Circuit/System Testing

Test the thermostat for correct operation. Refer to THERMOSTAT DIAGNOSIS
Test the engine cooling system for overheating. Refer to ENGINE OVERHEATING
Inspect the water pump and coolant flow for correct operation. Refer to WATER PUMP REPLACEMENT
Inspect the engine for worn/leaking/cracked cylinder heads and engine block. Refer to COOLANT IN COMBUSTION CHAMBER and COOLANT IN ENGINE OIL