No, I did not disassemble either hub. Given the right side did that itself when I was going to do the bearings on the original unit, I figured nothing would get worse from what I had. And same for rear. However, I did check the 3-9 and 6-12 movement on both when back together and all is good.

I do not have a separate overdrive/TCC unlock switch or "sport" mode button. Trans is stock GM 6L80E (76k when installed, 78k total miles now) and there's limited control over it with the Jaguar Specialties kit. I am leaning towards something with the transmission now as well, though there aren't any of the traditional, overt symptoms showing. Reason for my thinking is that the behavior when I shift into neutral while at 50mph +/- almost feels like it's unspooling.... (Best way I can describe it)...and the vibration continues throughout. Interestingly, the engine now stalls when I do shift it into neutral and I have to stop to restart it. It did not do this during the tuning mileage as my tuner was going in/out of gear while in motion to get idle parameters set. So, stalling in neutral is a new thing and whatever is happening, it's been getting worse over the last 2k miles as we'd typically not see vibration until 80-85+ early on. I also had very bad rear suspension bushings and shocks then too. Just don't know enough to determine whether failing torque convertor would cause this extensive of a vibration so consistently. It's not like the TCC is locking/unlocking in appropriately, that sound tends to be "clangy" and metallic. This isn't...

There's little out there on trans vibrations with the 6L80E in Camaro Gen5 chassis, mostly trucks, and when there is, it's torque convertor related. The newer 8L90E units had issues with OEM fluid (pre '19 spec) that supposedly stopped driveline vibrations similar to those I've been experiencing.

My next step is to run the car at speed on jack stands to see if anything presents itself visually. I'll do it with/without wheels on the back end and report back. Will also continue digging into the interwebs for more info...

 

I just read through this saga of a thread - what a pain! Sounds like you may be on to something with the transmission though, hopefully that's the right path. I don't have much technical insight, but did do a fair amount of research on changing from 2-piece to 1-piece driveshaft in an IRS vehicle. A lot of CTS-V and GTO guys ditch the 2-piece for a conventional 1-piece, but don't always have good results in terms of NVH. The last thing I wanted was a vibration or harmonics so I chose carefully and ensured my pinion angle and output shaft angles were exact (2 and 2 degrees if I recall). Since the IRS doesn't move like a live axle rear end, there is always a possibility of a range of shaft RPM giving you trouble. In the CTS-V/GTO groups, it was about 15-20% that had issues, and largely with guys getting mail ordered shafts, that could have been damaged in transit. Since you've observed a change in speed it occurs at, may not be an issue for you. Just some food for thought, check the angles best you can either way to rule it out if possible. Good luck!

 

I appreciate the thoughts! I did initially go straight to the driveshaft as that's the best way of describing the vibration, or at least to confirm the u-joints hadn't up and left early in their new life... I did confirm the driveline angles were not only in line with how the kit was engineered by Jaguar Specialties, but also within the 2 up/2 down rule you mentioned. I was most surprised when a) the driveshaft was found to be slightly tweaked given it was made to spec, although correction got it to "smooth as glass" at a highly reputable shop here in Austin and b) when the half shafts weren't binding and the hubs/bearings were ok (never mind that the right rear was definitely getting ready to do "something" without any bearing shims in place, I've really run out of ideas. Hopefully this evening will shed light and that I can "see" what's happening. I figure with no weight load to suppress things, it should manifest quite well.... I hope.

 

i have been following this thread with interest and surely we feel your pain.
It reminds me of an engine swap to a Chevy 454 from a 396. Everything
went smoothly until startup and the Chevelle nearly shook my teeth out.
Turns out the 396 was externally balanced and the 454 was internally balanced.
Changing the harmonic balancer was the answer. I hope you find your culprit.

 

Well, this saga took a turn last evening that should be categorized under "trust but verify - early and often - and more than once" or perhaps a variation of Occam's razor.

I put the XJR on stands last night and had a friend with a dial indicator come over to roll around under the car with me for a while. And what we found was ridiculously obvious and correlated exactly with what I'd been feeling these past few weeks. The driveshaft is not straight, like - seriously not straight. Even after it was sent out for this very reason.

Yes, I had it at a highly reputable shop two weeks ago. Yes, they found it was out of true at the time, fixed it and ran it to over 3k rpm within tolerances. (Stub was out .030, tolerance is <.005. Balance was out over .010 on each at at 1,000rpm, tolerance is <.002.) Yes, I called them this morning and sent them videos of what we saw. Yes, they're going to make it right. I haven't decided whether I should just get a new one made given that repairing this one once already led to it getting worse...I just want it "right". I'm no metallurgist and metallurgy isn't my thing, but this driveshaft doesn't seem like it wants to live life on the straight and narrow. Starting fresh may be the best option.

Two videos for your viewing pleasure:
1) The first video below (IMG_2719 3.mov) is the measurement on the machined part of the stub. Needle was swinging through .050. What we saw was more than when it was first assessed. Similar measurements were seen at other portions of the front end of the driveshaft. We measured at the rear as well and saw .007 variance.
2) The second video below (IMG_2722 3.mov) is with the car running at idle, in gear, indicated less than 10mph. The wobble is quite visible.

I'll take the driveshaft out again tonight or tomorrow (it's going to be 102+ for the rest of the week) and will be at the driveline shop Thursday morning. Will make the decision then about fix or replace. I'm leaning towards replace just have a fresh/good baseline, with a unit I will be transporting myself back to the garage to install. Fingers crossed....again.

 

I would love to see the methods your shop was using to true
the driveshaft.

 

Yeah - I get it. They are rather speechless at what I shared with them. It's a well known, highly regarded drivetrain shop (employee owned company actually with multiple locations in several states) and I saw the shaft on their machine. They invited me into the component shop while I was there and showed me the initial out of spec areas before addressing them. Their finished product was actually more "complete" than when I got it from the original source in that it now has the requisite alignment arrows to ensure the slip yolk and main tube are aligned. It wasn't originally made with that simple marking.

Everyone has a bad day, could be that there's an issue with the tubing itself, whatever. It's been frustrating that it's taken this long but as I mentioned earlier, they're committed to making it right and, I could have grabbed the dial indicator earlier after the initial repair and saved myself some time and perhaps at least one hub assembly. That right rear one was going to result in "something" soon enough without any bearing shims in it, regardless of its lack of symptoms.

 

So, at long last the vibration issue appears to have been resolved! The best summary of the root cause analysis is "a bunch of little stuff went wrong, which - when coupled with poor quality parts from a specific supplier, led to cascading failures."

More fully explained as: the initial driveshaft from Jaguar Specialties-recommended vendor in OK was made with a Spicer stub/slip joint. That original Spicer joint wasn't balanced/balancing properly for higher speeds (a known issue within the driveline building community here in Texas) and it caused a vibration that in turn exacerbated the slightly-out-of-true mounting of the Jaguar Specialties-supplied transmission flange adaptor to the transmission output shaft. That was due to it not having a chamfer to account for tolerances in how GM assembled 6L80E's for Camaro, where the nut that holds the flange to the output shaft is attached, and may not be flush to the end plane. (My transmission is apparently an example of that variation) That vibration from the slip joint was then amplified by the adaptor and transferred to the rearmost output shaft bushing in the 6L80E's extension housing, causing it to wear very rapidly and create excessive end play.

Despite numerous attempts with Spicer branded stub/slip joints, as soon as the driveshaft was spun up to 3k rpm, it would fall back into .020 or more out of round. Lower rpms presented no issues. So, they made a completely new driveshaft with a significantly stronger stub/slip joint from a different manufacturer with an entirely new tube. This driveshaft balanced perfectly and retained it at all rpms.

I replaced the transmission extension housing (less than $100 with new flange nut) and swapped in the new parts. At idle, in gear, on the jack stands, things looked very promising. I took it for a ride late Friday and the vibration is gone. Where it used to start vibrating at 45-50 and then increase closer to 60-65 and up, it's now smooth. Finally...

Pics with more details/comments below.



Wear is immediately visible once output flange and extension housing removed...


Best view of the wear induced from the vibration. The wear pattern is quite visible.


More wear elsewhere on the bushing.


New transmission extension housing with new bushing, seal(s) and flange nut. Pretty easy install once the transmission crossmember was loosened and mount removed.


The new driveshaft has a MUCH bigger stub/slip joint.

Next step is to get the tune revised to account for the new intake airbox/baffle I made to try and reduce the amount of hot air that is pulled into the air filter thanks to the high speed fan and surface of the sun temperatures here in Texas. I'll post that separately.

 

How nice to have no more bad vibration. (As opposed to the
Beach boy's "Good Vibrations") I suspect the quest yielded some
very valuable experience and a source of great information for those
who embark on a similar endeavor.

 

For sure - that's why I wanted to close out the thread. Most interesting part about this was actually tracking down the output shaft flange nut. For an $11 item, it was surprisingly hard to track down the part number online. Had to actually get to a dealer to get it - who then said they could get it for 2x the price in 3-4 weeks. EvilBay to the rescue for that... Also, was disappointing to learn about the rash of quality control issues/returns people are experiencing with Spicer products. Used to be one of the good ones.

 

Just like Interstate, Moog, Bosch, etc all used to be good too….

Glad to hear that your vibration is finally gone.

 

Nice one tracking it down. It seemed like a very frustrating process just reading about it on here.