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A cars radiator is a central part of the engines cooling system. Responsible for removal of heat from the engine coolant, the radiator system cools the liquid by allowing fresh external air to pass over a network of thin pipes, losing heat to the atmosphere.
Driven by the water pump the coolant circulates around the cooling system travelling through the engine and out via hoses or pipes to the radiator. The radiator acts as a heat exchange removing unwanted heat from the coolant before passing it back to the engine where it then runs back around the system.
Radiators without removable caps are topped up via a expansion bottle or tank, which is located within in the engine bay and connected to the coolant system via rubber tubing or hose.
The radiator expansion coolant bottle provides pressure relief to the coolant system allowing hot water and steam to expand. It also acts as a reservoir allowing the coolant system to refill as coolant cools and contracts.
Problems that can occur with a radiator expansion coolant bottle include cracks and splits due to repeated heating and cooling, damage to perished feed and return pipes and lost or damaged lids/caps.