Why does my car’s coolant leak?

Paul, thank you for sharing information on car maintenance in your weekly column. I am based in Jinja and drive a Toyota Corsa. Of late, the radiator cap leaks coolant when the engine is running. My mechanic has replaced the radiator cap twice with good used ones but it still leaks. What can we do next? Sam O.

Hello Sam, your car’s engine cooling system should not be leaking coolant as this will lead to overheating.
The engine’s radiator is a heat exchanger which transfers excess heat from the engine coolant fluid into the air.

When coolant fluid overheats it expands, which makes it pressurised. When coolant enters the radiator, its pressure increases.
The radiator cap works as a release valve designed to open at a specific pressure point. When coolant pressure exceeds 15 psi, the valve on the radiator cap opens to release excess heat and coolant fluid to the expansion tank or reservoir at the side of the radiator.
When the radiator cools down, coolant in the expansion tanks is sucked back into the cooling system.

When the radiator cap fails, the engine coolant pressure relief and control fails. This causes instantaneous overheating.
Radiator caps usually fail due to corrosion damage and should be replaced with new ones with the same pressure and temperature specifications so that they open at the right time.
In the event that leaking coolant continues after you replace the cap consider inspecting the whole cooling system.
I suggest your mechanic inspects the cooling fans, rule out a defective thermostat which would block coolant flow. Other causes of overheating should be ruled out such as leaking radiator or hoses, damaged water pump or warped cylinder head gasket.