Ok, here's where I'm at. My 87 GTA with 54,000 miles has a hellacious vibration problem at 70mph+. It "moves" at 80mph, meaning the dashpad shakes furiously but I can no longer feel it in the car itself. The vibration is not in the steering wheel at all at any point. It is not related to anything suspension or wheel related as the vibration exists when the car is up on the lift. The rear end has been eliminated as well through extensive testing.

The driveshaft is the obvious call here, except that I have had it balanced, balanced again, and rebalanced a third time. We swapped shafts with an aluminum one (that, granted, could have also been out of balance) but the vibration persists. Before I go and spend $300 on a brand new driveshaft which I have yet to do, I want to know any other possible culprits. Whatever it is, it's from the driveshaft forward. Is there something transmission-related that could cause this? It happens no matter what gear I am in (700r4). All motor and transmission mounts have been checked and look to be in great condition. I'm just trying to cover my bases here before I spend a chunk of cash.

 

have the u-joints been replaced at all?

 

U-joints have been replaced. motor and tranny mounts are in good shape.

What I'm noticing with more driving is that the vibration is not constant. Sometimes it's horrendous at 75mph, then it vanishes. Increases a bit on inclines, decreases on declines. My mechanic doesn't want to do anything else regarding this issue, I've had it to him three times and I could fill the rest of the page with the stuff he's replaced, checked, rechecked, etc. And he feels that when it IS there it's not bad enough to warrant anything more extensive. I suppose I could find someone else to hound about it but I don't trust anyone else with my car. I guess I'll just suck it up and deal with the intermittent shake at highway speeds.

 

In my case it was a cracked flywheel , busted 2 before I got a good 1

 

Try dropping the drive shaft and just revving the engine. This will possibly eliminate the trans. If the vibration happens at highway speeds and not at lower speeds then it's not engine related and exists somewhere from the transmission back. If it’s the engine then it would do it in a fixed RMP range instead of a speed range.

If it's not in the steering then odds are it's not in the front wheels either. Have you checked the rears to make sure they're both balanced correctly? A weight might have fallen off.

 

Could be a bad torque converter. Next time you're driving on the highway and the vibration is happening, unlock the converter and see if it goes away.

Unlock it by keeping your speed constant with one foot and tapping the brake with your other foot.

Your symptoms sounded nearly like mine when I figured out it was a torque converter that was messed up.

 

I'm going to buy a new driveshaft tomorrow. I need to know where to buy one from that will send me everything I need to just swap them, preferably with new u-joints. The ones I found on summit don't indicate that the u-joints are for a stock 700-r4, though I'm thinking they are. I could get one from spohn but it seems like overkill, the car is bone stock save for a chip and a flowmaster. I just want a nice driveshaft that won't shake my car when I drive at 70+mph. I'm not interested in used ones mainly because I don't want to go through the trouble of swapping them to discover the used one is also nerfed.

 

I think your wasting your money buying another shaft. If you tried a different one and got the same results, the odds that both are bad is pretty high.

Did you check the pinion angle? If you have the stock torque arm, maybe the bushing at the tranny end is not 100% and cuasing your diff to twist up and messing your pinion angle.

 

Well, sure enough, it was the driveshaft. I can't for the life of me figure out why my mechanic advised against buying a new one, i guess he figured the same thing. The old shaft came out and written in nice big yellow letters was "shaft bent" and it had a ton of weights on it. Threw in the aluminum one, BIG difference. Car feels smooth as butter all the way up to 90mph (gets a bit choppy there but my tires need air and maybe a little balancing). I'm just thrilled I can cruise at 60-80mph now and not shake like a washing machine.

 

How about a vibration that occurs when the car is in park and revved to 1,500 - 1,600 rpm's?

Any ideas?

The car does have a vibration while driving, but it can be found while it is sitting still as well! Above or below that 1,500 - 1,600 rpm range, it is fine. What might be in that harmonic range???