How does a radiator cap work?
A car radiator
works like a heat exchanger. The heat from the engine is released into the air via the coolant fluid. The coolant fluid, like all fluids, expands when heating up though. When expanding, coolant fluid becomes highly pressurised. This pressure increases even more when the coolant flows into the radiator. The radiator cap
works in fact as a release valve, which opens once the maximum pressure point is reached.
When the radiator cap
opens, the coolant flows into the overflow tanks on either side of the radiator. When the radiator cools down, for example when you switch off your engine, the coolant gets sucked back into the pump.