Americanmouse

iTrader: (7)

Hey Guys,
I have a 88 IROC thats been modified in just about ever aspect.
For the last couple of years maybe more, I've been stumped about a vibration that occurs around 45-50 mph. This gets worse the faster I go, and this is doing general acceleration. At those speeds to if I take my foot of the gas and just coast you can feel and almost hear the vibration through the whole car it seems. The vibration seems to go away once I drop below 45 mph. I've talked Dana at Pro Built and he doesn't think its the transmission which is rebuilt 700R4 that has the pro street kit I got from him years ago. I had the stock drive shaft balanced a few years ago, so I don't think that's it unless there's a bad u joint? The engine and trans mounts are also after market made out of polyurethane, could that be it? I have considered the 10 bolt rear which has been rebuilt and has a Powertrax unit in it, maybe the Powertrax is causing it? I also have considered my aftermarket LCA's which have 3 adjustment positions. I have it set to the highest position to raise the rear up more for clearance. Could this be causing a issue with the pinion angle that would cause the vibration? Other than that , I'm not sure were to go with this!
Any ideas suggestions would be appreciated!!!

1987transgta

This thread may be of some use to you
https://www.thirdgen.org/forums/tran...ison-pics.html

Americanmouse

iTrader: (7)

Thanks for the reply 87 transgta!
That was a very interesting thread, and I know I have energy suspension trans mount, even my engine mounts are energy suspension. I will definitely be removing that plate mostly this weekend.

I'll let you know outcome.

BrutalGTA

iTrader: (2)

I would also look at your strut mounts.

Americanmouse

iTrader: (7)

Im going to try and see if I can get a rubber mounted trans mount today, install it then go for a test ride. Also I do have after market drag struts but I cant remember what brand they are right now. Although I do remember reading some negative feedback about them sometime after I installed them.

-=Z28=-

iTrader: (3)

It could be a u-joint. If a series of needles (that are found under the cap of the u-joint) are dried out or rusty, it will prevent the joint working properly as the rear end/drive shaft move up and down. I had a drive shaft that did the same thing on my truck, took it to my balance guy and he said it was balanced fine. The u-joints felt great! It took me a while before I pulled a cap and looked in. I didnt think to!! They felt good and took grease. Anyways, 7 or so needles were all dried up, black sticky crud. Cleaned em and greased em and I was good to go.

What you are describing, to me, sounds like a drive shaft issue.

And, yes, the above about the energy can also cause noises/vibrations.

Barlow8869

iTrader: (1)

i had this problem caused by a bent rim where i had hit a pothole ... didnt think about it at the time i hit it because i almost never took my car out of town ... took it to my buddies shop and tossed it on the balancer (of course it was the 3rd one on) and it came back as bent

Americanmouse

iTrader: (7)

Thanks for the reply's!
I changed out the trans mount, and took it for a spin and the vibration is still there. I did have my tires balanced a year ago as I suspected that was the problem, but that didn't fix it either. I also had my drive shaft balanced 2 years ago as well thinking that was the issue, but the vibration is still there. I did check for play this morning at the u joint, but they seem pretty tight.
The needle bearing possibility is interesting. If I recall I have to remove the drive shaft to get at the bearing cap right? Also would it be better to just replace them if they are dried up, or does that require a new U joint?

paulo57509

Don't discount a bent wheel or one with excessive run out as a cause. I had a 2nd Gen that had a vibration issue like yours and it was a wheel with excessive run out. Re-balancing did nothing to help with this condition.

Spend an afternoon with your floor jack and spare. Swap the spare on in each corner and test drive it. If the vibration changes or goes away you've found the problem or someplace to concentrate further investigation. If the vibration remains the same you've at least eliminated the wheels and tires.

-=Z28=-

iTrader: (3)

Usually a bent wheel you can feel all the time tho and know what side its shaking from. Easy to check. rotate your tires. If it moves, you know your right on track.

Dont go making a bunch of changes. Change one thing. Worse or better or nothing. Next thing.

Yes, you will have to take the drive shaft out to look inside the caps. Careful as the needles can fall out. Need to be clean and greased and not missing any.

Yes, prob a good idea to have them replaced. I have seen u-joints that look fine on the outside, but on the in, the needles rusted to dust, allowing massive amounts of slop in the drive line.

I would start with the easy stuff and see if that makes any changes.

GL
-D

morgsie

iTrader: (1)

just because your tires balance OK on a tire machine doesn't mean they don't have a broken cord or some other internal defect that will cause vibrations. stay away from CHEAP tires if you want a decent ride and low noise.