Why is my car overheating?

Back when a common motoring accessory was not a Spotify playlist but a container of water in the boot, cars could boil like a kettle in hotter months.

You’d see them routinely on the side of the road in summer, with the bonnet up and steam rising out of the radiator overflow pipe as the driver stands poised with a water container to fill up the empty radiator.

Most often no damage was done, and the hardy old low-compression cast-iron engines that powered yesteryear’s cars would live to overheat another day.

Fast-forward 40 years and car engine cooling systems are so well designed that overheating is rare. Overheating can still happen, however, and if the cooling system is not maintained and checked properly, it’ll take more than a bottle of water in the boot to make things right again.

A conventionally fueled (petrol, diesel or LPG) car engine makes a lot of heat when burning its fuel to make the car go, and most of them employ a liquid cooling system to stop them getting as hot as a furnace.

This system pumps the liquid coolant though the engine, taking away heat and cooling the hot liquid by trickling it through a radiator, which uses air rushing though its fins to cool the liquid coolant running through its cores.

The system is run under pressure, so unlike your kettle, the coolant will not suddenly boil away once it hits 100 degrees centigrade, but much higher.

There are several circumstances that warn of an impending overheating situation and regular maintenance checks will help to make overheating less likely.

Find out where the coolant reservoir is under the bonnet and check it weekly. See that it is at least at the minimum mark when the engine is cold, or at about the maximum mark with a hot engine.

If the level is below the minimum, top it up to the minimum mark with the manufacturer’s recommended coolant – but only when the engine is cold. If you need to top up regularly, coolant is leaking somewhere and the leak should be fixed. It’ll only get worse.

Note that there are different types of coolant. If you have some spare coolant of a different colour to what is in the reservoir, don’t use it. Seek advice from your mechanic on which coolant type you need to top-up with.

When parked at home or another spot you know the ground is already dry, look for fresh coolant on the ground underneath the car (don’t get under the car though). If you see a bright green, red or blue-ish liquid, that’s the tell-tale signs of a coolant leak.

Most cars have a coolant temperature gauge on the dash (or at least a temperature warning light).

Check your car’s owner’s manual for the temperature gauge’s normal operating range. Notice if the gauge on your car is already sitting higher than the manual says it should when warmed up (after at least 15 minutes after starting from cold). That itself could be a warning sign that all is not well with the cooling system.

Manufacturers tend to design these gauges so that they don’t show a big variation and unnecessarily worry the driver. The gauge is ‘damped’ in its reaction to temperature change information it receives from the coolant sensor.

So if you’re driving up a steep, long hill in summer, with the car fully loaded or towing, the coolant will get a bit hotter than if you were just cruising along. But the gauge will probably show little if any difference, as it has been designed that way.

If the coolant temperature gauge is rising beyond the normal range, or a coolant warning light comes on, it means there is a spike in coolant temperature and overheating is looming.

Another potentially more dire warning sign is if the temperature gauge has been in the normal range but suddenly returns to cold when driving. This could mean the coolant level is so low in the system that the sensor cannot take a reading (or, if you’re lucky, it’s just a faulty sensor or wiring).

If the coolant gauge is heading for the red zone (or suddenly reads cold during normal driving), it’s time to find somewhere quickly where you can pull over and shut down the engine.

What to do if my car overheats checklist:

While you’re looking for somewhere to stop, turn off the air-conditioning (which will reduce the load on the engine) and turn on the heater to full heat and full fan speed.

This will help reduce the engine coolant temperature by cooling more of it though the heater core, which is like a mini radiator. It won’t fix the overheating problem, but in some situations it can help minimise the chances of damage.

With our busy roads and distracted drivers, it can be extremely dangerous to just stop on the side of a roadway, especially highways or freeways. While stopping an overheating car and switching off the engine as soon as possible is important, more important is not risking your life.

Don’t stop on the narrow shoulder of a busy freeway. Find a spot that is safe, well away from the road, even if it means you have to drive on further.

If the engine stalls or power is severely reduced before you can find a safe spot, just pull off the road as soon as you can and turn hazard lights on. As soon as you stop, shut down the engine. Keep your seatbelt on and call for roadside assist.

If you need to get out of the car, stay clear of the roadway (get out from the passenger’s side, if necessary). Get behind a safety barrier if possible.

If you’ve found somewhere well away from traffic to stop and shut down the engine, it’s worth doing an initial investigation if you are confident. Look for fresh coolant leaking on the ground around the front and underneath the car (don’t get under the car though). If you spot the tell-tale bright green, red or blue-ish coolant you’ve got a coolant leak. If it’s a big pool of the stuff, it’s time to call a tow truck.

It’s not just some roads that are dangerous – so too is the car’s cooling system when hot.

Do not try to open or touch anything to do with the engine until it cools down. This can take 30 minutes or more.

Once the engine has cooled down, carefully lift the bonnet. Don’t touch anything, just visually check the coolant level in the reservoir and also check for any obvious split or broken hoses.

If you can see evidence of coolant sprayed (from a leak) in the engine compartment, it’s safe to say it’s time to abandon any ideas of being able to top-up; time to call a tow truck.

If there’s no clear evidence of a coolant leak, and coolant appears low or non-existent in the reservoir, you can top it up if you have some spare coolant of the recommended type.

Once again, remember that a hot cooling system is under pressure, and it might end up spraying hot coolant out, so don’t attempt this until the engine has cooled. As an extra precaution, use a large rag or a towel when undoing the reservoir cap to absorb any coolant that might spray out.

Car engines are robust – up to a point. If proper maintenance and warning signs are ignored, a badly overheated engine will result in a blown head gasket or two (a repair job costing thousands) or in the worst case a seized engine.

Engine coolant doesn’t last forever. Some coolants are ‘long life’ which can mean they will last five years; some others need to be changed every two years.

Drive belts (which drive the water pump on many cars) and rubber or plastic coolant parts have a finite life. The plastic components – such as reservoirs, reservoir caps, heater pipes or thermostat elbows – can crack or break, causing leaks, while rubber hoses can burst or split, again causing leaks.

If the rubber drive belt breaks, it will likely mean the coolant pump stops working. The coolant pump is also a maintenance item; it will eventually leak at the seal and need to be replaced.

This article contains general information only. Seek independent advice that considers your own circumstances.