Vibrations at speed after rebuilding car. Coming from poly mounts or driveshaft?
 

I recently got my car running after sitting for 3.5 years. When I get to around 50, I start getting a significant vibration, that seems to get a little better after 70. I think its a little worse when decelerating, but not by a lot. It is independent of engine RPM. It feels like its coming from the back.

Over the coarse of the last few years I upgraded many, many things, but as far as this is concerned I think the only relevant thing is that I put in Himni poly engine and trans mounts, plus front poly bushings. I did check the diff mounts and rear bushings and they looked good. I have all new fluid in the transmission (ford motorcraft) and diff (RP 75-140 max gear).

From what I have read, it doesn't sound like its related to the poly bushings, but I wanted to make sure before I totally ruled it out. Based on what I have read, it sounds like the rear driveshaft u-joint. It was perfectly fine before I took the car apart, and the driveshaft has been quietly sitting on the floor of my garage for the whole time. Would it have somehow gone bad just while sitting there? Also, I didn't mark the location of the driveshaft before I removed it. Could that have caused a problem.

Also of relevance, I noticed that a boot on my axle is slightly torn. Could anything like this happen from a problem with a drive axle?

how are the tires? if the tire is out of round it'll do this stuff

Second the above. Sounds like a case of tires being out of balance.

Yeah, if its not the tires, check if your rear wheels get hot when driving. Could be bad wheel bearings.

Well now I feel stupid. Its been sitting with the front on jack stands for the past 6+ months, I bet that has something to do with it. I thought I was keeping the tires aired up, but they did get down to around 20, I think. It does seem like its better than when I first started driving it, so that matches up. I've put about 250 miles on in last week, and seems to slowly be getting better.

I've had bad wheel bearings and this definitely isn't it. There's no sound, just vibration.

The tires are old and need to be replaced, so I guess this will force me. I've got a set of 17" rims thats been in my garage for 2 years, but managed to spend the rest of my money on random miscellaneous stuff to get the car running. Maybe I can find a deal over Black Friday or something.

I've scratched the surface of the corvette world as of the past few years, and this is a common complaint by new owners on the forums with regularity.

The only other thing that causes a speed variable vibration like that is usually a U joint installed incorrectly, but that would probably be accompanied by some sort of related driveline noise.

My vote is out-of-round tires from sitting. In my experience it only takes about a year for them to develop a flat spot depending on the tire and temperature change in the garage or driveway.

No replaceable u-joints on the rx7, but they can get worn out. It's been down for 3.5 years, and it was definitely sitting for over a year at one point, I had just forgotten since it's been on jack stands since early '21.

a buddy of mine had a similar symptom, it would just vibrate sometimes, and it turned out to be the tires.
its kind of weird because my car at the time was all used junk and it was smooth as glass. my buddies car is a 30k mile car with everything in nice shape, but i guess it didn't like the tires, which were not even that old

I had a similar situation recently, at the exact same speeds. 50-70. Turned out to be bad ball joints and wheel bearings

Its definitely not the wheels, I swapped to a different set and no change.

Ball joints on the axles? That's what I'm wondering - how can I tell if its a driveshaft or an axle? I didn't see movement on either, but that's with it still on the car and trying with just my hands. Would I need to actually try prying it with something? FSM is pretty vague. I don't want to spend $400 on a new driveshaft and still have the same problem.

Switch the tires front/rear and see if any change.

Driveshaft and axle wear comes down to how much play in the joint. It's the 'click' when moving it by hand.
Is it coming through the steering wheel or do you feel it in the chassis? Generally steering wheel=front end, chassis=rear end.
If you raise the rear wheels and get it up to speed, does it vibrate? If you pop the clutch, does the diff move due to soft diff mounts?

There are cheap aftermarket ball joints, tie rods, wheel bearings, axles etc so you can start with those. Swap to better ones if they work.

Yeah, this definitely isn't wheel vibration, I feel it in the chassis in a different way from something on the wheel. This definitely feels like axle / driveshaft.

Mounts, bushings, tie rods, etc are all good, I had the whole rear end apart to replace them only to find out they were all fine. I'm honestly wondering if I didn't do something to one of the axles in the process of removing them, because when I took the car apart 3 years ago everything was fine.
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I know it sounds dumb, but have you jacked it up and shaken all the wheels?
While you're under there give the driveshaft a wiggle.

Next on my list! Gonna be a few days before I can get to it, though, I'll update once I try it.

One more stupid thing - I didn't mark the location of the driveshaft when I removed it. I didn't think it really mattered unless there were balance washers, but could that cause this?

Had a ‘02 Silverado howl at speed, turns out the axle shafts in the front were cooked. Maybe grab the ones in the back and try to rip them off the car, if the wiggle like heck there’s your problem. If it stops or quiets whenever you turn it’s definitely a wheel bearing.

There's no sound at all, just vibration. I've had bad wheel bearings before and that's not what's going on. Also, I already checked for movement and theres none.

If you put it up on jackstands and spin the wheels does it vibrate?

No, there's very little vibration until I hit about 50 mph.

this is exaclty what a failed u-joint on my old turbo driveshaft was. Little to no vibration at lower speeds and smoothing out past a certian speed. The u-joints are not INTENDED to be replaced, but they can be. Rockford makes replacemnt joints and bearing caps that secure in place with snap rings. I've replaced many u-joints with these and had no isses. You will need a press to press out the old joints, but otherwise it's actually a pretty easy procedure.

RX7 turbo is rockford part number: 430-11
RX7 non-turbo is rockford part number:430-10A

Installing the shaft w/o previous mark will not be a problem, and if the shaft was fine before, should be
fine now. Many years ago fought a vibration that was off/on and found to be a split r/s motor mount,
now that certainly isn't your problem with new mounts, but I installed Mazda comp mounts and after
the trans mounts were installed, a lot of vibration. They were in there 2 days and after replacement of
OEM new mounts no problem. If you have old mounts give them a try. As new trans mounts were
installed, I would also assume that the bushing in the crossmember is fine.
Keep us posted.

Very interesting, thanks for the info! I'll have to check that out. I'll probably end up just taking it to a driveshaft shop, though.

The shifter isn't really vibrating at all, so its probably not related to the transmission mounts. Definitely something I'll check out if the driveshaft checks out.

Been years ago, but the shifter didn't vibrate, just the opposite. As the mounts didn't absorb any vibration, the vibration became part of the entire shell, so back to OEM.

Vibration at speed poly mounts or driveshaft
 

I have a 1987 Turbo II and I had a vibration issue exactly as you described. I first tried a rebuild of the driveshaft universal joints but that did not fix the vibrations. I think that the folks who did the work on the universals were not up to what was needed to do the work properly for the car, I figure another repair shop with more experience would do a good job than just doing ordinary cars and trucks. I got a new driveshaft and it was butter smooth once again. That's my story, I can't say for sure that will be the same for you. Before the replacement there were vibrations that began around 45-50 mph, go away with a little speed and come back at 75-80. I hated the vibrations and it was great to get it fixed. Rotaries run so smooth, any vibrations are very noticeable.