Howdy all,
I'm going through the process of sorting my 99 XJR. I just took a 500 mile road trip and the car ran great. I did notice a sort of rhythmic highway vibration that would occasionally be through the chassis (trans tunnel mostly) and occasionally through the steering wheel. It was very intermittent, which makes it a bit difficult to sort.

From my reading, this could be everything from a bad tranny mount to a bad flex (jurid) disk to bad u joints in the prop shaft or half shafts.

I've gotten under the car and basically it looks pretty pristine. With the exception of the most minor superficial cracking on the giubos, the prop shaft looks pristine. There is no play in any direction. Axles similarly look good and I greased the ujoints before leaving. no play in the axles either. No wobble in the wheel bearings either.

So the only thing I'm a bit unsure on is the tranny mount. Its not completely shot, but the little pyramid pieces of the rubber are touching the metal bracket on the mount. If you move it with a prybar, there is some movement of the trans. Given that this seems to be the only questionable item, I'm inclined to replace it. It displays the original 99 build date, so its likely hard if not collapsed.

Any input on whether this is normal would be great.

Thanks.

 

Chances are that after 23 years (assuming originality of parts) both of the engine mounts are collapsed along with the transmission mount. I actually bought my 98’ XJR back in 14’ because the engine mounts were so collapsed that the transmission oil cooler aluminum pipes rubbed through on the subframe and leaked a lot of ATF out. It certainly couldn’t hurt. You said you felt a vibration in the steering wheel.... Depending if you’re braking, that can be front rotors. If it’s just while driving, it could just need the tires balanced. There are a number of things that can cause a driveline vibration. The above listed were just a few commonly known examples of what causes a vibration.

 

Knowing how bad are modern tyres when it comes to their roundness and how they can withstand car standing still for long periods of time. I would start from the tires. Gearbox mount supposed be pretty soft there is only rubber between gearbox housing and gearbox mount.That's why there will always be play when using praybar. The thing is to know how much there is a play.

 

thats all for input. I checked the engine mounts with the typical car in gear, brakes on, apply load to engine. No excessive movement at all. Hard to say about transmission mount, but some aftermarket options are fairly cheap, so may just throw one in to see if it helps.

Tires are brand new, seemingly balanced well, and actually lined up (yellow dot by valve stem). I'm sure they are not perfect, but given that its intermittent, I'm not inclined to say its a tire issue.

 

I need a little more information on this issue. What mph does this vibration start at? When you reach this mph, if you were to go 10 mph faster, does the vibration increase with the speed?

 

hey sorry for delay. vibration starts around 55 with what could be described as a wobble through the chassis. By 70 its more like a steering shake with the occasional chassis reverb.

Any thoughts?

thanks.

 

tranny mount is usually fine.

long overdue for engine mounts it's invisible until you unbolt them

 

Good to know. So even with a successful result when loading the motor while on brakes with no movement of the engine you think engine mounts should be replaced?

If so, will do them, no big deal

 

With no displacement of the engine under load, I would doubt that as the source of the vibration.
The symptoms you describe seem to be more indicative of suspension than anything else.
Out of curiosity, have you checked to make certain you have no wheels that are out of round.
I have a similar situation on a '82 "Bimmer" and it turned out to be an out of round wheel.

 

Todays tires are notoriusly bad build quality. No matter if they are known brand or not. It is not funny when you pay 800+ euro for a tire set and you get wobble and vibrations to the driver bottom and steering wheel just because the tires aren't round from the factory. Tires do balance out easily and without much balancing weights but unround does not show up on regular balancing machines. These kind of tires shakes the car chassis and or steering wheel from the 80kmh to the 110kmh range. Mostly at 100kmh. You can change everything to the suspension and all you have is bad tires from the start.

Dang I've had this kind of issues lately...

 

If I'm honest, just about every vehicle I own has a degree of steering wheel shake on the highway and it drives me nuts. I'm having the tires on my cayenne roadforce balanced tomorrow and will see what that does. highway vibrations are highly annoying.

Anyway, swapped trans mount. Old one was worn and had a bit of oil soak from a leaking transmission electrical connector, but it wasn't that bad it seems as the problem is not really any better.

I will certainly consider a road force balancing once I investigate all the bushings and such. I know the car for sure needs rear lower shock bushings (will just replace shocks) and the front lower bushings are "ehh", so likely shocks all around. Otherwise, my initial testing of the bushings indicated that they were in good condition.

 

yea engine mounts tend to be an annoying idle vibration and a little under load.