[RKDinOKC]

On the 928 motor most recommend you replace the waterpump when you put on a fresh timing belt, 60K miles. The impeller rotates againts the block. When the waterpump bearing goes the impellers tear up the block. There is an aftermarket waterpump that has plastic impellers to try to fix the problem, but the bearings in that pump don't seem to last as long as the timing belt replacement interval.

The waterpumps on the Cayennes seem to last around 140Kmiles. They start leaking from the shaft bearing. Usually just enough that it gets smelly, but not enough to effect cooling or get a low coolant warning.