The mechanical water pumps on BMW’s M5x series 6 cylinder engines have a horrible reputation of being a weak point in the cooling system.
Often they have to be replaced again in a very short space of time after a failure, due to worn bearings/leakage, which prompts people to opt for the upgraded Stewart pump which has larger bearings.
Careful examination of these situations has revealed that on the engines without a mechanical cooling fan bolted to the water pump that they have been lasting up to 150K miles…..As can be seen in the one on left in the upper right photograph.
Engines that use a mechanical cooling fan, with a viscous fan clutch , bolted to the water pump are more prone to failure. When the viscous fan clutch starts to fail and leak out it’s fluid-as shown in the photograph in the upper left, it can go out of balance, causing vibration and wear on the pumps bearings.
It is our recommendation that any time a mechanical water pump is changed, that the viscous fan clutch should be checked, and if any leakage is found, it should be replaced. This will help prevent premature wear on the replacement pump.