What does a raidiator cap do?

It is an opening where coolant liquid (a mixture of water and anti-freeze) can be added to a vehicle.

It is pumped through chambers in the engine block to absorb the excess heat and draw it away from vital areas so your car will not over heat. It is the nature of the internal combustion engine.

The cap is also a pressure release valve, and on cars it is usually set to 15 psi. The boiling point of water increases when the water is placed under pressure. (Source: howstuffworks.com)

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A radiator cap has 4 functions I can think of.

1) It provides a sealed cover for the opening which you use to add coolant/antifreeze on some cars. Others, the coolant/antifreeze is added to a container that is the coolant overflow tank.

2) It allows the excess pressure and fluid to be released from the cooling system/radiator into the overflow tank once heated to operating temperature.

3) It allows the excess coolant which was previously released to be drawn back into the radiator once the engine cools down to the point it actually creates a vacuum, syphoning the liquid from the overflow tank back into the radiator.

4) It provides a means for the cooling sysyem to operate at a greater than atmospheric pressure hence raising the boiling point of the coolant.

(If you were to use pure water in an unsealed/pressurized cooling system, the water would boil at approx. 212° F or 100° C. Since most newer vehicles {1970's and later} have a closed/pressurized cooling system the engine can be run hotter due to the pressure in the system raising the boiling point to well over 212° F or 100° C.)