High Mileage Bad Vibration(s)

My '91 350SD (450,000mi) has developed a come-and-go vibration under acceleration - cruise is smooth, engine runs like a top, no wrecks, no rear-end problems. It feels like it's coming from the below the rear seat
area - the effect is like running over slowdown warning grooves in the road. I assume the problem is driveshaft U-joint(s) which, mechanic says, means new/used or reman driveshaft. Anyone with high-mileage driveshaft advice, similar experience (or a driveshaft to sell)? Dankeschoen in advance.
(ps - this is a 350SD that hasn't been a nightmare - gets 25mpg at 75mph cruise, no leaks, no gasket problems, uses 1 qt every 2500 miles. I love the car in spite of its worrisome reputation and want to keep it.)
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How are the tires?
Have they been balanced on the car and has it been aligned recetnly?
A good mechanic doesn't suspect bad halfshafts, he tests for them.
Put two adults in the back anfd load up the truck with heavy stuff. Go for a drive. Is it WAY worse now? Can you hear a click click sound?
Take the wheel apart and rotate the halfshalft through 360 degrees. You'll know if you have a bad one by the awful noise it'll make in one position.
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Thanks for repy - tires are good, balanced, rotated, aligned. The problem began with one set of tires and has continued into another set, so I rule out tires. Drove 700 miles north this weekend with one passenger - returned 700 south with trunk/backseat heavily loaded. No effect on the vibration - it continues to occur only during acceleration and comes with a subtle "bbzzzzzzzt" sound (or resonance); feels like running over a strip of closely grooved pavement; there is no "clicking" sound. Duration of vibration is about 1 second or less per gear on the upshifts and does not happen under very gentle acceleration.
Mechanic checked shaft and supports last Spring and replaced worn or cracked bushings in the supports (small improvement for a while); said flexdisc(s) had minor age cracks but were not the likely cause; now he says one of the U-joints is binding a bit when he twists it and he wants to replace the driveshaft - new $900 from MB - no rebuilt to be found, nobody in Raleigh who will rebuild one. I am comfortable with his judgment - any remanufactured shafts anywhere? Mechanic says he has used junkyard replacements in the past and does not recommend them for me (or anyone).
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Well, your mechanic seems to have seriously done the leg work to pin this down to the U-joints, and he has replaced the easier more normal wear items on the shaft so U joints it is.
Technically, the U joints on my mercedes were not rebuildable either. As a matter of fact getting the joint itself was seriously difficult, but there are shops out there to do it. On my car the joint is peaned in when a piece of metal on the shaft is bent over the U joint and beaten into place to hold the joint. It's really only meant to happen once, but on my car we did it twice. I can believe that you can't find anyone to touch that.
I disagree about not using a junker for this. Alot of these cars died prematurely and therefore have around 100K miles or a little plus on them in mileage. The U joints are not serviceable on these vehicles so there is no maintenance issue. You should be able get the same 400K miles out of a salvaged part that you got out of yours. The only thing is that you have to have a driveshaft shop to dynamically rebalance the shaft with the new piece before you can reinstall the shaft, but you'll have to do that with a new piece anyway.
Me, I'd get a salvaged part. I think that it pays here as that $900 is alot of dosh. I also wonder if that's the whole shaft or just the rear half? I'm betting that it's just the rear half of the shaft that he wants to buy because that's what has the U joints in it.
Just my thoughts. I am also not besmirching your mechanic. The guy must be great to have kept this car going so long so well, and the choice to use a junk salvage part always carries the risk that you will be back doing this again soon. He's just trying to make sure that you don't end up unhappy if the part were to fail, so if you wanted a salvage part it is definitely you accepting the risk. Me . . . I'd do it. It might not be the right choice for you.
Good luck with that car!!! They're great and congrats on that mileage.
-H
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I think that's about they way he put it - mostly they're fine and sometimes they're not... and he knows I'll bop down another 50K miles this year. I'm thinking about it tho' - it's expensive, that new one. Mech said he can get the 2-part shave redone in town, but the shop won't do mine (3-piece?). I didn't ask why. Know someone who I can call in your area? For the right price, I'd strip it and ship it.
Thanks for the guidence.
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Mercedes has used three piece drive shafts on their cars off and on for decades. It was an option on my '72. Some cars had them and some didn't. They have some advantages in quietness and smoothness, but the disadvantages in mechanical complexity often off set that. I have no reason to believe that your mechanic is wrong and you most likely have a three piece shaft. I can also believe that noone wants to touch it, but not particularly because it's a three piece, mercedes shafts are more difficult to rebuild than most.
I'm afraid that I don't know who would do it for you. Mine was a two piece shaft, and my mechanic had to lean on his guy to get it done. It's his realtionship and not mine, and therefore I can't recommend anybody. I'm really sorry about that.
Balance the cost of labor for installing the shaft against the cost differential between the new shaft and the salvage part with potential risk of failure. Seems to me that you shouldn't have too much of a risk of failure, but it's your analysis not mine. There is always a risk using salvaged parts.
Best of Luck -
H
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<!doctype html public "-//w3c//dtd html 4.0 transitional//en"> <html> Check the flex disc(s).&nbsp; When were them last replaced?&nbsp; The front one usually bears more torque/damage than the rear one. <p>Enso wrote: <blockquote TYPE=CITE>My '91 350SD (450,000mi) has developed a come-and-go vibration under <br>acceleration - cruise is smooth, engine runs like a top, no wrecks, no <br>rear-end problems.&nbsp; It feels like it's coming from the below the rear seat <br>area - the effect is like running over slowdown warning grooves in the road. <br>I assume the problem is driveshaft U-joint(s) which, mechanic says, means <br>new/used or reman driveshaft.&nbsp; Anyone with high-mileage driveshaft advice, <br>similar experience (or a driveshaft to sell)?&nbsp; Dankeschoen in advance. <p>(ps - this is a 350SD that hasn't been a nightmare - gets 25mpg at 75mph <br>cruise, no leaks, no gasket problems, uses 1 qt every 2500 miles.&nbsp; I love <br>the car in spite of its worrisome reputation and want to keep it.)</blockquote> </html>
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Mechanic says he observes minor age cracks, but nothing to validate replacement.
the flex disc(s).&nbsp; When were them last replaced?&nbsp; The front one usually bears more torque/damage than the rear one. <P>Enso wrote: <BLOCKQUOTE TYPE="CITE">My '91 350SD (450,000mi) has developed a come-and-go vibration under <BR>acceleration - cruise is smooth, engine runs like a top, no wrecks, no <BR>rear-end problems.&nbsp; It feels like it's coming from the below the rear seat <BR>area - the effect is like running over slowdown warning grooves in the road. <BR>I assume the problem is driveshaft U-joint(s) which, mechanic says, means <BR>new/used or reman driveshaft.&nbsp; Anyone with high-mileage driveshaft advice, <BR>similar experience (or a driveshaft to sell)?&nbsp; Dankeschoen in advance. <P>(ps - this is a 350SD that hasn't been a nightmare - gets 25mpg at 75mph <BR>cruise, no leaks, no gasket problems, uses 1 qt every 2500 miles.&nbsp; I love <BR>the car in spite of its worrisome reputation and want to keep it.)</P></BLOCKQUOTE></BLOCKQUOTE></BODY></HTML>
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Sounds like a bad drive shaft to me too. I have not loked at the drive shaft of my father's '91 diesel, but my '72 exhibited similar symptoms before being rebuilt. The tale-tell for me was a lag on acceleration from a dead stop as all of the sloppy parts loaded up before going. There was also a whining noise as well as some clunking at high speed and a general sound of things rattling around down there.
Parts to look out for besides the u-joints on the '72 (and I have no reason to believe that this is different on a '91 since drive shafts are fairly universal) are the rubber flex disc, center support bearing, and possibly a support bushing.
One weird thing on the '72 was the way the U-joints were pressed into the rear half of the drive shaft. It was not the usual snap rings that I was expecting and my mechanic was fopced to send it to a driveshaft specialist who pressed new joints into the old shaft. This was far cheaper than getting a factory replacement or even new used. Further, if your car has this many miles on it, how's the rearend? It could be some of the rear axle components loosening up as well, and check for leaking oil back there just to be on the safe side.
As for keeping the car. Good lord, if it hasn't blown up yet, you must have gotten a good one. I doubt if my father's will be capable of topping 175Kmiles the way that it's running.
Sorry that I can't give you any specific advice, but a driveshaft is worth rebuilding, and my '72 feels and handles like a new car with its repaired.
Have a good one -
H
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<snip>

Thanks for kind reply - mechanic says nobody in town (Raleigh) will/can rebuilt the 3-part shaft. Looking like a new shaft is in my future unless someone knows who can rebuilt this one.
Rear end is quiet and very dry on the outside (minor dampness around the seal). An axle boot just tore up and I had it replaced - everything else appeared in good order. Never had a car I liked this much, kept this long or drove so far.
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