I replaced my WP a couple of weeks ago. The car had the original black impellar pump & 76k miles. The impellar looked fine untile I looked at it with a loupe. The vains were fine with no rub marks. There are, however, three hairline radial cracks starting at the hub. They are spaces about 120 degrees apart. One of them lines up with, and extends, to one of the two holes. It looks like there is a metal sleve that is cast into the plastic impellar, and that this is press fit onto the pump shaft. I suspect that this press fit is too tight and expands the sleeve to the point that too much force is put on the plastic and it eventually fractures. The tolerance variability on the interference fit probably determines how long the pump lasts. I guess the white impellars are of a different plastic that are less likely to stress fracture. No question that the best thing to do is put in a metal impellar pump.
