A radiator cap in a car is a crucial component of the cooling system. Its primary function is to seal the radiator, maintaining the correct pressure and preventing coolant from escaping. The radiator cap is essentially a pressure release valve that regulates the pressure inside the cooling system. It ensures that the coolant remains at the optimal temperature for efficient engine operation.

When the engine heats up, the coolant expands and increases the pressure inside the radiator. The radiator cap has a spring-loaded valve that allows excess pressure to escape, preventing the radiator from bursting. At the same time, the cap also maintains a constant pressure in the system, raising the boiling point of the coolant, and improving the cooling efficiency. Additionally, the radiator cap allows the cooling system to draw coolant from the overflow reservoir when needed, ensuring a continuous supply of coolant to keep the engine temperature in check.