When braking hard the car shudders once at the end right before stopping. Even at low speeds the car still behave the same way. Anyone have a clue to what it is I think it could be a warped rotor.

 

First part of your post makes sense,....car WILL shudder (better described as pulsating/vibrating brake pedal) just before the stop at hard braking, if the rotors have hard spots, or are warpped.

The other part doesn't make sense,....once the pedal is released (regardless of speed), there should not be ANY shuddering/pulsating/vibrating associated with the brakes (they are not engaged, and no contact between the pads and the rotor).

 

It's not a pulsating sensation... It's a hard jerk or thud that goes thru the whole car right before a full stop under a hard braking.

 

I think your problem is not with rotors. I would check all the linkage,.....it sounds like something is worn/loose down there. Put the car on the hoist, get a BIG prybar, and go at it. Imagine the forces on the linkage when you brake (wheels trying to stop, and the body trying to continue), and mimic this movement with the prybar. The loose part will reveal itself. If anything moves,.....time to retighten everything and/or replace.

If everything behaves as it should,....the only other culprits could be the calipers (check the mounting bolts) transmission, driveshaft, and diferential (in order from "could be", to "not likely").

 

I assume you've eliminated sticking caliper pistons, warped or cracked brake discs and terminal suspension bushes?

If it feels like one or more brakes grab - release with an almost metallic clunk - then grab again and stop the vehicle, you may be experimenting at the upper limits of the ABS.

Graham

 

Before I drove it again, I would check the caliper mounting bolts.

 

Sounds like a broken front subframe upper bushings.
During a hard braking the whole subframe with the engine goes down and than, right before the full stop, goes up with a loud thumping noise.
Open a hood and try to brake very hard from a quite low speed and if i'm right you'll see the engine goes up-n-down

 

Lol

 

I just had the same issue. Assuming you have ruled out the brake issues identified above, and you don't have a bowling ball in the trunk rolling around, I would look into the subframe mounts. It took me months to track this down, but Moscowleaper is right. Have somebody else drive the car with the hood open and watch the engine when the brakes are applied. You only need to be rolling slowly to see it happen. If the engine dives, its subframe mounts. Replacing them is a @#$%. I have had to do it twice. Expect to spend about five hours on it, assuming you are reasonable competent. The most important part of this post is that when you are reassembling the car, make sure you use a jack to get the subframe mounts to the exact position with respect to the unibody structure. The dowl pins need to be completely in, so that you don't wrench them into place. If you do, you too, like me, will get to do this twice. The mounts are not made to be pulled the way they are if you wrench them into place.

 

Exactly!
BTW it's a good idea to rebuild the whole suspension, since all the other rubber parts are not getting any better with ages... After the rebuild it's an absolutely different car

 

It indeed does jump up 3 inches....

 

do you mean the vee mounts?

 

IIRC the vee mounts comes only on XJR. On XJ8 there's only rubber mounts, but that doesn't matter at all, since it's the only a different option to hold a subframe.

If the engine jumps down for 3inches during braking - it's sure the problem with subframe mounts.

 

The vee-mount is common to all versions.

Are you referring to part #5 or part #2 in the following diagram?

 

nr.5

 

It would seem then that the action is the subframe/body pivoting on the rear mounts because the front vee mounts are delaminated and separating when the brakes hold back the subframe while the body rides forward/up. Then when motion ceases, gravity pushes the body back down and closes the separation resulting in the telltale thud.

 

Exactly. There's a rubber strap on a vee mount, that holds the upper (metal) part and doesn't allow the excessive movement in case the vee mount is broken. That's why the engine dives only few inches down during braking. But that rubber strap is not so thin and may, finally, broke too. After that there will be way too much movement of the engine, which may cause problems with the coolant hoses and other parts.
So i would not recommend to continue driving the car until the problem will be eliminated.
Sometimes it's hard (or even impossible) to remove vee mount bolts without braking them, so i recommend to buy new ones.

 

Good to know because there have been a few cases of this coming up lately. The cars aren't getting any younger so more may come as time goes on.

 

Front Subframe Mounts was confirmed today.. $140 for parts, $300 for labor.... Will be done tomorrow.

 

Not bad for what looks to be an unpleasant job.