The bi-metal spring in the BMW Thermostat is how this temperature control device opens and closes. This spring is designed to rigorous specifications so it will contract once a predetermined temperature is reached by the coolant in your BMW engine.
Each BMW thermostat is located following the top radiator hose back from the radiator to the engine block. The housing unit that the upper radiator hose is attached to on the engine block is the thermostat housing, of which the temperature control device lies underneath.
By having a correctly operating thermostat in your engine, the valve in it will remain closed so the coolant in the BMW will warm up sooner bring the engine to its optimum operating temperature faster than if the coolant was just permitted to flow freely. This valve being closed also makes it possible for the passenger compartment heater to produce heater faster in the cold winter months.
This temperature coolant control device only fails when the valve remains stuck closed or open all the time. When the valve is stuck closed and refuses to open, no matter what the temperature reaches in the coolant, the engine will overheat is a very short allotment of time. If this occurs the owner of the vehicle can take the top radiator hose off and manually open the valve with a screwdriver. This will permit the vehicle to be driven a short distance so a proper repair can be made.
If the thermostat on your BMW remains open all the time not only will the passenger compartment heater not function properly but the engine will never obtain its optimum temperature. This will reduce the performance of the engine and fuel mileage the vehicle is capable of producing.