I have posted my issue several times, and have removed all other potential causes, and now think I might have a problem with the passenger front unibody on my 1999 xjr. I am hoping this post is reviewed by somebody who has done extensive body work, or who has a pic of a disassembled front fender area.

The fact pattern is this: My kid hit a curb at 40mph at 70 degrees. The right front wheel was broken, and the front subframe was bent. I replaced the subframe with a used one in perfect condition, and replaced all the mounts, bushings, and other moving parts. The car was perfect for five thousand miles.

Now, upon heavy braking, the nose of the car rises, while the engine dives. The travel is enough to rip off the passenger side radiator hose. I have checked everything, tightened everything, and had a friend do the same. There is nothing loose, nothing worn, the brakes all work fine.

There is a rattle in the passenger front end area that occurs while driving normally, that we can not locate. It sounds like a shock mount, but it is not. It follows the bumps in the road.

My latest theory is that the unibody structure that the subframe mounts to is cracked and under extreme braking gives, causing the front subframe to submarine under the car, dragging down the engine, and forcing up the fenders and hood.

On other cars, I know they market after market stiffeners for unibodies. Does anybody have any theories or any suggestions? I simply can not figure this one out.

 

check your Engine Mounts, if you hear a clunking but cant identify the area, it could be the Engine Mount, have a look at this thread https://www.jaguarforums.com/forum/x...e-mount-81704/
as for the other problems i do not have an answer to, hopefully one of the Jag Techs on this forum can help further diagnostics

 

I agree that broken motor mounts are far more likely than the other scenario. Jack the car and support it well, or get it on a rack and have at it with a pry bar. If it is as you describe, you will find whats moving.

 

have a look at the last post in that thread #11 by Sean B, for a quick little test

 

John, there's a few things to check...see picture...
I've ringed the following to check,
subframe mounting bolts at the rear
subframe mounting bolts at the top going into the chassis legs
shock top plates and shocks
engine mounts
Then it's suspension on the side giving problems. Something may have worked loose over the 5k miles?
What pictures do you need of the front end?
The issue I see is front end loading and how you replicate the heavy breaking movement on a lift or stands. If the engine dives and the body lifts it's got to be subframe mounts at the front?

 

I have done the pry bar routine, and had a friend do it independantly, just to make sure I was not missing something. Everything is rock solid. The mounts are brand new, and I know that you can get one that is defective. Shouldn't the engine move if I power brake the car while I punch the gas in drive if it is the motor mounts?

I must say that this only occurs under hard braking, when tons of force are applied. I will look again, maybe even remove the motor mounts again, and see what gives. As far as my theory, does anybody have any unibody experience?

 

Try imagining that whole lump on the jack as a tire that suddenly stops and the car on the top of it keeps going. Doing that ... for the front end to rise, either the the subframe has to move away from the chassis legs or the back pivot mounts have to shift while the other bit stays fixed.

Connecting a laptop to a web cam taped to some strategic places while doing the heavy braking might come in handy.

Maybe a weld let go somewhere or the engine mounts are bolted tight .. but split.

 

I like the web cam idea. I was going to duct tape my droid under the hood and go for a drive....

As for the subframe mounts, they are tight, and don't budge with a prybar. I got to figure that there is a broken weld in that unibody structure. If I could figure out where it is, I would bond or weld a brace along the structure that would solve the problem. Maybe if I take the fender off I can get a better look at the unibody and see if there are any cracks.

I must say that I really appreciate the help.

 

Try using lines of masking tape as targets while filming to make the movement more obvious. Something like you see in those NHTSA crash test dummy films.

 

The more I think about it, the engine mounts are shot...but they're 5k miles? any signs on the chassis rails of chipped paint, or leaks from the mounts on the lower subframe?

 

What about the front and rear welded seam on the cradle that GGG cautioned about inspecting for rust when purchasing a used cradle?

 

I inspected the subframe pretty closely, but direct me to the thread...I will take a look. Sunday I am going to pull out the engine mounts and check them. I would just order two more, and replace them again, but this is getting expensive.

 

So I replace the motor mounts again. The mounts with five thousand miles on them were are fine. Now I have two gently used on the shelf for the future. Still have the problem....going to do the webcam. If anybody has to do motor mounts, I got the tricks down by now.....

 

And for the record, do any of you experts know what and where the plenum braces are found? I just saw two metal braces that are mounted to the BIW, what ever that is, that supply structural rigidity during a crash. Maybe I have one that is broken?

 

Problem solved. Subframe mounts were toast. Must have installed a bad one. Thanks for all the ideas. I am now a front subframe expert.