Boxstah986

Referring to your picture I see that Porsche finally put a metal ring to connect the fibre to the shaft. So even they believe that part of the impeller assembly should contain metal.

JFP in PA If there are only a 2 thousandths of an inch space between the engine face and impeller rear (I think you meant "impeller front") as you attest then the Porsche engineered pump is designed to fail and thus destroy the engine, regardless of the impeller's material substance. Instead, I believe the plastic impeller would loosen from the shaft long before the bearing wears enough to be torqued by the serpentine pulley to cause the abrasion you described. A more probable scenario would be: the plastic impeller will get lodged and stop turning, thus won't pump. The expensive engine will overheat and crack the heads, score the rings, etc. Also, both plastic and metal impellers have a slope to their design, so the bulk of the material geometry would not erode away due to abrasion as you suggest because the slope emanates away from the engine face. A metal shaft with a metal impeller is a much better design: it equates to similar material with the same thermal expansion coefficient. Also, plastic parts can break off in chunks to stop up a coolant block whereas metal parts will be tiny flakes . A coolant flush every now and then would wash away the metal flake risk and help lubricate the water pump bearing. I doubt if a flush would wash away any chunks of plastic. More likely, if the bearing wears enough to allow the shaft to wobble then the seal will leak, and you will notice the coolant fluid leaking from the water pump housing long before significant impeller parts are scattered throughout the coolant jacket. One can hear the rubbing noise if the impeller were metal. A plastic impeller would probably remain silent to the unsuspecting driver. This is why a well designed metal water pump had a hole in the housing so the fluid would leak out slowly to alert the unsuspecting driver/mechanic. Also, the impeller on the water pump would push the coolant outward thus thrusting the impeller away from direction of water flow (Newton's third law of motion - for every force action there is an equal and opposite force reaction. So, I can not buy into your abrasion theory. I'll stick to my fully metal water pump design on both my Boxster and M3!

If a Porsche brand pump is so great then why do they break? Are they using better bearings than everyone else. I do know that their fibered impeller approach is flawed.

Assuming the 2 front radiators have been cleared of road debris (must remove front bumper to gain good access), assuming the thermostat is new and system's new coolant fluid has been cleared of any air, and the coolant fans are working, then I would be suspicious about the "good" water pump. The only way to know about the state of the water pump is to remove and inspect. Quite possibly the impeller on the pump is not sufficiently tight on the pump shaft - if loose or damaged the impeller will not pump enough coolant to keep engine cool. For my car 98 boxster I ignored recommendations about replacing the water pump with a fibered impeller. Instead I ordered a Gates metal impeller water pump ($65) and that fixed my overheating problem. This is a common problem with the BMW's - on the bimmerforum most writers recommend a metal impeller for the water pump replacement. Also, by opening the coolant lid with car engine running and applying some short revving bursts one can see/hear the coolant fluid being pumped if the impeller is working. Another test - with the car warmed up, crawl under the engine and with your hand compress the inlet and outlet sides of the water pump hoses - they both should be warm/hot about the same temperature. If one hose is much cooler than the other then you have a bad impeller.