The cooling system removes about one-third of the heat produced in the combustion chamber. The exhaust system takes away much of the heat, but parts of the engine, such as the cylinder walls, pistons, and cylinder head, absorb large amounts of the heat. If a part of the engine gets too hot, the oil film fails to protect it. This lack of lubrication can ruin the engine. For these reasons, the cooling system is designed to stay out of the action until the engine is warmed up.
Liquid cooled engines have passages for the liquid, or coolant, through the cylinder block and head.
The coolant has to have indirect contact with such engine parts as the combustion chamber, the cylinder walls, and the valve seats and guides. This business continues as long as the engine is running, with the coolant absorbing and removing the engine's heat, and the radiator cooling the coolant. A cooling system pressure tester is used to check the pressure in the cooling system, which allows the mechanic to determine if the system has any slow leaks. The leak can then be found and fixed before it causes a major problem. The heater core is a smaller version of the radiator that is used to keep your toes warm when it's cold outside.