BMW: water pump, coolant line, water jackets

Question
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Followup To Josh, It was a plastic impeller, new pump.  There is nothing on the shaft, I did notice when I replaced the thermostat there was some darkness to the antifreze. I just cant imagine that the whole impeller disintergrated in so small of pieces.
Question -
I replaced a water pump on a 98, 2.8, Z3. It ran for about one hour, and over heated. I replaced the thermostat, and it did the same thing. I checked the water, it was not pumping. After removing the pump, the impeller is gone!!. any ideas as to what has happened? I have flushed the block, raditor and nothing. Has it lodged itself in the water jackets? Any ideas, Iam at loss as it seems to vanished.  Thanks for any advice.
Answer -
Kevin,
 Was it a plastic impeller or a metal one?  Was this the original pump or a new replacement unit (guessing the latter).  I've never seen this happen, but its definately possible.  if it's plastic it probably broke up into many pieces, it it was metal it's probably intact in the block.  It could not have gone car if it was a metal impeller, there's not enough room for it to make it around the water jackets.

 let me know what the material is, and I'll find a solution!
 Josh

Answer
Kevin,
 If it was a plastic impeller it "could" have broken apart.  I'll admit I've never seen it on a BMW, but I have seen it happen on other vehicles.  You might try flushing with a water hose to see if you can dislodge anything, other than that I am not sure how you will get it out, besides removing the engine and flushing it out with the front of the engine pointed down, so the peices will come out of the water pump hole.  I dont think these pieces will hurt anything inside the block, but you dont want them getting inside the radiator as they will stop it up. and create more problems.  I guess you could fashion a screen to fit over the input side of the coolant line to catch any of the possible debris that might enter the radiator, and check it periodically, but I dont know how much that would impede the flow to the radiator.
 Also I would call the manufacturer and have them pay for removing the pieces (if possible).  I would try to find a metal-impeller pump to replace it with, it will only add security/reliability to the pump.
 Hope this helps,
 Josh