Why is my car overheating?

First, the part that saves the engine

The gauge going up is not the damage. Continuing to drive is the damage.

Turn off the air conditioning, because its condenser sits in front of the radiator and adds heat. Turn the cabin heater up to hot with the fan on full, which sounds wrong and is right, because the heater core is a small second radiator and it will pull real heat out of the engine.

Then pull over somewhere safe and switch off. A tow is cheaper than a cylinder head, and it is not close.

Leave the radiator cap alone while the engine is hot. The system is under pressure and opening it releases boiling coolant upward.

Coolant loss

The most common reason, and the one with the widest range of causes. Hoses perish and split. Radiators corrode. Plastic expansion tanks crack along a seam. Water pump seals weep.

Some leaks are obvious as a puddle. Others only show under pressure once the engine is hot, which is why a pressure test is a normal first step.

The thermostat

A thermostat is a valve that stays shut until the engine reaches operating temperature, then opens to let coolant flow to the radiator.

Stuck shut, the coolant never reaches the radiator and the temperature climbs fast, often within a few minutes of starting. It is a small part and one of the cheaper causes on this list.

The cooling fan

At speed there is enough air through the radiator without help. In traffic there is not, and that is the fan's job.

The signature of a fan fault is an engine that runs fine on the Monash and creeps up the moment you are stopped in Springvale Road traffic. If your car only overheats when you are not moving, say so, because it narrows things down immediately.

Water pump and radiator

The water pump circulates the coolant. When its impeller corrodes or its bearing fails, flow drops and the engine cannot shed heat regardless of how much coolant is in it.

Radiators block internally over years, particularly where coolant has been topped up with water rather than replaced. The engine then overheats under load while looking fine at idle.

Head gasket

The expensive one. A failed head gasket can push combustion pressure into the cooling system, which forces coolant out and causes overheating that gets worse quickly.

White smoke from the exhaust that smells sweet, a milky film under the oil cap, coolant disappearing with no visible leak, or bubbling in the expansion tank all point this way.

It is also frequently the result of an earlier overheat rather than the cause of this one, which is the reason the advice at the top of this page is worth following.