blippy

Hi all, hoping someone can shed a light to my problems.

I'm getting a very pronounced vibration on deceleration ONLY (off gas and/or slipped into neutral) above 80 mph. Anything below 80 mph seems to be okay for the most part. Acceleration is smooth.
The sound is a low rumble, almost like what you would hear if you were to drive on a grooved shoulder (aka, the side of the freeway that makes a lot of noise when you drive on it). Felt around in cabin, seems to be more pronounced after gutting soft top.

I think this may have started after my gear install (4.77, from puddy) but either it's slowly getting worse, or I am driving at 80mph more often. Probably the latter as my commute sees more high speed driving recently. Could it have been the gear install (this was probably 5-6k ago). Diff fluid came out pretty clean on first swap with minimal shavings.

Differential mounts are in good order (less than 15k, OEM). Tires are almost new and balanced, and an alignment was done a couple of months ago. I've had the car on a lift and couldn't find anything to obvious.

possibly related mods:
- 4.77 geared diff, setup by Ben (aka puddy) himself.
- driveshaft spacers (solved vibration from pitting a few years back)
- replaced prop shaft (had my OEM prop shaft bored out to fit an AP2 pumpkin, but decided to go with an AP1 flange with the 4.77s)
- Lowered, eibach/koni's, though I doubt this is an issue.

Any ideas? Thanks

S2KPUDDYDAD

Is the prop shaft the AP2 and your using AP1 size bolts to hold to the AP1 diff flange? The differential it's self will not cause a vibration. A diff will whine if its going bad or not right. The issue I think you have is something is out of balance causing vibration at high speed. My guess is the prop shaft if it is not connected the the trans and diff using apples to apples parts and fasteners.

rob-2

Do you have hood dampers?

s2000maniac

What does hood dampers have to do with vibration?

blippy

No, it's an OEM AP1 shaft with brand new bolts and washers on both ends.
I figured there wouldn't really be much to cause a vibration, but thought I'd ask. Could it be a backlash issue? I tested it yesterday and it does seem to MOSTLY happen when I'm gassing it very slightly, just enough so I'm not accelerating or decelerating.. if that makes sense (again, above 80 mph).

There is no obvious whine, but the odd sound I noticed and talked to you about awhile back when I first installed the diff still persists. The fluid did look pretty clean with small shavings but nothing major.

blippy

No hood dampers.

realblag

Are you feeling ok?

S2KPUDDYDAD

In your first post you stated,
"replaced prop shaft (had my OEM prop shaft bored out to fit an AP2 pumpkin but decided to go with an AP1 flange with the 4.77s")

If you had to tight of a backlash you would hear it, you have no noise or whine, you have a vibration that is only at high speed above 80 mph. you could have a bad tire??

Is the AP1 prop shaft you are using bored so AP2 bolts will fit it too?

blippy

In your first post you stated,
"replaced prop shaft (had my OEM prop shaft bored out to fit an AP2 pumpkin but decided to go with an AP1 flange with the 4.77s")

If you had to tight of a backlash you would hear it, you have no noise or whine, you have a vibration that is only at high speed above 80 mph. you could have a bad tire??

Is the AP1 prop shaft you are using bored so AP2 bolts will fit it too?
[/quote]

Hi Ben
I meant my original prop shaft was bored out to fit an AP2 diff. After I replaced it with one that you built (ap1 flange), I replaced the modified prop shaft with an original AP1 unit just to be safe.
Basically, I'm running an unmodified OEM AP1 prop shaft with new AP1 bolts with the 4.77 diff.

When I feel the vibration, there is a "whirring" sound that accompanies it, but it's hard to pinpoint exactly where it's coming from or if it's caused by the vibration (or vice versa?).. especially with the clutch buzz I already experience.

Further testing shows the sound appears at around 80mph when lightly holding the gas, but also sometimes appears when I completely let go (also appears if I just stick it in neutral). The sound and mostly the vibration continues until the car slows down to about 73 mph, where it then disappears and the car is smooth again.
This isn't my video, but here's a similar sound to what I think I hear, albeit mine is more faint and muffled by my exhaust and clutch buzz (jump to 1:40 https://www.youtube.com/watch?v=tPkxAm_hf9k)

Inspected the prop shaft today, looked okay. No grease coming from the boots and all the weights from balancing look intact. Took off the prop shaft on the diff side and I didn't really see anything obvious. No leaks, diff oil level was good, and the flange itself doesn't seem to be damaged or loose side to side. I would've checked to see if the pinion nut was loose, but I don't have a socket that large. I reoriented the prop shaft hoping it might've just been a balance issue, but nothing changed.
I did notice some play when twisting the prop shaft with my hand (in neutral and rear wheels locked). Is this considered backlash? How much play should there be?
Here's a video of what I mean (not mine!)(https://www.youtube.com/watch?v=KxIpXVMMrM8) but my play is less, maybe about 2/3 of what that diff has. Is this normal?

I don't think it's the tires, they're less than a year old, balanced fine, no abnormal wear (and I would think that I'd be hearing it on acceleration also if it was the tires?)

Thanks again for the help and insight.

S2KPUDDYDAD

It is a strange issue you have and I'm betting it is something so settle that it would be easily missed. Something just tweaked enough to cause a high speed vibration but be gone at lower speed. You are very welcome to send the rear back for inspection. I can go through it to be sure everything is as it should be. If I find ANYthing I would think could cause your issue I will take care of it. No charge for the inspection or any adjustments even if I find nothing wrong