Radiator caps are essential for keeping your radiator coolant from escaping in a could of steam. In older cars the radiator cap is also the point at which you would top p your coolant. The extreme heats and pressures this particular cap is subjected to require a relatively sturdy construction. Even so, radiator caps sometimes fail. They also sometimes roll away and get lost. Regardless of why a replacement is needed, it is certainly not advisable to drive without a radiator cap. After the initial boiling mess of fluid escapes from the uncapped radiator, you could very easily overheat your engine. This would cause complete loss of power and possibly catastrophic damage.