Esteemed X308 colleagues - I've uncovered an issue with my 2000 XJR that has me a bit stumped. As background, the chassis has 111k miles currently and until I got it for the LS swap, it had sat for several years in south Texas heat and sun. The original engine, transmission and driveshaft where replaced with an LS3/6L80E combo, with custom one-piece driveshaft using kit from Jaguar Specialties. As I've been refining the tune for drivability over the past 2k miles, I've slowly been uncovering other things that need attention, but this one has me at a dead end.

My car has a vibration from the rear of the car at 50-60mph+, which increases somewhat with speed, but does so more dramatically after lifting off from strong acceleration at speed. The vibration feels "strong" in that it's steady and under light or part throttle at speed it subsides. At sustained higher speeds, it almost "thrums", if that makes sense. It's almost best described as classic rear u-joints being bad. BUT - they're not. Everything from the transmission to the differential is new (one piece driveshaft, Spicer u-joints, etc) and I just had the driveshaft out and rebalanced on Monday. It's now smooth as glass to 3,000+rpm on their machine. Everything related to the driveshaft is tight and true. There are no other suspension clunks, squeaks, clanks, knocks, bumps, thumps, etc.

Trans mount, engine mounts are all good. There's no engine/trans to body contact anywhere, nor is there for the exhaust. Axle half shaft u-joints look good as well and have been lubricated. Rear shocks (green Bilsteins) are new, donuts replaced when they were installed. Diff fluid changed 3 weeks ago with current spec for listed viscosity. No metal or untoward symptoms in the fluid when that was done. No leaks at pinion or axle shafts either. Wheel bearings aren't making any noise and the bushings at the rear all look ok.

Tires are new Pirellis and they were just rebalanced today (Hunter road force) and I know that made a slight difference because a) the weight pattern on the rims is different in both amount and locations and b) the vibration feels to be of a slightly different frequency now, but it's still present. Tire shop didn't indicate any issues with rims either.

If I hadn't just sold my 2001 XJR, I'd have swapped the entire rear subframe into this one as that car was smooth to 100mph+. But, that has a new home in Indiana now.

I'm open to any/all suggestions from the group - thanks in advance!

 

You’ve certainly done your due diligence addressing as much as you can. One thing I’d like to know is, what’s the driveshaft angle? You’ll need a magnetic bubble gauge for that.

 

Thanks for the feedback, I'll pick one up and shall report back. I don't have a lift so will have to do this the old fashioned way, laying on the ground...

Also, it occurred to me last night after reading your post that the driveshaft may be too long and is binding up at the front where the slip joint/splines are. Reason for this is that is exactly where the driveshaft was out of spec when at the shop. It was out of round at the neck and very top of the tube. As I've tightened up the suspension with the new parts, the rear is perhaps moving around less (correctly so) and over the past 2k miles of driving, this has stressed the driveshaft at those points. That would certainly give the vibrations I'm experiencing. Not sure why it didn't occur to me earlier when I saw where the driveshaft was tweaked. Based on this theory, I'm going to remeasure the length between the front flange adaptor (bolted to the transmission) and the rear one (bolted to the diff) and confirm things with the driveshaft shop, and will snap a pic of how much of the splined area is showing when in situ.

Stay tuned...

 

So, the "driveshaft is too short" theory is inaccurate... Plenty of room at the slip joint connection as shown in the first picture below.

When I measure the driveshaft angle with my handy-dandy new digital angle finder, I show 3 degrees with the car sitting on the ground. My garage floor has a 1.5 degree slope (thank you cookie cutter suburban home builder!), so I'd say that gets me to the target 2 degrees, correct?

After speaking with Andrew at JS, he indicated that engineered the driveshaft angles to have 0-2 degrees to the center bearing on the OEM driveshaft to ensure the Jurid coupling at the differential would remain at 0 degrees. Based on his comments, I'm going to check the seating of the machined adaptors at both ends. He said he's seen the front one not always sit flat on the center nub from the 6L80E output flange. If so, I'll just chamfer that hole and go from there. Will be fun (not) getting to the bolts that are on the transmission side of the adaptor. I will loosen and lower the transmission crossmember to try and get room for the wrench, let alone my hand, in there.


Plenty of space for flex here. I suspected as much, but - seeing is believing. The top arrow (pointing down) is towards the rear of the car. It is at this weld, and the machined neck, where the distortion was. If the front flange adaptor is, in fact, not seating flush, I think it'd stress the same area of the driveshaft...

This is the front adaptor to the driveshaft, looking towards the front of the car. Lowering crossmember looks to be the easiest approach to getting at the nuts that hold the countersunk bolts on.

Rear adaptor looks good as it sits. Once I see what's up with the front, I may as well remove and check this one...