As some of you may know, I've been grappling with this issue on my 1994 E320 for quite some time now, and I've managed to get some ways ahead in quelling some of it off, but so far I haven't been able to get rid of it entirely.
By "better" I mean that the vibration is now less severe, and starts to appear at higher speeds (it starts to get really noticeable - and annoying - at about 150km/h, but now not nearly as unbearable as it used to be.)
Generally speaking, it starts as a faint "rubbing" sensation, then develops into a resonating throbbing in the floor pan right under my left foot (when it's not on the dead pedal) and in the backrest of the driver's seat, along with a very slight wobble in the steering wheel, and then at higher speeds, it escalates into a vibration that seems to engulf the entire car.
Here's what I've done so far, in (a rather rough) chronological order:
- First off, when I first bought the car almost four years ago, I had replaced the OEM 15" wheels with (cheap) aftermarket 16" AMG replicas so I could run tires rated higher than H. At the time I thought that the vibration was caused by the El Cheapo wheels, but...
- I didn't have to worry about it for a couple of years because the car wasn't in my possession for that period (long story,) but the vibration really became an issue last year when I put the OEM wheels back on, after I finally found V-rated tires in the stock size on the local market (Bridgestone Turanza AR10).
- That had me looking elsewhere...
- When I first noticed it three years ago, I had the two rear differential mounts replaced (but not the third one at the top front of the diff.) Not much help, this.
- Much later (starting earlier this year,) I replaced a cracked flex disk at the transmission end. No change.
- I took out the prop shaft and had it checked at a specialist shop for the first time, which gave it a "pass" and said it's in "excellent shape."
- Then I replaced the centering bushings at both ends of the prop shaft, along with new front wheel bearings, which improved things by about 80%, but I still got vibes. So far, so good
- I also replaced all pulleys on the engine: tensioner (twice; first one failed,) main fan pulley (also twice; first one almost failed on a 1,000-mile trip to Amman, Jordan,) and idler pulley. In the course of all that, we found that the crankshaft pulley had been loose, so we tightened that. All of this did improve things, but not much.
- Then I replaced the center carrier, which had its rubber compressed downwards but still in good shape, with an OE spare part, along with the bearing. Nothing.
- I then took out the prop shaft for a second time and had it checked again by the same shop, about four months later than the first time, and the shop still gave me the same report: your prop shaft is fine!
- I had the rear bearings replaced three times: the first replacement bearings (Febi) failed after barely three weeks, one of the second replacements (Lemforder, IIRC) failed again a little later, so my mechanic in Riyadh "ordered" me to go get used complete rear wheel bubs from the junkyard, with which I duly complied. Good call there, but it still didn't resolve the original problem.
- After barely two months, the front wheel bearings (the new Febis I mentioned earlier) failed again, so in went new new ones, also Febis, but Made in Germany (as opposed to the Chinese ones earlier.)
- So, seeing as I did that I was quickly running out of options, I asked my old mechanics in Jeddah to have a go at it. I was a little surprised when one of them told me to go get a new transmission mount, but I went along for both curiosity and the desperation that was starting to creep in. I was surprised again when I compared my seemingly good old mount with the new one, and found that the old one had compressed by about 7mm. So in went the new one, which, again, improved things even more. The transmission now sits higher, which helps there being less of an angle between its axis and that of the prop shaft. Again, so far, so good
- But albeit much better, the vibration is still there. So I went ahead and replaced the perfectly fine diff-end flex disk just for good measure, but that didn't help at all - a total waste of money.
- Desperation finally having set in, I flushed even more money down the drain: yesterday I bought a set of brand new wheels, had them mounted and balanced, with the new set of tires I bought three weeks ago. No help there.
- Finally, after taking the car for a test drive yesterday evening, I went back to the shop that mounted the wheels and tires had the car up on a hoist. I took off the heat shield and had a look as the prop shaft's center carrier. Noticing as I did that it wasn't "sitting" correctly (it seemed to be mounted at a slight angle relative to the shaft,) I loosened the two bolts and pried it around with a large screwdriver to have it sit as the flexibility of its rubber allowed it to, and then tightened the bolts in that position. I took it for another test drive and found that that helped a great deal!
But the vibration is still there, albeit considerably less severe, and only gets annoying at higher speeds (about 170-180 km/h).
PS I scoured the local market for a used prop shaft, but couldn't find any (at least not in Riyadh,) and I was told not to bother searching, because I won't find one for my car; mine had the 14mm bolts, not the much more common 12mm variety. The local dealer asked for upwards of 5,000 riyals for a month-long preorder on a replacement, but frankly, the whole car isn't worth much more than that in the local market, so...
So, for now, two questions:
1. How should the center carrier be adjusted? Is there an "optimal position" for it?
2. What purpose does the big nut in the middle of the prop shaft do, exactly? I remember from my W123 days that it had to be loosened in order to take the prop shaft out of the car, but I never needed to mess with it when mounting/unmounting the prop shaft on my 124. Does it need to be tightened/loosened or anything like that? Can it play any role whatsoever in this whole vibration issue?