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.