Engine overheating

What to do straight away

1. Turn off the air conditioning and turn the heater to full hot with the fan on high. It sounds odd, but it draws heat away from the engine.
2. If the gauge keeps rising, steam appears or a red temperature light comes on, pull over as soon as it is safe and switch the engine off.
3. Get everyone out and away from traffic if you’re on a busy road.
4. Leave the bonnet closed until the engine has cooled, which takes at least 30 minutes.

Never open the coolant cap on a hot engine. The coolant is under pressure and can spray out and scald you.

Common causes of overheating

Low coolant or a leak

The most common cause. Look for puddles under the car (coolant is often pink, green or blue and smells sweet), damp hoses, or white residue around the radiator and hoses.

Faulty thermostat

A thermostat stuck closed stops coolant reaching the radiator. The engine can overheat quickly even with plenty of coolant.

Cooling fan not working

If the car overheats in slow traffic but is fine at speed, the electric cooling fan, its fuse or its relay may have failed.

Water pump failure

The pump circulates coolant. A failing pump may leak, whine, or simply stop moving coolant. On many engines it’s driven by the timing belt.

Head gasket problems

White exhaust smoke, coolant disappearing with no visible leak, or a milky substance under the oil filler cap can point to a head gasket fault. Stop driving and get it checked, as carrying on can cause serious engine damage.

Safe checks once the engine is cold

• Check the coolant level in the expansion tank against the MIN and MAX marks.
• Top up with the correct coolant for your car (see the handbook). In an emergency, plain water will get you to a garage, but tell the mechanic.
• Look for leaks, split hoses and loose clips.
• Start the engine and watch the gauge. If it climbs again quickly, don’t drive on.

Can I keep driving?

Not if the temperature light is on or the gauge is in the red. Driving an overheating engine can warp the cylinder head and ruin the engine. If topping up the coolant brings the temperature back to normal, you may be able to drive carefully to a garage, but keep an eye on the gauge. If in doubt, call for recovery.

What a garage will do

A mechanic will pressure test the cooling system to find leaks, check the thermostat, fan and water pump, and test for combustion gases in the coolant if a head gasket fault is suspected. They should tell you what they found and the cost before starting repairs.

Frequently asked questions

Why does my car overheat in traffic but not on the motorway?

At speed, air flows through the radiator on its own. In traffic, the cooling fan has to do that job, so a fan fault often shows up first in queues.

Can I use water instead of coolant?

Only in an emergency, to reach a garage. Coolant protects against freezing and corrosion, so get the system refilled properly afterwards.

How long should I wait before opening the bonnet?

At least 30 minutes, and longer before touching the coolant cap. Never open it while the engine is hot.