
Car engines like to run hot - but not too hot. Just like human beings, they have an optimal operating temperature that ensures different parts and systems work efficiently.
The cooling system allows the engine to reach and maintain this temperature quickly. But how does it work? That’s what we’re going to explore in this guide.
Cooling system components
Radiator
The radiator is a heat exchanger that helps the engine reach the ideal temperature quickly and then maintain it. It’s usually made of aluminium.
Features:
Small pipes with fins attached
Drainage plug
Inlet port
Outlet port
Sealed cover
Thermostat
The thermostat acts as a valve for the coolant. It contains a wax pellet that melts and expands when the car reaches its ideal operating temperature, forcing the valve open, which allows coolant to flow around the system.
Features:
Chamber
Rod
Valve
Water pump
The water pump moves coolant between the radiator and engine to facilitate the heat transfer process.
Features:
Belt
Impeller
Pulley
Bearing
Body/house
Seal
Other parts
Freeze plugs: Steel plugs that seal openings in the cylinder heads and cylinder block.
Coolant expansion tank: A tank with an inlet to the radiator and an overflow hole.
Head gasket: Seals major engine parts to prevent oil and coolant from mixing.
Fan(s): The fan is electrically powered and only comes on when the temperature is excessively hot or the car is stationary.
How does the cooling system work?
The job of the cooling system - to keep the engine at the right temperature - sounds pretty simple on the face of it. But consider this: a typical car travelling at motorway speeds will produce thousands of explosions every minute. That’s a lot of heat.
Learn more about how a car works.
When the engine is cold
A cold engine is bad news because it makes it harder for fuel to vaporise and means the engine oil doesn’t lubricate parts well. The cooling system helps the engine warm up quickly - here’s how.
When you start the engine, the water pump pushes cold coolant from the bottom of the radiator to the engine block.
The coolant travels through channels to the cylinder head and back to the pump.
A thermostatically controlled valve sits near the pump. If the coolant returns too cold, the valve stays closed, preventing the fluid from reaching the radiator.
The coolant heats up quickly and carries heat around the engine, accelerating the warm-up process and boosting efficiency.
Find out what coolant you should use for your car.
When the engine is warm
Most engines have a maximum operating temperature of around 120C. Here’s what happens when the engine and coolant heat up.
When the coolant reaches around 90C, the wax in the thermostat melts, and the valve opens, allowing hot coolant to flow into the radiator.
The temperature change is monitored by a sensor that sends information to the engine control unit (ECU). The ECU uses this data to make minor adjustments to the ignition and fuel systems.
In the latest cars, the engine management system operates the thermostat, allowing for precise control of the coolant temperature.
How the coolant expansion tank works
When the coolant’s temperature rises, it expands, increasing the pressure in the cooling system. This raises the coolant’s boiling point.
However, there must be a way to relieve the pressure; otherwise, the system will eventually explode. That’s why the radiator has a cap or pressure valve. It opens when the pressure reaches around 15psi, allowing more coolant to flow into the expansion tank.
The system is sealed, so you should rarely need to top up your coolant. If the fluid level drops, there may be a leak - especially if this happens repeatedly after topping it up.
What does a car radiator do?
Now you know how an engine gets up to temperature and avoids overheating. The next thing to understand is how the radiator helps this process.
A bit like the radiators in your house, a car radiator contains a network of pipes running from the top tank to the bottom tank. Around these pipes is a honeycomb structure of dense aluminium layers.
Heat transfers from the coolant to the aluminium.
Air is drawn in from the front grille as the car moves.
The air passes over the aluminium layers, and these layers boost the cooling process by increasing the radiator’s surface area.
The coolant loses heat as it travels from the radiator’s upper tank to its lower tank before being pumped back into the engine.
Frequently asked questions
It takes about 30 minutes for the engine to cool to a reasonably safe temperature after use and several hours to cool completely.
A car typically takes five to 10 minutes to reach the optimal operating temperature. The time varies depending on the external temperature.
Coolant enters the radiator at the top and flows to the bottom.
Coolant circulates around the engine when the thermostat is closed but does not enter the radiator.
If you found this guide helpful, you might also like:
- Related topics:
- Engine And Cooling
- Engine Coolant
- How It Works
Written by Ellie
Ellie is FixMyCar's content manager. She has over three years of experience writing about cars and regularly collaborates with automotive experts to provide trustworthy advice for drivers that is easy understand. Her work has been featured in Yahoo! Finance, iNews, The Daily Express and The Sun. She has a BA in English literature and an MA in creative writing from Durham University. Outside of work, Ellie follows F1 and eagerly awaits Ferrari's next era of dominance in the sport. She drives a Suzuki Swift.