In the pressurized cooling system, there is an important part called radiator cap. When the threshold has been reached some point, some vapors are allowed to escape after the radiator cap seals in coolant to a particular pressure point. The ejected fluid, or some vapors, is piped into the overflow bottle. The coolant level will drop inside the radiator while the engine cools down. At the same time, an opposite-direction-flow valve in the radiator cap opens to allow fluid from the bottle to return to the radiator. A failing radiator cap can cause too much or too little fluid to escape the system. Therefore, if you notice your BMW car is overheated, of if your car's overflow bottle fills up, and these are all signs showing your radiator cap goes wrong. You should stop driving immediately and check it. Once you decide to do replacement, we are glad to offer you all BMW genuine OEM radiator cap with a cheapest price.