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Old 04-18-2010, 09:33 PM
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Replacment impeller for Oberdorfer not making the grade

Last year I bought a replacement impeller for the Oberdorfer water pump on my late model A4. At the time I noticed that it was about 1 mm shallower than the old impeller. (By that I mean that if you laid the impellers on a flat surface, the old impeller was about a mm "taller.")

I didn't think too much of it at the time, but when I replaced the impeller I right away noticed that it didn't push as much water as the old one, and at low idle in gear the engine got noticeably hotter than usual. I mentioned this here at the time, and Don recommended soaking the new impeller in some MMO and trying again.

Well, I did this, but then I put the new impeller in a ziploc bag in my toolbox and left the old impeller in for the rest of last season, and happily motored the rest of the year. At the beginning of this season I replaced the old impeller with the new, MMO-soaked impeller, and for a while things seemed OK. I still didn't think it was putting out as much water as the old one, but the engine temps were good and since I'd put in a bypass valve I figured that I could always send more water through the block with that.

Now we come to today. I was down at the boat upgrading some rigging (imagine that -- working on actual sailing stuff!), and at the end of the day I started up the engine to give her a warmup, since it had been a week or so since I started her.

Now usually I open the raw water thru-hull first (because she always starts right up), but this time for some reason I forgot. So I started her up and listened to her for a couple of seconds, pushed in the choke and then remembered I hadn't opened the valve -- so I dived below and cranked it open. No more than a minute had gone by.

I noticed that the temp gauge was off the peg already, but I put this down to running dry. I looked over the transom at the exhaust and noticed that it was definitely putting out less water than usual, and the temp was already closing in on 180, when it usually has to run in gear for nearly a half-hour to get that warm.

So I thought great, I've trashed the new impeller by running it dry. But I still had the old one (still looking great: no lobe memory, no cracks, very flexible), so I figured I'd pull the new one and if it had gone, I'd put the old one back in.

When I pulled it out it looked fine; not a thing wrong with it. So, since the old one still looked great I put it back in, and when I fired her up it gushed water as of old and stayed cool.

So, at the end of this long tale, what's my best course? Could it just be that I got a slightly smaller impeller from an odd batch, and that another one would work better? Or have the impellers changed over time so that a 2010 impeller just won't work in a 1975 A4? I'm mighty tempted to just buy a new water pump, but it gripes me to think of laying out more $$ because replacement impellers won't work for me.

One other thing I did notice: when I put the old impeller back in, it extended beyond the pump housing exactly the amount that the new impeller was short, and tightening down the screws compressed everything into place. It seems hard to believe that such a small difference in height could make such a big difference in water flow, but I can't think of anything else that could cause it.

I'm going to keep running with the old impeller, since it looks and feels fine; but I need to resolve this issue for good.

Has anyone else with a mid '70s A4 had any problems at all with replacement impellers in an Oberdorfer water pump?

Thanks!
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