Replacement thermostat only. BMW only sells the thermostat as part of the water pump assembly. This allows you to replace a faulty thermostat and save you money in the process by not being forced to by a water pump which is still in good working condition. Your engine coolant thermostat sits between the engine and the radiator, in a thermostat housing. The thermostat blocks the flow of coolant to the radiator until the engine has warmed up. Once the engine is warm enough, usually around 200-220 degrees F, the thermostat opens and allows the coolant to flow. The engine coolant thermostat is the most common component of the cooling system to fail, which can cause your car to overheat in the unlikely event it were to be stuck closed. More commonly, the thermostat can be stuck open, making it difficult for your engine to warm up. This can result in your car running cool, causing an overly rich fuel mixture and an inefficient running engine. It should be replaced anytime you have major work done to the coolant system.
