Radiator cap

If no leaks are apparent, the radiator cap should be pressure tested to make sure it is holding the specified pressure. If the spring inside the cap is weak the engine will lose coolant out the overflow tube every time it gets hot.

Exhaust

In some cases severe exhaust restriction can produce enough backpressure to cause an engine to overheat. The most likely cause of the blockage would be a plugged catalytic converter or a crushed or damaged pipe. Checking intake vacuum and/or exhaust backpressure can diagnose this kind of problem.

Water pump

In a high mileage engine, the impeller that pumps the coolant through the engine inside the water pump may corroded causing the blades to become loose or eaten away. If such is the case, the pump must be replaced.

Most pump failures occur at the pump shaft bearing and seal. After tens of thousands of miles of operation, the bearing and seal wear out. Coolant starts to leak out past the shaft seal, which may cause the engine to overheat due to the loss of coolant. A sealer additive will not stop this kind of leak. Replacing the water pump is the only cure.

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