Gauge in the red, warning light or steam? Pull over now
Signal, pull over somewhere safe and switch the engine off. Don't lift the bonnet while steam is pouring out, and never open the radiator or coolant cap on a hot engine: the system is under pressure and can spray scalding coolant. Hazard lights on, and on a motorway stay belted in on the side away from traffic unless it's safer to get out. If anyone is hurt or in danger, call 111.
Once you've stopped
Let it cool properly
Wait until the bonnet is cool to the touch, not just until the steam stops. That can take a good while.
Look underneath from a safe spot
A puddle of green, orange, pink or blue fluid with a sweet smell is coolant. AA New Zealand says makers use those bright colours so a coolant leak is easy to spot.
Check the overflow bottle when it's cold
The see-through plastic bottle shows the coolant level against its marks. Empty or nearly empty means it's lost coolant somewhere.
Decide whether to move it
If the bottle was empty, the gauge shot up fast, or you saw white smoke, have it towed or get a mobile mechanic to come to you.
What does the temperature warning mean?
Most cars have a gauge that settles in the middle once warm, a red warning light, or both. Either one heading for hot means the cooling system can't carry heat away as fast as the engine makes it.
The system runs under pressure on purpose. Wikipedia's radiator entry explains that a valve in the cap keeps the coolant above atmospheric pressure, which lifts its boiling point. A water pump moves the coolant round, and the thermostat opens once the engine reaches its working temperature.
Why do cars overheat?
| Clues | What fixes it | |
|---|---|---|
| Coolant leak (hose, clamp, radiator, water pump) | Puddle, sweet smell, low overflow bottle | Replacing the leaking part and refilling |
| Thermostat stuck shut | Gets hot quickly, even on a cool day | A new thermostat |
| Radiator fan not running | Fine on the open road, hot in traffic | A fan, relay or sensor repair |
| Blocked radiator | Runs a little hotter month by month | Flushing or replacing the radiator |
| Broken drive belt | A squeal, then sudden heat and a battery light | A new belt, and finding why it went |
| Head gasket | White smoke, bubbles in the coolant, milky oil | Head gasket repair |
Why it often happens in traffic or on a hill
Cooling systems work hardest when the engine is working hard and the air isn't moving: a summer queue on the way to the beach, towing a boat or caravan up a long hill, or a motorway crawl with the air conditioning on. A system that's already a bit weak, a little low on coolant or with a tired fan, shows it first then.
If the gauge creeps up in traffic and drops again on the open road, say so in the note. It points the mechanic at the fan.
What will the mechanic do?
Pressure-test the cooling system to find any leak, check the thermostat, fan and water pump, and test the coolant for combustion gases if a head gasket is possible. AA New Zealand names overheating as the most common cause of head gasket failure, so after a bad overheat that test matters.
Ask the mechanic to explain what failed and why, and to quote the repair in writing before starting. If the engine ran hot for a while, ask what else they'd want to check.
What you can do, and what's the mechanic's job
You can
- Check the coolant in the overflow bottle when the engine is cold.
- Look for bright-coloured drips where the car is usually parked.
- Keep coolant away from children and pets: Wikipedia notes ethylene glycol is poisonous and its sweet taste can tempt them.
Leave to the mechanic
- Anything involving the radiator cap or a hot engine.
- Pressure testing, and thermostat or water pump replacement.
- Checking for head gasket damage after an overheat.
If it were our car
We'd stop at the first sign of the gauge climbing and let it cool right down. Then we'd send it as a cooling job with every clue in the note (puddle, steam, smoke, when it climbed) and ask for a pressure test before anyone replaces parts.