Engine overheating: what to do and causes

Engine overheating is a serious situation: driving with an overheated engine can damage the block, head or head gasket, with very costly repairs. Here is what to do and the most common causes.

Immediate action

As soon as the temperature warning light (red thermometer) comes on or the needle goes into the red: reduce load (slow down, avoid hills and hard acceleration), turn the heater on full (this sheds some heat from the cooling system). If temperature does not drop: stop as soon as possible, engine off, and do not open the radiator or expansion tank cap when hot (risk of burns and spray). Wait for cooling (often 30 min or more) before opening and checking coolant level. If in doubt, call breakdown rather than restarting.

Most common causes

Coolant leak or low level: perished hoses, head gasket, cracked radiator or expansion tank. Thermostat stuck closed: coolant no longer flows to the radiator, engine overheats. Radiator clogged or blocked: poor heat rejection. Water pump failure: no coolant circulation. Fan (electric or viscous) not engaging: problem in town or when stationary. Timing belt (if it drives the water pump) broken: engine overheats and must not be run.

After overheating

Even if the level is topped up and all seems normal, have the system checked by a professional: overheating can weaken gaskets and hoses. Check for no oil/coolant mix (mayo on oil cap or in expansion tank), a possible sign of a failed head gasket.

Order of magnitude for costs

Coolant fill and bleed: €50–120. Thermostat: €80–200. Water pump: €150–400. Radiator: €200–500 (parts and labour). Head gasket: €800–2,000 and more depending on engine. My Auto Check-up helps you prepare your visit by gathering useful information for an initial estimate.