Car overheating: what to do right now and why it happens

Car overheating: what to do in the first two minutes

The order matters. Do these in sequence and do not skip the last one.

1. Air conditioning off, heater on full heat with the fan on high. The heater is a small radiator and pulls heat out of the engine. It will be unpleasant for a minute.
2. Pull in somewhere safe: a lay-by, a gateway, a hard shoulder. Not a live lane and not a bend.
3. Switch the engine off. On a motorway, do this on the hard shoulder and get behind the barrier.
4. Open the bonnet if it is safe to, to let heat out, and stand back in case of steam.
5. Do not open the coolant cap. The system is under pressure and the coolant is well above boiling, so the cap on a hot engine sprays scalding liquid over your hands and face. Leave it at least thirty minutes.
6. Look, do not touch. Steam, a puddle under the car, a sweet smell, a loose belt. Note what you see for whoever comes out.

The coolant warning light and the temperature gauge

Most cars have a gauge that sits in the middle once warm and a red thermometer symbol for when the engine is too hot. Some newer cars have no gauge, just a blue light when cold and a red light when overheating, so the red light is the first you know. Either way, a red coolant warning light means stop, not slow down. An amber coolant symbol on some cars means the level is low rather than the temperature high, which is a check-it-soon warning rather than an emergency, unless the gauge is climbing too.

If the gauge suddenly drops to cold while driving, that can be a coolant problem rather than good news, because a sensor sitting in air instead of coolant reads cold. Pull in and check the level once it has cooled.

Engine overheating causes, from cheap to expensive

There are five usual suspects. The cheap ones are more common than the dear ones, which is the main reason to stop early.

Head gasket symptoms

Here is what a failed head gasket actually looks like. White exhaust smoke that smells sweet and keeps coming once the engine is warm. Coolant that keeps disappearing with no leak you can find. Creamy, mayonnaise-like sludge under the oil filler cap or on the dipstick. Bubbles rising in the expansion tank with the engine running, or a tank that is rock hard with pressure moments after starting. Overheating that comes back within minutes of topping up.

One of those alone, especially the mayonnaise on a car that only does short winter trips, is not proof. Three together is a head gasket until a garage proves otherwise, and the car should go on the truck, not be driven.

What you can and cannot do at the roadside

Once the engine has cooled for a good half hour, open the cap slowly with a rag over it, check the level and look for an obvious leak. If the level is low and there is no big leak, topping up with water from a bottle is fine as an emergency measure to reach a garage. Never pour cold water into a hot engine, because the shock can crack the head or block. If a hose has split, a top-up buys you a kilometre at most.

What you cannot do at the roadside is fix a thermostat, water pump, fan or gasket, and neither can we. Our roadside fixes are belts, hoses that can be patched or re-clipped, fuses and a top-up. Anything else, we carry the car, and on the M9 we always recover first and look at it at the next junction.

Why driving on destroys the engine

Nearly every modern engine has an aluminium cylinder head, and above normal operating temperature aluminium warps. Once the head is warped the gasket cannot seal, so a car that was overheating because of a cheap thermostat now has a head gasket failure as well. Keep going and the pistons seize in the bores or the oil thins and the bearings go, and the engine is scrap.

The sums are simple. A thermostat or a hose is a modest repair. A head gasket on a modern engine is a four figure one. A seized engine on a ten year old car is usually a scrap car collection. We would far rather lift a car with a split hose than a dead one, and so would you.

Car overheating in traffic, on hills and when towing: Kilkenny examples

The ring road at five o'clock, the queue back from the Hebron Road roundabout, the crawl around MacDonagh Junction on a Saturday: this is where fan failures show up. The car was fine on the N10 at 100 km/h and climbs to the red as soon as it stops. Heater on, windows open, and pull into the next car park rather than sit in the queue watching the gauge.

The long climb out of Castlecomer on the N78 and the hills around Windgap and Clogh find weak water pumps and low coolant, because an engine working hard uphill makes far more heat than on the flat. Then there is towing. Summer on the M9 means horseboxes heading for Gowran and caravans heading for Waterford and Wexford, and a car pulling near its limit on a hot day works much harder than its owner is used to. If the gauge is creeping up with a trailer behind you, come off at the next junction and let it cool, because a horsebox on the hard shoulder is a problem on top of a problem. We recover car and trailer together, so ring 087 288 2280 and tell us what is on the back.

When to ring for recovery

Ring if the coolant is gone and you cannot see why, if it overheats again within minutes of topping up, if there is any sign of a head gasket, if the car is on the M9 or a national road, or if you are not sure. If you have breakdown cover, ring your insurer first, because cover usually only pays for recovery they arrange. If you have none, or the wait is too long, ring us. A flatbed run from anywhere in the county to your garage costs a fraction of an engine, and Kilkenny Recovery Truck has no call-out fee. Price agreed before the truck moves, 24 hours a day.