A functioning radiator cap is an important part of your car's cooling system. It isolates the radiator from foreign elements, helps regulate the pressure in the radiator, and allows for movement back and forth of coolant in response to changes in pressure. A bad cap can cause cooling malfunctions. In addition, a faulty radiator cap can cause problems that look like head gasket issues, so it is a cheap place to start if your car is overheating at unusual times, such as when idling.
Overheating can come from many causes, but an improper seal of the radiator cap or pressure malfunction of the cap is an easy (and cheap) source to check first. If the pressure in the radiator is not high enough, the fluid will boil at a lower point and the resulting steam cannot flow through the cooling system.
Coolant Won't Return to Radiator
If the coolant does not return to the radiator, and remains in the expansion reservoir, that is a sign that one of the seals on the radiator cap could be faulty. The coolant should flow easily back into the radiator from the reservoir as the car cools.
Head warping is another sign that can mean the radiator cap is not keeping the pressure in the radiator high enough. If the lower temperature coolant is forced through the engine, it can have bubbles or be foamy, which inhibits the cooling function, which can lead to warping in the engine head, an expensive problem.