Hello,
Not sure how to describe this problem accurately but will try.
Our 1999 XJ8 has about 75K miles on it and has for the most part a great car with few issues.
That said, we have an issue where the car will run strong and smoothly when first starting out.
On occasion, usually after we stop at a light or two and then continue, the car will start shuddering as if the front end wheel alignment suddenly went bad.
The steering wheel will jerk back and forth ant it will also feel as if a heavily loaded trailer was attached to the rear and is holding us back.
If we pull over and pump the brakes and then proceed again the problem will sometimes go away right away and sometimes after a couple of stops.
Eventually the shuddering will stop and car will run as strong and smoothly as usual.
I have been wondering if somehow the ABS is activating even though the brake is not applied?
Has anyone heard of or experienced anything like this?
Thanks.

 

Any faults in the ABS module??
I once had a X100 SuperCharged (XKR) that would do something very similar when I worked at the dealer.
A used car lot brought the car in for warranty repair for a 'shudder' at speed and I test drove to verify.

After some diagnosis I discovered that someone had 'rotated' the wheels/tires front-to-back.

The larger/wider wheels/tires were now on the front and the smaller/narrower ones were on the rear.

The 'traction-control' was attempting to keep the rear wheels from going faster (slipping?) than the fronts. (different diameters)

I called the car lot to come pick up the car as I could not help them with a warranty repair. The problem was 'self-inflicted'.

bob

 

Tires are same size all the way around so not traction control inflicting pain - at least not for that reason.
Will check to see if there are any ABS codes when car comes home tomorrow. Got a report that shuddering happened again and went away after braking a few times.
Same as it always does.
Read that brake fluid should be changed every few years (hydroscopic). Have not changed for some time - wonder if that can cause that issue.
Also, we have to do a "hard" reset on the battery every month or so to get sunroof to stay open, so BCM may be on the way out and maybe ABS module?
Thanks

 

Stuck caliper, perhaps? symptoms could fit, if it's sticking after being used, and binding, it'll cause the vibration and explain why it hadn't done it upon first starting out. The easy way to test is to drive it, let it do it, pull over, and put your hand near the wheels. If you have one wheel that's putting off much more heat than the others, it's a good bet. I never had it happen on the Jag, but did on cars prior. Shouldn't take much to test, or at least rule out.

 

Obviously, and as CharlzO suggested, the problem is brake related. Looks like one of the brake pads on one of your front wheels, after braking to stop at lights, does not release (gets stuck). After pumping the brake, it gets unstuck and everything is good again. This may happen if the brake discs are a bit overskimmed and when the pads are close to fully worn. The caliper piston comes out too much and gets skewed in the bore and temporarily stuck. If the brake pads are rather worn (thin), replacing them with new ones may cure the problem. Otherwise, you would have to rebuild the brake calipers by thorough cleaning of the piston bores and replacing the pistons (if corroded/damaged) and the seals.

 

Checked the ABS with a scanner & found no fault codes.

Yes, when this has happened, putting hand near the wheel picks up lots of heat.

Will check the pads to see how worn they are and how much travel is on the calipers..

Thanks for giving me some more ideas to check out... Just wanted to have a reasonable expectation that it was not the ABS module somehow braking as if the brakes were jammed on when they were not.

Will post with solution when found. Won't be able to tear it down until Father's day weekend...

Thanks!