An engine runs within a fairly narrow temperature band. Push it beyond that and metal expands past its tolerances, gaskets fail, heads warp and, in the worst case, the engine is beyond economical repair. The difference between an inconvenient roadside stop and a new engine is usually how quickly the driver switched it off.
So this guide starts with what to do, not why it happened.
What to do, in order
- Turn the air conditioning off. It adds heat load and its condenser sits in front of the radiator.
- Turn the cabin heater to maximum heat with the fan on full. It feels wrong on a hot day, but the heater core is a second radiator and it pulls real heat out of the engine. This is a way to buy a minute or two, not a fix.
- Pull over as soon as it is safe. Off the carriageway, away from traffic, on level ground if you can.
- Switch the engine off. If the temperature gauge is in the red or a warning light is on, do not keep driving to the next exit.
- Get out, away from the car, especially if you can see steam.
- Leave the bonnet closed for now. If there is steam or spray, opening it immediately puts your face over it. Once things have calmed, releasing the catch and leaving the bonnet ajar helps it cool.
- Wait. Half an hour is a realistic minimum before anything is safe to touch, and longer is better.
Only once everything is genuinely cold should you look at the coolant level. If the expansion tank is empty and there is a visible leak, that is your answer and it needs recovery rather than a top-up and a hopeful drive.
Why it happens
Not enough coolant
The most common cause, and usually a symptom of something else. Coolant sits in a sealed system and does not get consumed, so a low level means it is escaping — a perished hose, a weeping radiator, a failed water pump seal, a loose clamp, or a leaking heater core, which often shows up as a sweet smell inside the cabin or a film on the inside of the windscreen.
The cooling fan is not running
At low speed and in traffic, the electric fan does most of the work. If it has failed, or its relay, fuse or temperature sensor has, the car will run normally on the open road and overheat in traffic. That pattern — fine when moving, hot when stationary — is a strong clue.
A stuck thermostat
The thermostat opens to let coolant circulate to the radiator once the engine is warm. Stuck closed, it traps the coolant in the engine and the temperature climbs quickly, often within a few kilometres of a cold start.
A blocked or dirty radiator
Leaves, insects, plastic bags and road dirt packed into the radiator fins stop air passing through. Internal blockage from old or mixed coolant does the same from the inside.
Water pump failure
The pump circulates the coolant. When it fails, everything else can be in perfect order and the engine will still overheat. A whining noise or a coolant drip from the front of the engine can precede it.
A broken drive belt
On many engines, one belt drives both the alternator and the water pump. If it snaps you usually get a battery warning light and rapid overheating together. That combination means stop immediately.
Head gasket failure
Often a consequence of overheating rather than its cause, but it can be both. Signs include bubbles in the expansion tank, coolant disappearing with no external leak, white smoke from the exhaust that does not clear once warm, or oil that looks like coffee with milk.
Low oil
Oil carries away a surprising amount of heat. A very low level raises temperatures as well as risking direct engine damage.
After an overheat
Do not simply top it up and carry on. An engine that has overheated once should be checked properly — both to find the cause and to confirm it has not already done damage. Mention to the mechanic how hot it got, how long it ran hot, and what you saw.
Your manual has the final word
Coolant type, capacity, where the caps are and what each temperature warning means are specific to your car. Where this guide and your owner’s manual disagree, follow the manual.
