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Discussion Starter #1
Yet another GTV6 drive shaft balancing thread..

I'm trying to gather some ideas on a strange problem. A good friend has a GTV6, the drive shaft was rebuilt a couple of years ago, and was perfectly vibration free at all RPM's. More recently, he had the engine pulled to fix a leaky rear main seal, and since then the car has developed a vibration at around 2k rpm, and also at some higher rpms.

We put the car on the lift, everything is tight and snug. The rear and center drive shaft and donuts are all clearly marked as to their alignment, so obviously somebody took great care to realign things at some point. I could find no such markings on the front donut. I'm assuming that the front donut was disconnected to get the engine out and in.

So, what are some possible causes of this new vibration. I know that it's likely that the front donut is out by 120 degrees in either direction from when it went in, but why would that cause a vibration? If the engine was balanced before, and the drive shaft was as well, then rotation the drive shaft relative to the crank shouldn't matter. And if it does, it means that both the engine and drive shaft are out of balance, right?

I can try rotating the drive shaft relative to the crank, but I don't want to spend the time trying something that shouldn't matter. For what it's worth, we found neither lower motor mount nut had been installed, when we installed them on both sides that seemed to make a little difference. I also tried loosening the pinch bolt and running the motor for a bit, that may have helped a slight amount also.

For the record, this car has been meticulously kept, everything on the undersides looks almost new. All of the nuts, bolts, and washers are there, none are loose. Center support looks like it was replaced when the rest of the work was done, and is secure.

Any ideas are welcome.

Thanks,

bs
 

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Were the engine mounts changed? I believe a misalignment of the engine &/or transaxle will affect the smooth turning of the driveshaft.

When the rear main seal was replaced was the flywheel re-installed in its original position? Can it be re-installed in the 'wrong' position? Would that affect balance? (can you guess I haven't got a clue...)
 

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I doubt that it is the front Giubo. I broke the front yoke when the Giubo exploded and installed a spare one and a new Giubo and there was no difference in vibration. I suggest that you loosen the pinch bolt on the yoke, run the motor at 2000 rpm and then tighten the bolt.
 

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Discussion Starter #4
I suppose the flywheel could have been installed with a different orientation. But I'd like to think that you could change a flywheel without having to rebalance the entire crank/rods/pistons/flywheel assembly. People change clutches and pressure plates on these things all the time, right? That shouldn't affect the engine balance (one would think).

Ugh. If that's the problem, then hopefully the old hose clamp on the drive shaft trick will offset it.

bs
 

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Discussion Starter #5
I ran it with the pinch bolt loose, then tightened it. That did seem to help a tiny bit, but the vibration is still there.

bs
 

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Discussion Starter #6
Oh, forgot to mention - yes, I think one or both of the motor mounts might have been changed. Assuming that is the problem, I'm guessing the solution would be to loosen them, then try 'twisting' the motor one way or the other?

As with the flywheel, people change motor mounts on these things without having balancing issues, don't they?

Keep the ideas coming!

bs
 

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I don't think the motor mounts would change required driveshaft balance, no.

I had a similar problem with vibration at 2K after a driveshaft change and it was the pinch bolt adjustment. Hopefully you lubed that well on assembly with anti-seize so that it can slide. I'd loosen it and rev it up and down a bunch of times (preferably with the weight on the wheels...don't know if it'll make a difference but better to keep it in the driving condition and that's how I did it.)
 

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Discussion Starter #8
There is never seize on the splines. I didn't check to see if the front donut slides fore and aft easily with the pinch bolt loosened, that might be worth trying.

I did run it and retighten the pinch bolt with the car on the lift, so trying it on the ground might be worth a shot.

Do you happen to remember if there is supposed to be a washer on the pinch bolt? There wasn't one there. I tried adding one, just for chuckles, and it didn't make any difference. Seemed like a real easy way to play with weight, although only in that one particular place.

bs
 

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V6 flywheels, front dampers and crankshafts are balanced as an assembly, in contrary to the 4 cilinder engines which are separately balanced. So yes, the position of the flywheel relative to the crank does matter. If that causes the vibration though, I don't know. For me the transaxle driveshafts are always somewhat a mystery though. There are driveshafts with completely worn out donuts, all the engine/tranny mounts shot that 100% vibration free, and there are cases where everything is done how it should and it still vibrates.
 

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Good luck finding someone to balance the driveshaft with the flywheel attached. I wonder if anyone has been able to do that.
 

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Mounting holes on the flywheel are offset. It will only go in one way. If they pulled the driveshaft it might have been forced and bent?
 

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Having been through all of this (I'm the one who ended up using hose clamps in the end), I would play around with the engine to trans alignment. If the driveshaft was smooth before the engine was pulled and reinstalled and now has a vibration, it is much more likely to be an alignment issue. Greg Gordon's site has great info about this.

In the case of my GTV6's persistent vibration, I deduced that it had to be an actual shaft balance issue because shimming the engine and transaxle every which way barely made a difference in the vibration.
 

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Discussion Starter #15
Thanks!

I read your thread. When the car comes back, I will try to use a dial gauge to see if the drive shaft is bent in any way. If the car had sagging motor mounts, then shimming might help, but with new motor mounts, if anything is the problem the motor would be too high. But I'd think the height should be good with new motor mounts. It came from the factory that way, after all.

Thanks!

bs
 

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I could lend you my laser driveshaft alignment tool. It's not super accurate because the laser in its own casing isn't mounted true to its axis of output. But you can get the ballpark runout measurement. It's better than doing it by eye, or in most cases nobody even bothers to align the engine, center bearing mount and transaxle. The driveshaft could, at steady state, already be misaligned at all those mount points.
 

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Do you know if they completely removed the driveshaft from the car? At the rear there is a bearing that is mounted on the end of the driveshaft that is supposed to go inside the part from the clutch, I have had it get pushed further on to the driveshaft instead of going into the clutch section. I ended up with a slight vibration and getting that inside the clutch section fixed it.
 

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There is a bearing on the rear driveshaft that keeps the driveline centered. When removing or reinstalling the driveshaft it is very easy to damage this bearing especially if it is your mechanics first time doing this. Usually this center bearing is pressed into place. And just let you know the flywheel has only one mounting position. And I think the yolk has only one position or else the bolt won’t go through.
 

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Discussion Starter #19
I don't know, but I don't think they did. The marks on the rear look like they're untouched since the giubos were done, which was a few years ago.

You're talking about the splined section only having one position, right? Yes, that is correct.

bs
 

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I don't know, but I don't think they did. The marks on the rear look like they're untouched since the giubos were done, which was a few years ago.

You're talking about the splined section only having one position, right? Yes, that is correct.

bs
right near the center support where the two drive shafts meet there is a bearing ( not visible) that slides within the the two shafts. You have to angle the driveline when reinstalling or removing the driveline lots of times that bearing brakes very delicate piece. At low rpms the center donut or Guibo keeps the driveline centered, under load naturally the rubber flexes causing vibrations if that bearing is damaged. My most unfavorite job to remove and reinstalling the driveline on the transaxle cars. Hope this helps. If you need a complete marked driveline let me know.
 
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