Been having an issue of late with Superblue. Seems sometimes whenever I hit certain dips in the road, even at relatively low speed, the back end will “hop up”, even to the degree that it feels like the rear tires have left the pavement momentarily. Of course, what goes up must come down, and with a JOLT to the back end. That result, of course, can’t be good for the rear suspension.
Any idea what is causing this (e.g. bad springs, bad suspension bushings, etc.), or is this normal response for a face-lift XJS (w. 84K miles)?
Well, they do have some bounce to them, but it’s not like the car is bouncing all over the place and multiple times, unlike my old '96 X-300. btw, I thought someone posted earlier that the “bounce test” does NOT work (i.e. is not a valid indication of condition) on XJS (or is that Jag?) shocks … Supposedly they can only be tested after removal from the vehicle … Is that true?
Kirbert
(Author of the Book, former owner of an '83 XJ-S H.E.)
6
The bounce test cannot confirm that the shocks are in good order, since they may have lost all their gas pressure and still pass. You have to take the shocks out and note the gas pressure pushing them to full extension before declaring them fit for duty. However, the bounce test can indicate that your shocks are completely shot if they fail! Make sense?
My 94 4.0 coupe had the same issue. 2 of the 4 shocks were out of gas. Replaced all 4 and still had a slight issue. Found all four of radius arms bushings worn and torn. Once replaced the car stuck to the road as designed.
Yes, so in other words it is possible that the test indicates a “false positive” (i.e. they’re O.K. when they’re really not) but if the test indicates negative they really ARE bad … Thanks for clarifying that for me, Kirby …
Likely they are OEM, based on other “consumable” parts that I have run across in the past year or so on her that appear to be original when they should have been r/red. btw, what were the original brand of shocks on the '94 4.0s, if anyone knows? Bilstein? Boge? I know for awhile Jag installed shocks that were branded in their own name with the leaper logo on them (same for oil filters).
I usually find shocks only last 50-90,000 miles. On my two Volvos, their oem were also Boge. I replaced them but I found them too soft. I then tried Bilstein. I found them too harsh. I’ve now discovered Koni orange (str.t) and really like them as a compromise. If they make them for the XJS, I’m going to buy them.
I had read somewhere back that shocks are generally expected to only last about half that long (e.g. 40,000 miles), Greg … That really surprised me, as I figured unless they developed a fluid leak they should last the life of the car (or at least 100K miles).
Unless the shock blows out, generally the shock just slowly wears down, so the driver doesn’t even notice over time. Until they hit a bump and bottom out, or feel airborn Put new shocks in, and you’ll notice a huge difference.
That result, of course, can’t be good for the rear suspension. 
btw, what were the original brand of shocks on the '94 4.0s, if anyone knows? Bilstein? Boge? I know for awhile Jag installed shocks that were branded in their own name with the leaper logo on them (same for oil filters).
Put new shocks in, and you’ll notice a huge difference.