BucketSpeed

Howdy, and thanks in advance for the help! Acquired a 2000 XJ Classic in October of 2021, with the intention of cleaning it up and giving it to my son for his first car. It has just under 250k on the clock. So far, I've replaced shocks, leaf springs, coils, various gaskets etc. I also drove it without the front drive shaft installed, and replaced the rear drive shaft as it looked pretty bad and figured it was time anyway. Also has new tires. Yesterday I swapped the transmission mount, a step in chasing a rather nasty vibration at highway speed. The old trans mount was completely disconnected on one side. My current gremlin is a discomforting vibration that starts around 64mph and grows with speed, essentially shaking the body and causing a low frequency hum that makes driving at that speed feel like you're riding in a propeller plane. I've had the tires balanced and rotated, and the vibration persists. There is almost no sense of the vibration in the steering wheel, but shakes the entire car otherwise, at a consistent frequency to the speed of the car. I was hoping it was a wheel/tire, but since I had them rotated and balanced, the sensation remains the same. Not sure what to check next. Maybe motor mounts???

Side note, there is a feeling at idle as if there is an engine miss-fire, but I'm running a bluetooth OBD sensor that hasn't shown any errors. I do plan on cleaning all my grounds, but I don't imagine the high speed vibration and the idle shake are related. But then again, I'm a part-time moron...

Thanks in advance!

That Jeep Guy XJMJ

Welcome and congrats on your first post!

Check them motor mounts and report back.

Have you had an alignment done?

4.3L XJ

Did you lift it?

BucketSpeed

thanks! No alignment, as it tracks straight on the highway.

BucketSpeed

I went the "UpCountry" clone route. Nothing dramatic.

DustyWagoneer

Never mind!
LOL

Rogue4x4

Are you still without a front driveshaft? Or is it installed now? Is the rear shaft brand new, or rebuilt? Were they installed after your UC clone lift?

There's a method to madnesthat...

...also, just because it's tracking straight, doesn't mean it's properly aligned, you might have a slight tire scrub happening, but we'll come back to that...

BucketSpeed

Front DS is installed. Rear is brand new Tom Woods. Front looks stock, rear was measured after the UC lift and ordered. The vibrations were there before, with the stock DS that looked pretty bad.

Red1992XJ

Just curious, have you given the brakes a good check? Sometimes an out of round drum or bad rotor, or even a sticking caliper can cause issues. If you have not had them checked, now is probably a good time to get it checked.

I recently had an issue with my Mercury Grand Marquis (I know, different car) where I would get a vibration starting at about 65 and getting worse as I sped up. Under 60 it was undetectable. It was a bad front rotor and a sticking caliper.

It can also be a tire that may have a broken belt. If you jack up the car and spin the tire, watch the tread for any shimmy that could indicate a problem. You can also try rotating them to see if it may show up at a different corner of the vehicle.

BucketSpeed

All good points!

The drums are on the suspect list. Otherwise, I'm thinking it's not front brakes, since the shake is almost unnoticeable in the steering wheel, but practically giving me a massage through the seat, floorboards. I had the tires rotated mid-trip yesterday and the vibrations didn't change or "move" with the wheels. I know the knuckles have the usual notching where the pads rest, and it's on the list of things to fix, but I would think if it was front brake-related, I'd definitely feel it in the steering wheel, no?

4.3L XJ

So, how much lift on this upcountry clone?

BucketSpeed

It measured out to about 1.5" over stock. The suspension was pretty shot when I bought it. The XJ has just under 250k on the clock.

Rogue4x4

So it still has vibes with no front driveshaft installed? My thinking is it's somewhere in the driveline. Even a slight change in angle in the u joints can cause a vibration.

If it did this while the front was out, and after the new rear shaft was installed, start looking at the output shafts on the yokes, both axles, and transfer case. Make sure there's no play or slop in them, might be bad bearings/races. Your broken mount may have put stress on the driveline. Also make sure the front DS is balanced correctly.

BucketSpeed

Yeah, still has vibrations with the front DS in. I'll put the output shaft(s) on the list! Thank you!

4.3L XJ

1.5" is not much, but the 2000s are very sensitive to rear lift. Can you take measurements on the angle of the face of the tcase and the yoke?