BMW models before the mid-1990s typically overheated because of failing fan clutches. The fan clutch is in the hub of the engine driven fan . It uses thick silicone oil and a bimetal strip controlling a internal valve to allow the fan to slip more when cold. Over time a poor seal allows the silicone oil to leak out, resulting in a fan that appears to be working but spins too slowly to cool the engine when hot.
Models after the mid-1990s improved the fan clutch seal, but many engines were shipped with water pumps that fail without external symptoms. BMW changed from a metal impeller to a lighter, more accurately shaped, non-corroding plastic composite impeller. With long exposure to hot BMW-specified antifreeze this composite would degrade, eventually breaking apart into the engine. Once the impeller breaks the engine rapidly overheats. Combined with the computer-controlled engine temperature indicator that holds the needle in the middle position until the engine starts to overheat, the driver has very little warning before the engine is dangerously overheated.
First answer by ID2044629076. Last edit by Djbecker2. Contributor trust: 9 [recommend contributor]. Question popularity: 16 [recommend question]
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