The idea
Beginner explanation — switch to Technical for the engineering detail.
A combustion engine turns only part of its fuel into movement. A large share leaves as hot exhaust, and a large share soaks into the metal around the cylinders. Without cooling, the metal would quickly get hot enough to distort the cylinder head, break down the oil and damage pistons.
Liquid coolant — water mixed with antifreeze — flows through passages around the cylinders called the water jacket. A pump keeps it moving. The hot coolant then passes through the radiator at the front of the car, where air flowing through thin fins carries the heat away. A fan helps when the car is slow or stationary.
Engines also must not run too cold: they wear faster and waste fuel. A thermostat acts like a gate. It stays shut while the engine warms up, keeping coolant inside the engine, then opens to send coolant to the radiator once the engine reaches its working temperature. The same hot coolant also feeds the cabin heater.
What you will learn
- Explain where engine heat comes from and why it must be controlled
- Trace the coolant circuit through pump, jacket, thermostat and radiator
- Explain how a pressure cap raises the boiling point of coolant
- Recognise the main causes and warning signs of overheating
How it flows
Read the diagram as text
- Water pump → Water jacket
- Water jacket → Thermostat
- Thermostat → Radiator & fan (when warm)
- Radiator & fan → Water pump
- Water jacket → Heater core (always)
- Heater core → Water pump
- Water jacket: Heat picked up
- Thermostat: Opens ≈ 85–95 °C
- Radiator & fan: Heat to air
- Water pump: Circulates coolant
- Heater core: Cabin warmth
Step by step
Read each step in order.
Step 1: Combustion heats the metal
Burning fuel heats the cylinder walls and head far beyond what metal and oil can tolerate for long.
Step 2: The pump moves coolant
A pump, driven by a belt or an electric motor, pushes coolant through the passages around the cylinders.
Step 3: The thermostat decides the route
Cold engine: the thermostat stays shut and coolant skips the radiator. Warm engine: it opens and coolant flows to the radiator.
Step 4: The radiator sheds heat
Thin metal fins pass the heat into air flowing through the radiator; a fan pulls air through when the car is slow.
Step 5: Pressure keeps it liquid
The cap keeps the system under pressure so the coolant does not boil, and an expansion tank takes up the extra volume as it heats.
Go deeper
Optional. Open a panel for the engineering detail — or switch to Technical above.
The engineering explanation3
At typical operating points, roughly a quarter to a third of the fuel's chemical energy is rejected to the coolant, comparable to what reaches the crankshaft. The heat flow carried by the coolant is Q̇ = ṁ × c_p × ΔT; with a pump circulating about 1–2 kg/s, a temperature drop of only 5–10 °C across the radiator removes tens of kilowatts.
A wax-pellet thermostat begins to open at a set temperature (commonly around 85–95 °C; some electronically mapped thermostats run hotter at light load for efficiency and cooler at high load to protect against knock). While it is closed, a bypass keeps coolant circulating through the block so hot spots do not form. The radiator is a cross-flow or down-flow heat exchanger with aluminium tubes and louvred fins; the fan is switched or speed-controlled by the ECU.
The system is sealed and pressurised: a radiator or expansion-tank cap holds roughly 1–1.5 bar above atmospheric. Raising pressure raises the boiling point, so a 50/50 mix of water and ethylene glycol that would boil a little above 100 °C at atmospheric pressure stays liquid to around 120 °C or more. Glycol also lowers the freezing point (to about −35 °C at 50/50) and carries corrosion inhibitors that protect aluminium, iron and seals; those inhibitors deplete, which is why coolant has a service life. Nucleate boiling at hot spots aids heat transfer locally, but bulk boiling creates steam pockets that insulate metal, which is why a low coolant level can cause rapid overheating.
Key concepts4
- Heat carried by coolant
- Heat flow equals mass flow times specific heat times the temperature change.
Q̇ = ṁ × c_p × ΔT- Thermostat
- A temperature-operated valve that holds the engine at its design temperature by controlling flow to the radiator.
- Pressure cap
- A spring-loaded valve that keeps the sealed system above atmospheric pressure, raising the coolant's boiling point.
- Antifreeze (glycol)
- Lowers the freezing point, raises the boiling point and carries corrosion inhibitors that protect the engine's metals.
Common misconceptions3
MythRemoving the thermostat makes an engine run cooler and therefore better.
RealityThe engine may never reach its design temperature, increasing wear, fuel use and emissions — and in some designs uneven flow can even cause local overheating.
MythPlain water is a better coolant than a water-glycol mix.
RealityWater carries heat well, but it freezes, boils sooner and offers no corrosion protection. The correct mix protects the whole system.
MythIt is fine to open the cap to check the coolant on a hot engine.
RealityThe coolant is under pressure above its atmospheric boiling point; releasing the cap can make it flash to steam and scald. Check only when cold.
In the real world3
- The temperature gauge settles mid-scale within minutes of starting: that is the thermostat opening and holding temperature.
- In stationary traffic on a hot day you may hear the radiator fan run on — there is no ram air, so the fan provides the airflow.
- Turning the cabin heater to full hot with the fan on maximum can buy time if an engine starts to overheat, because the heater core is an extra radiator.
Common questions
Short answers to the questions people most often ask about this topic.
How does a car's cooling system work?
A water pump circulates coolant through passages around the cylinders and head, where it absorbs heat. The hot coolant flows to the radiator, where air passing through the fins carries the heat away, then returns to the engine. A thermostat controls the flow so the engine warms up quickly and then stays at its working temperature.
What does the thermostat do?
It keeps coolant away from the radiator while the engine is cold, so the engine reaches its operating temperature quickly, then opens progressively to hold that temperature steady once warm.
Why is the cooling system pressurised?
Pressure raises the coolant's boiling point well above 100 °C, so it can run hot without boiling. The radiator or expansion-tank cap holds that pressure, which is why it must never be opened while the engine is hot.
Check your understanding
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0 of 3 answered
Question 1 of 3
What does the thermostat do while the engine is still cold?
Almost there
- Overview — Read the introduction to the end.
- See it work — 5 steps left to read
- Go deeper (optional) — Optional: open a deeper panel or switch to Technical.
- Check — Answer the quiz questions.
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Earlier in this route you skipped The Four-Stroke Cycle, Pistons, Connecting Rods and the Crankshaft, Valves, Camshafts and Valve Timing, Spark Ignition and Combustion Timing, Fuel Injection, Engine Layouts: Inline, V and Flat, Diesel Engines.
Sources & attribution
Further reading
- How an engine cooling system worksHow a Car Works · consulted 2026-10-21(external site, opens in a new tab)
Further reading on an external site. Linked only; AutoVerse lesson text is written independently.
- Checking hoses and the radiator capHow a Car Works · consulted 2026-10-21(external site, opens in a new tab)
Further reading on an external site. Linked only; AutoVerse lesson text is written independently.
- Licence
- AutoVerse original content
This lesson is original AutoVerse writing. The links above are suggestions for further reading, not the source of this text.