2000 BMW E46 Subframe/Floor Damage

[Zaradomerix]

My 2000 328ci has the telltale cracks on one side of the rear of my subframe mounting location on the underside of the car. This is an LS swapped, manual trans car. The drivers side rear mount must have taken a solid beating after cracking because the "mount" has actually pulled out of the body along with the bolt and the subframe. So there is a square "pocket" in the bottom of the car where the mounting point has pulled through... is this fixable? Will welding a steel panel or using the parts from the "subframe mounting point repair kit" i purchased fix this properly?

If I have to buy a car and move all my parts over I can do that, I'm not DEEPLY invested in this specific rolling chassis, but after hundreds of hours of labor I'd rather not have to start over again 😅.

Thanks for any help guys!

[E39 Newbie]

The cracks happen, IIRC, because of the increased torque of the 328 and 330 engines. Another BMW goof. I can imagine how much more torque an LS puts out. I suspect the break was inevitable. We currently have a 2000 328i in the shop with the exact same issue - the mount completely pulled out of the chassis.

As you know by now, the subframe repair kit has plates you weld in from the top to strengthen the mounting points. However, you'll probably have to fabricate metal to replace the bottom section. Obviously the entire subframe has to come out to do this properly. But, yes, it is fixable.

[Zaradomerix]

I've done lots of research on the topic of repairing. I've also got a kit for front and rear shock tower reinforcements, as well as the cross bar for the rear struts. I will be getting a full roll cage and plan to fab some extra reinforcements for the rear with that as well.

Like I said, the car had some rust to begin with. The car has only been driven about 3 times with the LS engine in it, but I'm sure one snappy corner is all it took. The front bushings were basically putty so I'm sure the whole front end of the subframe swung up and down and put tons of stress on the rear causing it to pull out (since the rear bushing were still strong enough to apply force, being in better condition than the front bushings).

I've seen many repaired with cracks but couldn't find anyone with advice to fix an actual hole, much less the pulled out mounting point. Thank you very much! My friend is a trade welder and said he believed it would be alright if we fabricated a plate to replace the missing metal. Since that seem to be the group concensus I'll move forward with that plan.

Thanks again!

[vinyldude]

I fixed my 2000 328i, whose one subframe mount (driver's side rear) had broken exactly the same way as yours. I first prepped the "hole" where the mount had been torn off to be as rectangle shaped as possible. I then cut and prepared a same size rectangle "cover" from slightly thicker sheet metal, cut a big hole to it for the bolt and smaller holes for spot welding. I put the mount piece back in and welded the cover in place. I had stripped everything off the rear subframe since I was replacing all the bushings too so I was able to use it as a "jig" to align the mount and bolt hole and weld it to the right place. There are actually three layers of sheet metal there and the mount piece is between the bottom two. You need to cut a hole to the top layer inside the trunk so that you can weld the mount piece from both top and bottom.

Rust prevention is a challenge. I used weld-through primer on the metal before welding and sprayed seam sealer type product through holes between the layers after the welding was done.

If you know how to cut and grind sheet metal into shape and spot weld well enough then I think it is quite doable.

[enf1945]

can this happen to a 2004 325i or 330i as well ?
or did they reinforce the sub frame in 2002 ?

best of luck on your project

thanks

[vinyldude]

can this happen to a 2004 325i or 330i as well ?
or did they reinforce the sub frame in 2002 ?

I don't think BMW really admitted any fault or manufacturing defect so there likely isn't any accurate data available.
I would say anecdotally though that the early year models (-99 and -00 in the US) are more affected than others.
I think I've read somewhere that they improved their welding techniques somewhat for the later models (at least).

My analysis of what happened in my car is basically that the spot welds on top of the subframe mount piece (attached to the middle sheet metal layer of the three layers) failed first. This allowed the mount piece to begin to wiggle and cause cracks to the bottom sheet metal layer and eventually break free.