Engine Overheating — Causes, What to Do and When to Call a Mechanic

An engine temperature gauge in the red zone is an emergency. Continuing to drive an overheating engine risks warping the cylinder head, blowing the head gasket or, in extreme cases, engine seizure. The repair cost for an overheated engine that has been driven too far is significantly higher than the repair cost for the original fault.

What to Do Immediately

1. Switch off the engine — pull over safely and stop as soon as it is safe to do so. Do not keep driving to reach your destination or a garage.
2. Do not open the bonnet immediately — pressurised coolant is dangerous.
3. Wait 30 minutes — let the engine cool fully before opening the bonnet.
4. Do not remove the coolant cap when hot — the coolant system is pressurised at 1.2–1.4 bar. Removing the cap on a hot engine releases scalding steam.

Only after the engine is fully cool:

5. Open the bonnet and check the coolant level in the expansion tank.
6. Look for any visible coolant leaks under the car.
7. Check the radiator fan has not broken.

Most Common Causes of Engine Overheating

Low Coolant Level

The most common cause — and often the most easily fixed if caught early. A slow leak can go unnoticed for weeks. Check the coolant level regularly (monthly), especially as summer temperatures rise.

Low coolant is a symptom, not the fault. The underlying leak still needs to be found and repaired.

Faulty Thermostat (Stuck Closed)

The thermostat controls when coolant flows to the radiator. If it sticks in the closed position, coolant cannot circulate through the radiator to cool down. The engine overheats rapidly — often within 10–15 minutes of starting.

Thermostat replacement is an inexpensive repair: from £75 including parts.

Cooling Fan Failure

The electric radiator fan should activate when the engine reaches operating temperature (typically 95–100°C). If the fan does not come on — due to a failed motor, relay or temperature sensor — the radiator cannot cool the coolant at low speeds or at a standstill. Overheating typically occurs in traffic or on slow roads, not at motorway speeds where airflow through the radiator is sufficient.

Radiator Blockage or Leak

A blocked radiator cannot transfer heat efficiently. A leaking radiator (tank cracks, core damage) causes coolant loss and subsequent overheating.

Water Pump Failure

The water pump circulates coolant throughout the engine. A failed water pump impeller means coolant is not moving even if the thermostat is open. Signs include overheating and sometimes a squealing or whining noise from the front of the engine.

Head Gasket Failure

A blown head gasket allows combustion gases to enter the cooling system. This raises system pressure rapidly, pushes coolant out through the overflow and causes the system to overheat despite appearing full. Signs: white or sweet-smelling exhaust smoke, milky oil, rapid coolant loss with no visible leak. We carry out a block test (combustion gas in coolant) to confirm or rule this out.

Safe DIY Checks (Cold Engine Only)

• Check coolant level in the expansion tank
• Inspect hoses for visible cracks or weeping
• Check radiator cap condition and seal
• Look for puddles under the car when parked

What a Mechanic Checks

• Coolant pressure test (identifies small leaks invisible to the eye)
• Thermostat function (does it open at the correct temperature?)
• Cooling fan operation
• Water pump output
• Combustion gas test (head gasket)
• Radiator condition and flow

Book Overheating Diagnosis in Reading

Call 07305 523333. Do not drive an overheating vehicle to us — we come to you. We cover Reading and all Berkshire towns.

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