Originally Posted by asawadude

BMW now sells an OEM replacement water pump for 3.0i with a composite impeller, replacing the previous metal impeller model which replaced the original plastic impeller model.

The plastic impellers would basically deteriorate from heat, contamination, ice, etc. All it takes is one strategic crack on a blade or the shaft and the impeller would start the process of disintegrating.

Waterpumps with the metal impellers are not always the answer. There are several aftermarket brands as well as some BMW out of date OEM stock. The impeller is solid metal and therefore much heavier; the bearings are not up so spec, so they wear out quickly. Watch out for the cheap brands - if you want a good pump with a metal impeller, go to a reputable source like Turner Motor Sports, Pelican, or Bavarian Auto, all of whom who sell the highest quality replacement parts.

The latest version of the OEM water pump uses a composite impeller. My independent mechanic says the composite pump has none of the issues as described for the 2 earlier models. His preference is to install the composite instead of the metal impellers. Based on what he told me, I put in a composite water pump in my '99 328i earlier this year. No issues thus far in 5 months.