Hello Paul, I have a Toyota Hiace with a 3L diesel engine. Lately, it has a persistent overheating. My mechanic has failed to find the problem after changing the engine fan and radiator. Please advise what we can check. Hajj Semakula

Hello Hajj Semakula, there a number of factors that can cause the overheating of your 3L Toyota engine, all are related to the engine cooling system. Start with the simple and obvious checks.

Leakage in the engine cooling system is one of the leading causes of overheating. Ensure that all the coolant hoses are not cracked, loose or leaking. Confirm that the cylinder head gaskets, water pump seal, thermostat seal are all not leaking.
Occasionally, the ventilation heating pipes can cause a leakage of coolant, inspect them. Check that the coolant reservoir cap or radiator cap is not damaged by corrosion, does not leak and can still release their thermistor switch to allow return flow of coolant.

It would be a good idea to check whether the thermostat still works at the specified temperature by dipping it in boiling water. It should open or release if it works. If it does not, replace it with a new one (do not discard completely). Confirm that the water pump runs well without making noise or showing signs of restriction. It is prudent to confirm that the new cylinder head and gasket you fitted are working well. Check that the engine oil is not mixing with coolant, a sure sign of head gasket failure.