Overheating and Cooling System Problems: Radiators, Water Pumps and Coolant

What to do if the gauge starts to rise

Act straight away. Turn the heater to full hot and the fan to maximum, as this draws some heat out of the engine, and then find a safe place to stop. Switch off, put the hazard lights on if needed and let the engine cool.

Never open the radiator or expansion tank cap while the engine is hot, because the system is under pressure and can release scalding coolant. Do not keep driving to see how far you get. A hot engine can warp the cylinder head or blow the head gasket, and the damage rises quickly with every mile.

• Pull over somewhere safe and switch off
• Do not open the coolant cap on a hot engine
• Wait for the engine to cool before checking the level
• If it is low or the problem returns, get the car recovered rather than driving on

Thermostat and coolant problems

The thermostat is a valve that controls coolant flow. If it sticks closed, the coolant cannot circulate to the radiator, and the engine overheats quickly. If it sticks open, the engine may take ages to warm up. It is a small part with a big effect.

Coolant itself matters. It is a mix of antifreeze and water that prevents boiling and freezing and protects against corrosion. Old or wrongly mixed coolant loses its protection and can lead to rust and blockages. Top it up with the correct type, and have a falling level investigated, because it has to be going somewhere.

Radiators and leaks

The radiator sheds heat from the coolant to the air passing through it. Over time the core can become blocked with deposits, the plastic end tanks can crack, and the fins can corrode or be damaged by stones. Leaks show up as damp patches, a sweet smell, or coolant stains under the car.

Hoses and clips also perish, and a split hose can empty the system very quickly. A cooling fan that does not turn on can cause overheating in slow traffic even when the car is fine at speed. At Banks of the Tyne in Hebburn we repair cooling systems, radiators and water pumps, and we can pressure test the system to find the leak.

Water pump failure

The water pump circulates the coolant around the engine. When its bearing wears or its seal leaks, you might see coolant weeping from the front of the engine, hear a whine or rumble, or find that the engine heats up for no obvious reason. A failed pump can stop the flow entirely.

On some engines the pump is driven by the timing belt, so a failure can also threaten the belt. That is one reason the pump is often replaced at the same time as the timing belt.

• Coolant seeping near the front of the engine
• A whine, grinding or rumbling noise that changes with engine speed
• Overheating despite a full coolant level
• Steam or a sweet smell from under the bonnet

Knock-on damage: head gaskets, turbos and injectors

The worst outcome of overheating is damage inside the engine: a blown head gasket, a warped cylinder head or scored cylinders. Turbocharged engines are particularly sensitive to heat, and overheating can damage the turbo and its oil supply. Heat can also affect injectors and fuel system parts.

That is why a cooling fault is never just a coolant top-up. Have it diagnosed, and let us know about any earlier overheating, because it changes what we check. The sooner it is looked at, the better the chance of avoiding a bigger engine repair.

Frequently asked questions