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Old 12-01-2012, 10:12 AM   #27
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Join Date: Jul 2006
Location: north babylon NY
Posts: 2,573
My Ride: 99 323i
Originally Posted by WDE46 View Post
I just don't see how it's the water pump or the thermostat. The water pump doesn't just fade out. The impeller always works. The thermostat doesn't control flow to the heater core at all. The heat works if the thermostat is closed or open. In my opinion it has to be the valve or something in that part diagram I posted.

OP, can you tell us your cars water temperatures while at idle warmed up and while driving (at say 60-70 mph) warmed up? This will tell us if your thermostat is broken.
Sadly mistaken I'm afraid, and this will lead to an overheating situation. My water pump (only 2 years old) had this "fade" problem, where it would circulate and then not circulate coolant. I was finally annoyed and pulled it. Here is a pic of what happened:

The impeller was just sitting there while the shaft spun freely. And to reinforce this even more, the bearings in the pump were quieter than a mouse, so there wasn't any indication that the pump had failed! I found a cheap replacement at Pep Boys for I think $60 made by Bosch with a metal impeller. Good luck with your problem.
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