To prevent overheating and coolant loss, cooling systems are pressurized raising the coolant's boiling point about 3-degrees F for each additional psi above atmospheric pressure. The cooling system is pressurized as the engine warms up and the coolant expands. The radiator cap's pressure relief valve allows pressure to build up to a specified level, then lets excess pressure to escape. In an "open" systems found in older vehicles, excess pressure escapes to the atmosphere through an overflow tube. As the system cools, air enters through the overflow tube and coolant is lost. Therefore, "closed" or reservoir cooling systems are used in later model vehicles. Here, as the coolant expands it goes through the over- flow tube into a reservoir. The radiator cap now serves as a vacuum relief and siphon valve allowing coolant to be siphoned back into the radiator as the engine cools and the coolant contracts.
Radiator caps also serve as pressure relief valve to prevent excessive pressure in the cooling system after the engine is turned off. Unchecked high pressure could cause damage to the radiator, heater core, hoses or water pump seal. The pressure cap also prevents radiator hoses and tanks from collapsing due to the partial vacuum which would be created if air was unable to enter. | 
2000 Mustang v6 replacing radiator 
