Cooling Systems
Cooling system fundamentals
The cooling system moves heat from the engine to the radiator and heater core. A belt- or electric-driven water pump circulates coolant; the thermostat blocks flow until the engine reaches operating temperature, then opens progressively. The radiator transfers heat to air, assisted by a mechanical fan clutch or electric fan. The pressure cap raises the boiling point and allows expansion into the recovery bottle. Coolant must be the manufacturer-specified OAT, HOAT, or conventional formula mixed 50/50 with distilled water unless a pre-mixed coolant is specified.
Traps: coolant color is not a reliable identifier of chemistry; mixing incompatible coolants can gel and plug the heater core. A thermostat installed backward or stuck open causes slow warm-up and poor heater output. Air pockets after a drain and refill can cause overheating and false low-coolant readings; bleed per OEM procedure. Electrolysis from bad grounds or mixed coolant can eat aluminum. Do not open a hot pressurized cap.
Rule of thumb: each 1 psi of cap pressure raises the coolant boiling point about 3°F, so a 15 psi cap adds roughly 45°F. A 50/50 glycol mix at atmospheric pressure boils near 223°F; with a 15 psi cap it can reach about 265°F.
Worked example: A vehicle overheats at idle but cools at highway speed. Coolant level and mixture are correct. The likely fault is airflow, such as an inoperative electric fan, a slipping fan clutch, or a debris-blocked condenser/radiator. Verify fan command with a scan tool and check the cap with a pressure tester.