A bad radiator cap can cause the coolant to boil over from the reservoir or the engine to overheat. The cap is an integral piece of an engine's cooling system as it retains the coolant´s pressure.
A radiator caps primary function in a cooling system is to retain the engine´s cooling system pressure. A standard cap will have a weight scribed on top of it. The number refers to the boiling point which is increased by degrees for every pound written on the cap. For example, a cap rated at 15 pounds will increase the boiling point of coolant an extra 30 degrees. Listed below are the symptoms of a bad radiator cap.
Reservoir boils over
The reservoir tank is where the coolant flows when it expands. Pressure releases the radiator cap which sends the coolant to the overflow tank. A faulty cap will release the coolant too soon resulting in an overflowing reservoir.
Air in the system
If the cap does not properly seal the cooling system, air will enter it. When air enters the system pockets form inside the cooling hoses, thermostat and heater core. When there is air in the system the heater will fail to produce heat and the engine will not be able to retain a consistent temperature, which will cause it to overheat.