Differential Mount Cracks & Trunk Floor Separating from Sub-Frame
Well, I suppose it should have been obvious that the rear-end our Z3s borrowed from the relatively low-powered E30s wouldn’t hold up well to the brute force of the larger engines in the Zs. Even in the E30s there are many incidents of broken differential mounts on more modified examples.
I was further prepared for the diff mount issue as my mechanic advised me a few years back that installing the solid mount bushing would certainly add additional strain to the mount and would probably accelerate the occurrence of cracking. I added the solid bushing after the second weak rubber bushing had failed, and I was tired of replacing them.
After noticing the cracks in the differential mount, the car received limited usage (much to my chagrin) until I could get it in the shop for a new diff mount (not a small chore).
In hindsight, I’m glad I did wait as the issue of cracked diff mounts and trunk floor separations in the Z3 has had time to come to light as well as testing of repair solutions and the outcome of warranty claims.
Initially it didn’t appear as if mine was suffering any trunk floor separation. Initial inspections found the joint compound in the trunk intact and minus any cracks, and there was no apparent deformation of the trunk floor spot welds. A fact both the shop and I found rather interesting given the rash of Z3s experiencing the problems and how rough I can be on the rear-end.
The differential mount cracks were not unexpected…particularly with the solid mount bushing, performance modifications, and a heavy right foot.
Of course, we’d have to start with the same flimsy aluminum stock diff mount…but wanted some extra reinforcement.
The shop I was working with has some very talented fabricators and welders.
Fabricating a couple pieces of steel to mig-weld to the sides was a fairly easy solution.
They drilled, cut, and ripped out the old mount and prepared the surface for welding…
And welded the reinforced diff mount into place. Room for sway bars and diff mount bushing became real tight once the thicker mount was in place. After a bit of prodding and prying, everything fit well.
But…it wouldn’t be going back together quite yet.
While they were welding the diff mount, and the welder was steadying his hand against the underside of the trunk floor, he noticed the floor lifted very easy. Way too easy.
The diagnosis …a total of 10 failed spot welds. They seemed to have failed so cleanly, that they didn’t even look deformed…they just weren’t actually holding anything anymore. Even in the picture above you can’t tell from looking that the sub-frame spot welds to the left of the diff mount were completely loose.
Initially we were going to weld in some beefy structural braces.
It wasn’t until the tech had visited another shop and had a chance to inspect a (track modified and used) Z3 coupe that his ideas changed. Apparently the coupe had sustained similar cracking and weld problems, and had some beefy structural braces similar to those planned for my Z3 welded into place.
The result was that the normal flex was removed from the sub-frame/trunk floor area…and spread to the very ends of the sub-frame. This resulted in the stress simply ripping out other, more difficult-to-repair, welds further out toward the sides.
The new plan had to provide for flex between the entire length of the sub-frame and anything attached to it. It also had to provide reinforcement of the welds.
What the tech came up with was to fix the existing spot-welds (stripping, prepping, and welding), and then to add small strips of steel bolted in place on the top and bottom to sandwich the the welds, the trunk floor and the sub-frame together.
This provides for the necessary flex as the steel isn’t welded in place….but allows the steel strips to spread the stress of the flex over a small area to help protect the spot welds.
Of course the bottom received the same professional weld prep, treatment and finish.
The whole job cost a few hundred. None of it I could do myself (except make the initial cracks and test the fix with some spirited driving). I am quite certain I will not break the new diff mount. I don’t understand why BMW insists on these flimsy mounts even after all these years.
As for the trunk floor repairs, I suppose it is only a matter of time (and spirited miles) before we see how well they perform. The car now feels better than new.