Modern cars are far less likely to overheat than older vehicles, but it can still happen. Engines normally overheat in the summer months, but can potentially do so at any time of year. You may have a problem if your temperature indicator starts to rise, or a malfunction light flicks on. You may also see vapour emerging from the bonnet, or notice a smell from the engine.
Cool your engine
Immediately turn off the air-conditioning and open your windows, which will reduce the load on the engine.
If this doesn't work, turn on your heater and blower, which will transfer heat from the engine to the cabin. As you might expect, doing this on a hot summer's day will make the cabin rather uncomfortable.
If your car overheats in traffic, change to neutral and rev the engine. This will operate the water pump and increase the fan speed, which will suck more water and air through the radiator, cooling the engine.
Avoid using your brakes unnecessarily, as this increases drag on the engine.
If you're worried the engine will boil over, pull up in a safe place, raise the bonnet and wait for the engine to cool down.
Call your roadside assistance provider and await help.
Identifying and fixing the problem yourself
For most motorists, taking an overheated car to the nearest garage is the easiest way to get back on the road. It’s important to fix the issue quickly, as overheating can badly damage your engine. But for some, addressing the problem yourself is relatively straightforward.
One common reason for an overheated engine is low levels of water and coolant. Open the radiator cap and check inside; if it's dry, you need to add more fluid.
Pressure test the radiator system for leaks
You can buy a pressure leak testing kit affordably. Attach it to the radiator, then pump up the pressure. If water comes out of the holes in the radiator, you have a leak. To find a less obvious leak, pump up the tester and watch gauge; if it starts to move down, there's a leak.
No leaks? Check your radiator cap
Radiator caps are supposed to hold pressure in. But if they are worn and rusty, you may need a new one. You could pressure test the cap with a special system, but they are so cheap you're probably better off buying a new one.
If overheating persists, check your cooling fan. Turn it on full blast, then check under the bonnet to see if the fans are working. If they are not, there won’t be enough air being sucked into the radiator, causing the engine to overheat. The most likely reason for a fault is that the motor has burnt out.
You can check the fan motors with a jumper wire: attach one end to the positive cable on the battery and the other on the red cable that feeds the motor you're testing. If the fan fails to spin, the motor is burnt out.
If the cooling fans are operating normally, you may have a faulty thermostat. These components close as the engine heats and open when the engine is very hot. Sometimes thermostats get stuck. Most thermostats can be located by following the radiator hose to the thermostat housing. This has a bolt on the top and the bottom; simply pull the thermostat out and replace it with a new one.
Is your car still overheating?
With luck, one of the above components caused your car's engine to overheat. If the problem persists, there are two likely issues - neither of which is cheap to remedy.
When a radiator gets very old, it may no longer be able to dissipate heat. Modern units are manufactured from plastic and aluminium, the latter of which can corrode over time, meaning the radiator no longer dissipates heat. The radiator may not leak and otherwise look to be in good condition, but if it cannot remove heat, your engine will suffer.
Head gasket starting to blow
While replacing your radiator is costly, fixing a faulty head gasket is even more so. The engine will need to be stripped down, rebuilt and reinstalled, which may cost up to £1,000 - more than some cars are worth in their entirety.
Combustion leak test
Before getting your engine stripped down, it's sensible to carry out a combustion leak test so you know for sure if the head gasket is leaking. This involves adding a special blue liquid to a plastic tube, which is attached to the radiator. The engine is turned over and if the liquid turns yellow, you have a head gasket leak. Your local garage will perform this test before suggesting the engine is stripped down.