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Old 05-24-2010, 03:18 PM   #27
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Join Date: Jun 2009
Location: Boston
Posts: 720
My Ride: ZHP / E30 M3
Originally Posted by nathancarter View Post
Since the OEM pump changed from plastic to composite, the weak point is no longer the impeller, but the bearings. The new OEM composite impeller doesn't shatter like the old OEM plastic ones did.

So, your question should be more specific: Which one has the best bearings and bearing seals?
Anecdotal evidence suggests that the bearings on the aftermarket pumps aren't as reliable as the bearings on the OEM (reman) pumps. I know of no empirical evidence to back this up, though.

The Stewart pump, regardless of its "improved" pumping volume, has a lifetime warranty. That doesn't mean it'll last forever.
Unfortunately to scientifically answer this question we would have to take a bunch of cars, equip them with all different pumps then run them trough an identical test cycle over a few years and then watch the sequence at which they fail. Needless to say this is an impossible task. Stewarts lifetime warranty is a marketing strategy and while that in itself might not indicate fail safe bearings, they do explicitly say that the bearing have been redesigned to be better and more durable.

On my M3 there is no tensioner. The tension of the belt is adjusted by moving the alternator angle away from the crank pulley. I found that the life of the bearing is directly proportional to how loose this belt is. So the key is to make it as loose as possible just before it start slipping at high RPMs. Clearly with E46s we don't have that choice so a bad tensioner pulley could also have a significant impact on this. One would hope that with age the tensioner will get looser and not tighter but I wonder if manufacturing tolerances cause slight differences in the actual tension imposed onto the water pump bearing.

Given the relatively low cost of the pump itself I think the best approach might be choosing the OEM-Composite pump and changing it every 60K.
'04 330i ZHP - OEM Clears - ///M Strut bar - CoolantSnitch
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