A defective radiator cap can cause an engine to overheat because the faulty part reduces the functional efficiency of the radiator. The coolant-system temperature is lowered in the radiator by passing through a series of fins and tubes, so anything that disrupts this process decreases cooling, which can lead to an increase in engine temperature.
When checking the radiator cap, make sure the sealing ring is undamaged. If it is a rubber ring, ensure it hasn't become brittle and cracked. The cap's pressure-relief valve can be tested by depressing it against its spring; effort to do this is required. The valve itself should move freely.
The owner can test the cap's efficiency by running the engine for a few minutes, then using a heavy cloth and a thick glove, can undo the cap to its first stop; never fully remove the cap as the pressurized hot water will erupt. If no escaping air or steam is present, the cap is likely defective and a new one needs to be fitted.
Ensure the new cap is rated correctly; the pressure rating is marked on it. This is important because the coolant system is pressurized, which increases water's boiling point. Fitting a cap with too low a pressure rating means decreased system pressure, which may again lead to overheating, while fitting one with too high a rating may lead to over-pressurization of the system.