How an engine cooling system works
Coolant absorbs heat from the engine, a pump circulates it, a thermostat holds the engine at operating temperature, and the radiator and fan release the heat to the air.
The circuit
The pump pushes coolant through passages around the cylinders and head. When the engine is cold the thermostat stays closed so it warms quickly; once warm it opens and coolant flows through the radiator.
Why pressure matters
A pressure cap raises the coolant's boiling point. A weak cap or leak can cause boiling at normal temperatures.
Overheating causes
Low coolant, a stuck thermostat, a failing pump, a blocked radiator, a fan or fan-clutch fault, air in the system, or a head gasket problem can all cause overheating. Stop safely if the gauge climbs — continuing can damage the engine.
Common questions
- Can I open the radiator cap when hot?
- No. Hot coolant under pressure can cause serious burns. Let the engine cool first.
Sources & review status
- Motor Service's Automotive Encyclopedia, Jud Purvis and William K. Toboldt, 3rd edition, The Goodheart-Willcox Co., Chicago, c. 1958 (copyright dates 1954, 1956, 1958). PDF p. 16 and following. Supports: Cooling system configurations. Historic source.
- Dyke's Automobile and Gasoline Engine Encyclopedia, A. L. Dyke, 12th edition, second run, A. L. Dyke, St. Louis, Missouri, c. 1920. PDF p. 7 (contents). Supports: Organisation of cooling and lubrication systems. Historic source.
Modern coolant types, electric pumps and electronic thermostats are not covered by the historic sources. Use the maker's coolant specification.
Original Repairius explanation. Not yet reviewed by a qualified technician; not a substitute for the maker's service information.