I've been chasing a squeal at startup on my 93 300e 2.8 for way too long now. New belt, new tensioner, new idler pulley, and I still get a horrendous squeal for the first couple of minutes. I've disconnected the air pump clutch leads, and I think I've isolated it finally to the air pump clutch itself. When disengaged the outer pully rotates around the inner clutch, and I think I have some sort of interference there until things get toasty. I want to remove the air pump and run it without it for a few days to really isolate it as I'm tired of routinely replacing every single rotating part.

So, does anyone have experience with running a M104 engine without the air pump? Any idea what length belt would then be needed?

Typically the problem here is the bearing in the clutch. If the squealing stops after 1-2 minutes after a cold start, then it most likely is this bearing. There are threads on replacing this bearing, a chinese one that is easily available. I went the other route and bought a rebuilt air pump. I am starting to get a little noise already and it has only been a year or two, so this bearing isn't the best. Either that or the rebuilt wasn't done very well.

1991 300CE
2000 C280

The bearing in the pulley is spinning all the time, so if the noise is only on start up for a few minutes, when the pump is engaged the noise is most likely in the pump not the pulley bearning. There is a check valve to keep exhaust gases from entering the pump, if it fails the pump will seize-up with rust. If that has happened the pump and check valve must be replaced. It also is normal for the pumps to make some noise when engaged, it is moving air both being drawn into and out of the pump. But should not be a belt noise. My name is Chris Brown, I own C & M Hydraulics we have been is bus. since 1994 and all we do is remanufacture MBZ power steering gearboxes, p/s pumps and air (smog) pumps with electro-magnetic clutches. We deal with only larger parts dist. however most online dist. can get our parts if you ask. if you have any questions feel free to call. 702.734.8090.

Chris

Thanks, guys. I pulled the air pump off yesterday and through a LOT of trial and error and trips to the parts store I found that a 6PK922 (2345mm) belt works best. I had read several threads here stating that a 2257 mm belt was the right replacement, but I can definitely say that on a 93 300e 2.8, if you remove the airpump but keep the same belt routing it takes a 92 to 92.5 inch belt.

So far so good, but I've gone a day or two without squeals before right after tightening the tensioner. I'll probably replace the pump bearing and put it all back on if this proves to be the long term fix. I don't think I'm going to notice any mileage or performance gains either way, so might as well do the right thing.

However, on a related topic here, I removed the tensioner assembly yet again to make sure that the tensioning element wasn't "spun", and it's fine, but what a lame design! Between rotating freely all the way clockwise and hitting the underside of the water pump stuff, and rotating all the way counterclockwise and hitting the power steering bracket or something, there is only about 1/2" of travel. Is this right? How are you supposed to tension much of anything with that?