My 90 XJ-S with sportspac has developed a strange motion. When stopping it rocks forth and then back when it comes to rest unless I stop very gently. Radius arms are new. Rear cradle rubbers look ok both on the two poster lift and with car sitting on the ground. Similarly front mountings incl engine mounts appear sound. GAZ adjustable dampers, still capable of a rock hard ride. Rust free body aprt from some rust in front suspension bridge but passed MOT equivalent. Not particularly wayward sideways. Any ideas?

 

Sounds like an American car to me! Are you sure that the shock absorbers are in good condition? I had an MGB once where the front shocks went and it was like trying to drive a rocking horse after a while, highly amusing but nearly impossible.
Try the bounce test on each wing: push down firmly and let go and the car should only bounce about 1 1/2 times; if it does less then the springs are shot (which should be visible by the gap above the wheels to the wheelarch lip) and if bounces more then the shocks are shot or on their way out.
Good luck.
Steve

 

Vertical movement is as it should be according to the chosen settings of the knurled wheels on the dampers. This is only a horizontal back and forth. Dead dampers I recognise, this is something else.

 

Just put a General Motors badge on it and now you have a proper American car. It really does sound like a shock issue. I have seen shocks pass the bounce test. That does not mean they are good. Shocks can change when they get hot and the car puts much more force then you can ever create doing a bounce test.

Springs will not cause this. You check a spring by measuring height. The stiffer the shock the less times the spring can oscillate.

 

Have a look at Ian Clements' recent thread in the X308 section. In his case it turned out to be the four bolts on the pendulum frame being loose. No one would expect those bolts to be loose, but they were.

 

Greg, the knurled wheels I referred to are the adjusters for the dampers. I can set them rock hard until the fillings fall out of my teeth but the horizontal movement is still there.

 

Plums, no clonks or suchlike. Just the gentle horizontal movement.

But wait! When using kickdown it feels like the engine/box needs to think and wind up and the the car launces itself forward. In other words suspension windup.

 

On the X308 there are two BIG bushings between the body and the suspension located just in front of the rear wheel arches. When they go, you can get that windup action. Same or similar on XJS?

There are lots of mounting points that could feel fine when pushed by hand but move under more force. The best thing would be to get it on a hoist and start checking with a pry bar.

 

Assuming the dampers really are OK; then the movement must be coming from one of these points, I think (save a catastrophic bodywork fail!):

1. Loose diff bolts to rear axle cage top
2. bad rear axle cage to body rubber bushings/bolts
3. radius arms bushings shot
4. rear suspension 'wishbone' fulcrums (inboard and/or outboard) really poor condition
5. 6 shot bushings at front of front suspension subframe shot/loose or front subframe actually has failed somewhere. This is a real possibility.
6. rear front subframe V mounts loose/shot
7. Front suspension wishbone mountings loose/shot
8. engine mountings loose/shot
9. gearbox mountings loose/shot

None of these things are that expensive to fix or renew, even if time consuming so I suggest do all of them! I feel something serious must be going home, or the matter would not be so pronounced.



Greg

 

Greg and plums, I think you observations on bushings are on the right track. Some I know to be good, others are looking OK but may be old and gone. I will do one last thing before starting a general replacement regime: Driving past a videocam when stopping and using kickdown to see if the wheel to body movement is pronounced enough to see clearly. I have found a good front suspension member to replace the rust damaged one so new bushings all around is on the cards anyway as soon as the V12 swap in the XJC is completed.

Considering that the movement is felt both when stopping and using kickdown I suspect I have a multiple issue here. If only the rear was bad the stopping rocking shouldn't be quite as pronounced. If only the front was affected the kickdown should not show much windup.

Thanks for all your input.

 

Playing video games with your own car pays off


The main culprit is indeed the dampers: Rebound is way off all around. The whole car lifted, that is what gave me the horizontal movement feeling. Bump can be compensated and so masking the deteriorated dampers. In addition the rear cradle mounts are on their way out. I have a complete set of dampers bought for another one which I sold before the job got done (heard that one before ).

So next weekend will see the job done I hope. I enjoy a good old rear wheel drive, when you know it an XJ-S can be treated like a compact sportscar coz the limits are lowish but utterly predictable.

 

Glad you found the issue. My point being if a shock fails the bounce test it is junk. If it passes the test it does not mean it is good. Good luck!

 

I noticed my front dampers were gone when I used to brake and the front would bounce about. Replaced both and as a bonus the ride at the front is higher and clears my drive without scraping the spoiler

 

I managed to fool myself because the GAZ adjustables only adjust the bump, not the rebound! I have always been playing around with the settings according to mood and road conditions. Thus I haven't noticed that they gradually deteriorated. But the rebound finally got so bad I had to notice. Oh well, live and learn. The sad thing is they were quite expensive and only lasted for two years.

 

That is the problem with shocks and tires. They wear slowly so you do not notice the change until you replace them. Then you notice a major improvement.

I never used GAZ shocks. I owned my own shop for years and have tried most of the major brands, Monroe, Gabriel, Boge, KYB and Bilstein. I have never used Koni shocks. Not all of these brands were tried on the same vehicle. My findings were that Monroe and Gabriel are garbage. NAPA is Monroe and I think Carquest is Gabriel but been many moons since I dealt with either of those two so I could be wrong. Boge is hit or miss, some last a long time some don't last any longer than Monroes. KYB is about the same as Boge. Bilstein seems to be the best. I never had to replace a Bilstein under warranty. They are usually more money of course.

As a shop owner doing the job under warranty hurts on 2 fronts. First is the parts store supplies the part but not the labor so that is absorbed in the shop. So while the shocks are being replaced a second time the paying job sits and waits. Lowering the effective labor rate. Second is the perception that my shop does lousy work. The average consumer thinks I sold them junk or did the job wrong. Neither of which makes me happy.

I know there are some who are of the opinion just buy the Monroe shocks and warranty them every year. I prefer to do a job once and move on. I do not want shocks to become a regular service item.

You may find that the only place that carries Bilstein is an online vendor.

 

I have an American-made GM car and it doesn't do that... but this Jag does.