1983 300CD

I swapped the water pump on my car because it was bleeding profusely from the weep hole. The replacement water pump seemed to match the old one exactly. During reinstallation, I noticed that the water pump pulley was rubbing against the dampener plate during part of its revolution and rubbing against the neighboring, outermost pulley during the other part of its revolution.

I pulled the pulley off, bolted it to the old water pump (which was not rubbing while it was installed), and clamped the water pump vertically into a vice such that the pulley could spin parallel to the ground. I got down at eye level and spun the pulley and could clearly see about 1/8" of up/down oscillation during the rotation. In other words, the pulley was bent.

I went to the junkyard and got a smaller pulley off a W126 car and did the same test only to find that it too had about 1/8" of up/down oscillation during rotation.

So now I'm wondering if the 1/8" is within normal range for such pulleys, or if I just got extremely unlucky and ended up with two bent pulleys. Again, the old larger pulley was not rubbing during its previous installation on the failed water pump. I don't think I was rough enough with it during disassembly to bend it, so the only thing I can think of is that the new water pump is slightly longer or shorter than the original--even though it appears exactly the same side by side. However the slot where the pulley rides, between the dampener plate and the outermost pulley seems to have such a tight tolerance, I can't understand how the original pulley with 1/8" play ever worked in that space.

I think the smaller pulley will fix my issue because it is too short to reach down into the tight slot, however, it will require smaller v-belts, which means disassembly of the power steering system, AC compressor, etc. in order to get the old alternator/water pump belts off. I tried installing the smaller pulley with the existing v-belts, but the alternator adjustment does not have enough range to accommodate. So I'd rather use my existing original pulley if possible.

Any thoughts on the situation? Thanks.

It isn't really clear from your description. Is the pulley wobbling in/out (towards and away from) in relation to the pump body? Or is it out of round?

If it is wobbling in/out, either the pulley is bent, the shaft on the pump is bent, or there's something behind the pulley preventing it from seating squarely on the shaft. Running it like that will tear up your belts and put undue strain on the pump bearings.

Try your test with the new pump and see if it also wobbles with both pulleys.

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Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

The pulley should not wobble. It is bolted solidly to the water pump. If the shaft does not wobble and the pulley is on straight and bolted solidly, it will not wobble either. Back the tension off the belt and examine how the pulley fits onto the shaft.

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85SD 240K & stopped counting painted, putting bac together. 84SD 180,000. sold to a neighbor and member here but I forget his handle. The 84 is much improved from when I had it. 85TD beginning to repair to DD status. Lots of stuff to do.

The original Water pump pulley is oversized for the space and rides behind the crankshaft pulley. The clearence between the two is tight, almost looking like they are touching. The trick is to slip the pulley behind the crank pulley, then mate it up to the waterpump flange.

I ran into the pulley interference when I replaced my water pump last year.

Hammerwerfer

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1985 300DT 241,000 miles
1979 300SD 202,000 miles

The wobbling is in/out in relation to the pump body. It is not visible while the pulley is on the car, which is why I mocked it up by putting the old water pump in a vice vertically and attaching the pulley such that it spins parallel to the ground and looking at it at eye level. In my mock up, the oscillation is up/down. In relation to the pump body, it would be in/out. I did this test with both pumps and both pulleys and I see the up/down oscillation in all scenarios which is why I'm concerned that I got two bent pulleys.

The pulley fits on the shaft just fine. It bolts down evenly and is flush. With no tension on the belt it is difficult to turn by hand for about half the rotation and nearly impossible to turn the rest of the rotation. The car won't start with the pulley installed because of the drag the pulley is causing. As a test, I took the pulley and fan off and the car fired right up...obviously I didn't let it run in this scenario for very long, but it did confirm that the pulley is obscenely tight.

It took me a while to learn this trick but that is what I am doing to install it. The nipple that protrudes through the center of the pulley is sticking out the same distance as it does on the old water pump and is symmetric within the hole.