Car Overheating? What to Do When the Temperature Gauge Rises

What to do straight away

If your car's temperature gauge rises towards the red or the coolant warning light comes on, switch off the air conditioning, turn the heater to full heat with the fan on high, and pull over as soon as it is safe. Turn the engine off and let it cool down. Do not open the bonnet if you see steam pouring out, and never open the coolant cap while the engine is hot.

Carrying on driving an overheating engine can cause serious damage, such as a warped cylinder head or a failed head gasket. Stopping a few minutes early is always better than pushing on to get home.

Why turning the heater on helps

It sounds odd, but the heater works by passing hot coolant through a small radiator inside the dashboard. Turning it to full heat with the fan on high takes some heat away from the engine. It will not fix the problem, but it can buy you a little time to find a safe place to stop.

If you are stuck in slow traffic, avoid sitting with the engine revving, and pull over when you can. Many cars overheat in traffic because there is no air being pushed through the radiator at low speed.

What not to do

Never remove the radiator or expansion tank cap when the engine is hot. The cooling system is under pressure, and boiling coolant can spray out and cause bad burns. Wait until the engine is properly cool, which can take half an hour or more.

Do not pour cold water into a very hot engine, as a sudden temperature change can crack engine parts. And do not keep driving on and off, letting it cool then overheating again. Each time it overheats, the risk of expensive damage goes up.

Common causes of overheating

Low coolant is the most common cause, and it is usually low because of a leak somewhere. Look for drips under the car, a sweet smell, or crusty residue around hoses and the radiator. Split hoses and loose hose clips are frequent culprits.

Sometimes the problem is with the gauge rather than the engine. A faulty temperature sensor can make the needle jump around or read high when the engine is actually fine. It is still not worth guessing, because the symptoms can look identical to a real overheat. Treat any rise into the red as genuine until the car has been checked. A garage can compare the sensor reading with the actual engine temperature to tell the difference.

A thermostat stuck shut stops coolant flowing to the radiator, so the engine heats up quickly. A failing water pump cannot circulate coolant properly, and on many engines it is driven by the timing belt, so it is often replaced at the same time.

The radiator cooling fan may not be switching on, which shows up most in traffic. A blocked or damaged radiator cannot shed heat, and an air lock in the system after a repair can cause hot spots. More seriously, a failing head gasket can push combustion gases into the coolant. Signs include white exhaust smoke with a sweet smell, bubbling in the expansion tank, and coolant disappearing with no visible leak.

Once the engine has cooled

When the engine is cold, check the coolant level in the expansion tank. If it is low, you can top up with the correct coolant, or water in an emergency, to get to a garage. Keep an eye on the gauge and stop again if it starts to climb.

If the coolant is very low, there is a large leak, the level drops again quickly, or you see signs of head gasket trouble, do not drive the car. Call your breakdown provider instead. It is not worth risking the engine to save a recovery.

If you topped up with plain water, get the coolant mix checked soon, particularly before winter, as too much water weakens the frost and corrosion protection.

Get your cooling system checked

Overheating rarely fixes itself. Even if the car seems fine after cooling down and a top-up, the cause is still there. We can pressure test the cooling system to find leaks, check the thermostat, fan and water pump, and test for signs of head gasket failure.

If your car has been running hot, get in touch with MF Autocentre in Walsall. Call 01922 636820 or book online and we will find out what is going on.