Have you ever noticed how you can spin your BMW engine fan when your motor is turned off, even though it appears to be solidly bolted to the engine? That's because of the mushroom-shaped device that fits between the fan and the engine, called a BMW Fan Clutch. Your BMW fan clutch uses a neat property called viscosity to either drive the fan or let it freewheel, depending upon the temperature. The BMW fan clutch is filled with a silicone fluid that thickens when it gets hot and thins when it's cool. When it's hot and the fluid thickens, the fan spins with the engine due to viscous coupling. When it's thin, the fan freewheels so the engine doesn't have to use power to drive it. Make sense?

The thing that usually kills a BMW fan clutch is leakage. As it ages, the fan clutch develops hairline cracks and the silicone fluid escapes. When that happens, the fan freewheels all the time, since there's no fluid to create the viscous coupling. If your car is overheating and you can spin the fan even when the engine is hot (don't try this with the engine running!), you most likely need a new BMW fan clutch.