How a car cooling system works

Confirm the specification in your owner’s manual before buying. Product examples are not a fitment guarantee.

How your car's cooling system works

An engine turns only part of its fuel energy into motion. Much of the rest becomes heat, which the cooling system must carry away. The water pump circulates coolant through passages in the engine block and cylinder head, where it absorbs heat. A thermostat stays closed while the engine warms up, then opens so hot coolant flows to the radiator. There, air passing through the fins (helped by the cooling fan at low speed) carries the heat away.

The system is pressurized by the radiator or expansion-tank cap, which raises the coolant's boiling point. As coolant heats and expands, the excess moves into the reservoir, and it returns as the engine cools. Part of the flow also goes through the heater core behind the dashboard, which is how your cabin heater works. Sensors report the coolant temperature to the engine computer and the dashboard gauge.

Each part depends on the others: a weak cap lowers the boiling point, a leak lets in air, a sticking thermostat or a failing pump stops circulation, and depleted coolant lets corrosion build up. That's why a symptom in one place often has its cause somewhere else.

The parts and what goes wrong

Coolant flow, step by step

1. Cold start: the thermostat is closed, so coolant circulates only through the engine (and heater core) to warm up quickly.
2. Warm-up: the coolant temperature rises, and the engine computer adjusts fuel and ignition.
3. Operating temperature: the thermostat opens and hot coolant flows to the radiator.
4. In the radiator: air passing through the fins cools the coolant. The fan helps when the car is slow or stopped.
5. Return: cooled coolant goes back to the pump and the cycle repeats.
6. Expansion: as coolant heats, excess volume moves to the reservoir, and it returns as the engine cools.

Coolant standards: ASTM D3306 and D6210

ASTM D3306 is the standard specification for glycol-based engine coolant for automobile and light-duty service. According to ASTM's summary, it covers ethylene glycol, propylene glycol and ethylene glycol/glycerin coolants, as concentrates and prediluted products. Concentrates are generally intended for use at 40–60% by volume in water (70% is permitted by agreement), and prediluted products contain at least 50% concentrate. Its requirements include freezing and boiling point, density, ash, pH, chloride, corrosion, foaming and cavitation-erosion performance.

A coolant that meets D3306 meets a general performance baseline. It doesn't automatically meet your carmaker's own specification, which may add requirements. Use D3306 as a minimum, and the OEM spec in your manual as the real target.

Heavy-duty engines use ASTM D6210, which adds cavitation and liner-pitting protection requirements. See heavy-duty coolant.

Five-minute cooling system check-up

1. With the engine cold, check that the reservoir level is between MIN and MAX.
2. Look at the coolant's color and clarity through the tank. It should be clear and bright, not murky, rusty or oily.
3. Squeeze the upper and lower radiator hoses. They should feel firm, not mushy, cracked or swollen.
4. Look under the front of the car for drips or a sweet-smelling damp patch.
5. Check the radiator fins for debris such as leaves or insects, which block airflow.
6. Note the temperature gauge on your next drive. It should settle in its usual position.

Glossary

Where to go next

• Which coolant does my car need?
• OAT vs HOAT vs IAT
• How to add coolant
• Coolant flush guide
• Coolant leaks
• Coolant warning light

More guides: Which coolant does my car need? · OAT vs HOAT vs IAT · Coolant (water) pump