Possible terminal rust in rear subframe mounting points

[HayekFan]

Got some bad news about my car while it was at the shop last week -- rusty rear subframe mounting points. The car was in for a repair to the power steering bracket and once that was done they inspected for a clunk sound in the rear. I figured it would be something minor like the sway bar links since it wasn't loud and wasn't accompanied by weird behavior like the back end moving out on its own.

But nope, the rear subframe bushings are shot, and replacing them has the potential to be greatly complicated by some nasty looking rust at the mounting points. The shop is concerned they may be so compromised that it may not be possible to reinstall the subframe. They're preparing me for the possibility that the car may be a total loss due to the difficulty in repairing the mounting points. They don't do that kind of fabrication and said they didn't know of local body shop who could do it.

I'm taking the car back down there next week and they're going to put it on a lift so we can all take a look at it and decide what the next step should be. Yesterday I pulled the back seat to inspect the inner mounting points in the unibody, and they look okay. I've seen pics where the inner mounts fatigue and break but these look sound. So it seems to be a matter of how intact the structure is on the underside where the bushings mate to the car.

My thinking is the only real option is to attempt the bushing replacement and see what happens. I've gone ahead and ordered the bushings and a Forsvara bushing removal tool. I guess one question is whether to leave the pins in and drop the subframe or hammer out the pins.

Very interested in any thoughts anyone might have on this. Thanks

[1st 5er]

Sounds frightening.

[stuartinmn]

After you have a chance to look at the issue, check around to see if you can find another shop willing to do the repairs (just because this place says they don't know of anyone doesn't mean there isn't someone who can.) It's all a matter of how much you're willing to pay...anything can be fixed with enough money, but if it's just the mounts themselves it may not be that catastrophic. New mounts can be fabricated.

[harrypalmer]

I'm with Stuartinmn on this. My car has had an underside dried rust (not clunking) for a while which I'll be addressing very soon.
If you follow this link https://www.flickr.com/photos/109547372 ... otostream/ you'll see many insightful pictures of the metallurgic surgical nature of both the fabricating and actual repair made.
The pictures are not mine (except the ones numbered 1 to 5 of my actual car) as I grabbed them from someone else's website a while ago. Don't recall the URL or his name, otherwise would give him due credit for his brilliance and indirect help). The point is, they might aid in predetermine your or any other shop what they'll up against, and also enable them to have a better idea on how to proceed. Hope you can fix your car and erase your E28 sad news. Best.

[Mike W.]

As long as the bushings aren't seized on the studs, I'd go the low tech sawzall method of bushing replacement. That way you're not beating the pin out and not really moving stuff around much. Not to discourage you from a proper repair, but in this case the less invasive the better at least for now.

Curiously there was a thread about this on First Fives recently, and while not identical, Es 12 and 28 are very similar, so it might be worth looking at.

https://www.firstfives.org/phpBB-3.2.1/ ... php?t=6488

[HayekFan]

Thank you all for the replies. These mounts are kind of mysterious with their various layers...it's hard to tell what's truly structural. I was just under my car poking at them, and between that and the info in this thread I think I have a better sense of what's going on down there.

The driver side mount seems okay. The passenger side is going to be the issue. It has lost some of its outer layer (the floor pan). The layer under that (the lower bracket) is mostly intact but does have an area where rust has penetrated it. It seems to be holding the pin rigidly in position rather than letting it flop around. My sense is that it's intact enough not to give way during a subframe bushing replacement, but I could be wrong. I can see how the sawzall method might avoid being too rough on things.

Here are pics the shop took:





Here are pics I took of the mounts under the back seat:

[HayekFan]

I took the car back to the shop today and they put it up on a lift and we talked it over. The plan is to drop the subframe and replace the bushings but not do any structural repair beyond cleaning things up and hitting the area with rust sealer. We're going to try to schedule it for next week.

[4DSC]

The thread harrypalmer references pictures from definitely helped me understand what was going on with all the layers. It's located here and is a true masterclass in e28 welding that I found kind of inspiring. You are looking for pages 9 and 10.

I had a similar disappointing rust revelation when I dropped my subframe to do my bushings last year, but probably not as severe as your pictures. Some surface rust and pitting, and a small hole eaten through on one side.

The issue I ran into figuring out how to address this, as it sounds like you're finding too, is that most mechanics that will be willing to take on removing a subframe (at great expense probably) won't be willing to tackle the welding/fabrication, and welders aren't going to drop the subframe. Since I dropped mine in my garage, I was kind of stuck and had to find a mobile welder since I can't weld myself. Perhaps try to do the same -- maybe someone will be willing to come and do it at the shop while your subframe is out?

It's definitely possible to repair "right", but it took me forever to find a mobile welder that was willing to tackle it, and it wasn't cheap. Had a few agree to come and ghost me. The guy I found to do it eventually cut/ground the area from below (under the car) and welded a super thick steel plate around the pin dowel where the body sheet metal was failing.

Also -- I was kind of concerned about the little drain holes (I guess that's what they are?) that were put in the mount area from the factory as sources of future problems, and maybe they contribute to the rust buildup around the mounts in the first place. My welder's advice was to periodically spray this stuff in there to prevent any future rust buildup.

Good luck! Been there, it sucks, but its not terminal if you can find some professional help.

[HayekFan]

As far as I can tell about the structure of the mounting points, the floor pan layer isn't as critical as the next layer in. The next layer in is referred to as "stiffening plates" on page 9 of Harry Palmer's linked thread, and it seems to be what's really responsible for bearing the load. The importance of this layer/structure seems to be confirmed by the pics and discussion in the thread Mike linked to.

I definitely have an area in my floor pan around right side mounting point that has rusted through. I can see through that layer to the next layer in, which also has rust penetration, this time about the area of half a nickel or so. And I'm sure the metal around the holes has been made thinner even though it hasn't been totally consumed.

Despite the damage, I think there's enough structure intact that the bushings can be replaced and the subframe reattached without things caving in or breaking loose and that the car will be drivable. I'm looking at this as a stop-gap until I can make arrangements to do a proper repair.

Of course I could be wrong and am prepared for them to drop the subframe and find it's worse than I'm thinking...we'll just have to see. Things are on hold due to some scheduling complications but I'm hoping to get the car back in the shop the week after next.

[HayekFan]

The shop was worried about the dowel pins twisting out when they loosened the nuts with an impact wrench, but it seems like the upper stiffening plate or the weld holding the insert would have to fail for that to happen, and those seem to be in good shape as seen in the pic.

They were also worried about the insert pulling through when they tightened the nut during reinstallation of the subframe, but again it seems like that would require the upper plate or the weld to fail which seems unlikely.



I also don't see the role of these brackets talked about much, but it seems like they do a lot to control the lower end of the pin. For instance during braking the bracket would be under tension to resist rearward force on the pin.

Overall it seems like the critical structure is the upper stiffening plate and the weld holding the insert. The lower stiffening plate seems secondary, and the floor pan layer seems to act mainly as a covering that doesn't directly bear the load of the pin.

Does anyone agree/disagree with this?

[HayekFan]

Well, good news. The shop was able to replace the subframe bushings and put everything back together with no problems. I was able to stop by right after they dropped the subframe to take a look. The mounting pins and sleeves seemed solid despite the rust damage in the lower wall of the boxed area. As far as I can tell, the pin & sleeve just pass through the lower wall. There is no load bearing mechanical connection between the lower wall and the lower end of the sleeve. What matters structurally is the upper wall where the pin & sleeve are mounted. In my car, the upper wall was in good shape. The shop removed as much rust from the lower wall as possible and then treated the area with rust converter. It's not exactly beautiful, and ideally I'd get some new metal welded in at some point, but I don't think it's urgent. In fact, I think it would be fine for the foreseeable life of the car to leave it as-is, which is probably what I'll do. The new bushings did sharpen up the road feel of the car...less floatiness in the back end now.

[HayekFan]

Bumping to the top for comments anyone might have on this.

My conclusion is that the lower wall of the mounting point just acts as a pass-through for the pin and sleeve. It doesn't handle any mechanical load, so rust damage there isn't fatal. But it must be limited to the lower wall.

The critical part is the upper wall where the sleeve and pin are anchored. If that part fails or is compromised, you're in trouble.

Agree/disagree? Thoughts?

[Fanclutchnut]

This rust and the requisite repair have been pretty well documented here and on YouTube. Most cut away the first layer from below that’s rusted and weld in a replacement panel. Both sides need to be coated with weld through primer. Next apply undercoating and that should be good to go.

YouTube Memphis e28 and you will see him do a similar repair when he removes the rear subframe.

[HayekFan]

Thanks, that Memphis E28 video clarifies it for me at around 3:30:

https://youtu.be/D5id42b9d8E?si=lRkFuN0tsrk-m3-X&t=200

Contrary to what I was thinking, the lower wall of the mount is *not* just a pass-through for the sleeve. While the sleeve does pass through its outer layer, it is welded to the inner layer, so there is a stressed mechanical connection there.