Vibrations at 80+

[OldZeuski]

My 95 gt has been sitting for 8 years and I finally decided to restore it. After all the work I've done, there is a vibration in the stick at about 80+, it gets pretty violent and is rhythmic/harmonic/resonating/pulsing, however you like to describe it. It comes on in 4th or 5th gear, and persists even if I shift to neutral and coast at 80+. it has nothing to do with the RPM. It reminds me of a worn U joint, but more on that below.

New parts:
aluminum driveshaft (helped a little)
U joints
Motor mounts and trans mount
axles (28 spline)
30k miles on the 3.73 rear ( we checked the backlash and changed fluid )
brakes
Harmonic balancer (again, not RPM related)
wheels and tires all balanced dynamically

pinion angle is 2* down, the car is lowered but this vibration didn't exist 8 years ago.

I am wondering if there may be an internal issue with the t5, maybe the tail shaft bushing, or a bearing. However, it shifts smooth and makes no noise other than a little bit of whirring from time to time. The harmonic weight on the pumpkin is still in place, I've heard this is designed for the 2.73 ratio and may cause issues after a gear change, but this issue didn't exist with these 3.73's 8 years ago before the car got parked.

Any ideas?

 

[Terrorist 5.0]

My 95 gt has been sitting for 8 years and I finally decided to restore it. After all the work I've done, there is a vibration in the stick at about 80+, it gets pretty violent and is rhythmic/harmonic/resonating/pulsing, however you like to describe it. It comes on in 4th or 5th gear, and persists even if I shift to neutral and coast at 80+. it has nothing to do with the RPM. It reminds me of a worn U joint, but more on that below.

New parts:
aluminum driveshaft (helped a little)
U joints
Motor mounts and trans mount
axles (28 spline)
30k miles on the 3.73 rear ( we checked the backlash and changed fluid )
brakes
Harmonic balancer (again, not RPM related)
wheels and tires all balanced dynamically

pinion angle is 2* down, the car is lowered but this vibration didn't exist 8 years ago.

I am wondering if there may be an internal issue with the t5, maybe the tail shaft bushing, or a bearing. However, it shifts smooth and makes no noise other than a little bit of whirring from time to time. The harmonic weight on the pumpkin is still in place, I've heard this is designed for the 2.73 ratio and may cause issues after a gear change, but this issue didn't exist with these 3.73's 8 years ago before the car got parked.

Any ideas?

View attachment 43968

That is a beautiful car you have there. My car has similar symptoms when under some load, and I have chalked it up to internal transmission wear.

The transmission shift forks have a sort of padding on them to dampen any vibrations that can transmit up through the stick. As they wear, and with shifting over time, they can wear down and even fall off. When this happens, you can get some vibrations and play in the stick.

I made a post almost a year ago after I put a Tri-Ax handle on my stock shifter (that way I can have the more direct feel without the annoying clunkiness and tightness of a short throw kit) and started experiencing a very violent vibration at high RPM and load. I never fixed the issue, only dampened it. I had a bit of a vibration before, but the rubber block in the stock shifter handle took care of it pretty well.

You didn’t mention though, are your tires the same ones that were on the car while it was sitting? The way you word it doesn’t make it seem that way, but if they are they could have a flat spot. Wouldn’t produce the exact symptoms you mention, more so it would shake the car as a whole, but always a safe thing to check.

What I have mentioned above is mostly when the car has load and is a little up in the RPMs. In your case, sounds a lot like a bent shaft, or incorrect angle. You could try taking the driveshaft dampened off, and see what happens?

 

[OldZeuski]

That is a beautiful car you have there. My car has similar symptoms when under some load, and I have chalked it up to internal transmission wear.

The transmission shift forks have a sort of padding on them to dampen any vibrations that can transmit up through the stick. As they wear, and with shifting over time, they can wear down and even fall off. When this happens, you can get some vibrations and play in the stick.

I made a post almost a year ago after I put a Tri-Ax handle on my stock shifter (that way I can have the more direct feel without the annoying clunkiness and tightness of a short throw kit) and started experiencing a very violent vibration at high RPM and load. I never fixed the issue, only dampened it. I had a bit of a vibration before, but the rubber block in the stock shifter handle took care of it pretty well.

You didn’t mention though, are your tires the same ones that were on the car while it was sitting? The way you word it doesn’t make it seem that way, but if they are they could have a flat spot. Wouldn’t produce the exact symptoms you mention, more so it would shake the car as a whole, but always a safe thing to check.

What I have mentioned above is mostly when the car has load and is a little up in the RPMs. In your case, sounds a lot like a bent shaft, or incorrect angle. You could try taking the driveshaft dampened off, and see what happens?

Thank you. It looked a lot worse about a year ago when I started restoring it. Long story short I didn’t own the car for the years it was sitting. I sold it to a guy who decided to abandon the car and let it sit neglected for 8 years in a parking lot.

The wheels and tires are new as of last month. But I was also running a spare set before that with the same issue.

All of the new parts I mentioned are new as of the last several months. This issue has existed ever since the day i got the car back on the road.

We did pull the engine at the start of the project to fix some strut tower rust, and to do the motor mounts. I’m wondering if maybe we got something wrong when putting it back in. I assume the trans mount would kind of force the trans to align with the pinion left to right. If that makes sense.

I’ve basically replaced everything but the trans itself so other than an alignment issue I’m starting to think an internal problem is all that’s left.

 

[OldZeuski]

Some photos of the car before I started fixing it. Someone smashed the quarter window and broke the door lock out.

 

[Terrorist 5.0]

If you’ve ever had the driveshaft out, did you make sure to mark it? With the work you’ve done to the car, you must have a pretty strong grasp on this. You’ve dug into your car more than I’ve dug into mine. However, have you ever had the driveshaft off? Did you mark it? I understand you have an aluminum driveshaft, so I guess no weights on it?

Just spitballing here, but if there are any weights, try removing your driveshaft and putting it on another way. May smooth out your vibration. Also, try taking your driveshaft to a driveshaft shop. If it is new this is unlikely, but if it is old, it may be slightly bent and out of balance. I’m really thinking it is driveshaft related.

 

[OldZeuski]

so the guy who let it sit put a used aluminum drive shaft on it before parking it. That was the only thing he changed after I sold it to him. That shaft was in fact bent which is why I replaced it first thinking it was the issue. I have no idea if he bent it or if he bought it like that, but I installed the new one and only noticed a slight improvement. The bent shaft was making the shifter wobble at much lower speeds, but these high speed vibrations have remained the same. For all I know that bent shaft may have damaged the tail shaft while I was driving it.

I totally agree that this all seems like the usual drive shaft issues. And I would happily just ignore them if i wasn’t going to be driving this car at those speeds as often as I do. But the vibration Is bad enough that I worry something is going to grenade on me on the highway or when I’m making a pass at the drag strip. I’m thinking I might yank this t5 and let someone who knows how to inspect them give a look over.

 

[Terrorist 5.0]

so the guy who let it sit put a used aluminum drive shaft on it before parking it. That was the only thing he changed after I sold it to him. That shaft was in fact bent which is why I replaced it first thinking it was the issue. I have no idea if he bent it or if he bought it like that, but I installed the new one and only noticed a slight improvement. The bent shaft was making the shifter wobble at much lower speeds, but these high speed vibrations have remained the same. For all I know that bent shaft may have damaged the tail shaft while I was driving it.

I totally agree that this all seems like the usual drive shaft issues. And I would happily just ignore them if i wasn’t going to be driving this car at those speeds as often as I do. But the vibration Is bad enough that I worry something is going to grenade on me on the highway or when I’m making a pass at the drag strip. I’m thinking I might yank this t5 and let someone who knows how to inspect them give a look over.

I would say in that case yes. If you are willing to take the trans out, let someone look it over. That seems in my head at least the next culprit.

 

[Joeminnesota]

Before you pull trans or anything major - 3 things. Could be rear u-joint to yoke being 180 degrees out (kind of doubtful). Could be rear input yoke (doubtful). I’ve experienced bad vibration like this at high speeds due to incorrect pinion angle. If you have adjustable rear control arms (to adjust angle after lowering) also check that your rear/yoke is centered in the car. If in doubt, check center and tweak the pinion angle a degree or two and see if the problem is worse or improves. I would almost bet it’s pinion angle due to lowering.

 

[OldZeuski]

Before you pull trans or anything major - 3 things. Could be rear u-joint to yoke being 180 degrees out (kind of doubtful). Could be rear input yoke (doubtful). I’ve experienced bad vibration like this at high speeds due to incorrect pinion angle. If you have adjustable rear control arms (to adjust angle after lowering) also check that your rear/yoke is centered in the car. If in doubt, check center and tweak the pinion angle a degree or two and see if the problem is worse or improves. I would almost bet it’s pinion angle due to lowering.

I have checked the pinion angle, and the pinion is lower (opposite but 2 degrees beyond parallel) than the trans. So I know it’s not good, but I’ve seen worse. However, I have no idea how to check for the centering of the pinion (left to right). I was wondering if maybe my trans/motor mounts caused the engine&trans to be crooked just a few degrees.

It has new OEM upper control arms and new UPR lowers, non adjustable. The torque boxes look fine. But I would not be surprised if I screwed something up angle wise without noticing, either at the engine/trans or the diff.

 

[96blak54]

4 jackstands and some much needed caressing the under side. Could be as simple as something loose or not properly aligned and tight. Just get under there and try to move things around to see what gives. Your eyes might catch something you didnt notice before. Hope you figure it out!

 

[RAU03MACH]

All your bolts tight on the drive shaft
And transmission the whole center line
If vibration is in the middle of vehicle

 

[OldZeuski]

All your bolts tight on the drive shaft
And transmission the whole center line
If vibration is in the middle of vehicle

Bolts are tight. I’m not sure what you mean about the transmission center line but the vibration is in the stick, and it gets transferred through the trans cross member just a little because I can feel it in my seat, but not nearly as intense.

I’m not dismissing anyone’s ideas here just filling in what I know as you guys suggest things.

Car is going up on jacks this evening.

 

[96blak54]

We spitballin ideas. Its easy to forget or overlook something. Im one to tighten and retighten/bolt check everything. Its redundant but thats me.

 

[OldZeuski]

Trans is 2.8 degrees down from level. Pinion is 2.2 degrees up from level. Drive shaft is 2.4 down from front to back.

These all seem like good numbers to me. Going to pull the shaft and see if I can figure out the center line.

 

[OldZeuski]

Scoring on the brand new yolk. I’ve got a tail shaft bushing and puller already so I think I’m gonna replace it while I’m here.

 

[OldZeuski]

 

[RAU03MACH]

loose clutch bolts will do the same thing
loose transmission bolts will do the same thing
transmission mount
motor mount
all in center line category

 

[Terrorist 5.0]

These are all suggestions that mostly crossed my mind. That’s why I love forums.

 

[RAU03MACH]

harmonic balancer will also give some vibration if its coming apart