I've been having the typical W123 interior leakage where water from rain or washing the car pools up in the rear of the floorboard. As I mentioned in another thread I have started addressing this issue for the second time. The first time, I did some POR15 sealing around the battery and cowl drains. This time I found out it was the windshield seal and those rubber boots in the door jams. I also did some poking around with a screwdriver and I was rather shocked to find out that the corners where the rear suspension support bolts are located are rusted through in some places, as shown. All this on a car that has been a southwest car all its life. Even cars like this are not immune from rust as you can see.

I'm still in the process of cleaning up the area to evaluate the damage and come up with a repair plan. So far I'm thinking of POR15, some Epoxy Putty and maybe a few rivets. But I need to know exactly what that brace in the second picture is for and how much force those two bolts in the floorboard are subjected to. From my analysis from below the car it seems to me that the brace is only for emergency support of the rear suspension if the rear suspension mount bolts and differential mount bolts break off. Is that correct? Or is it subjected to some force all the time? Does the suspension mount bolt go through the chassis or does it only connect the suspension with the floorboard? I hope it's the former. Suspension is really not my area of expertise.

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2004 VW Jetta TDI (manual)

Past MB's: '96 E300D, '83 240D, '82 300D, '87 300D, '87 420SEL

The 2 bolts on the right side of the second picture are loaded in shear from that bracket. It takes the shear so the rubber subframe mount sees compression. Get the rust out of that area (and what looks like old bondo) and weld in some new steel above those bolts. You should be fine.

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'82 300SD - 361K mi - "Blue"

"Good judgement comes from experience. Experience comes from bad judgement."

listen, look, .........and duck.

That is what basically pushes the car. Yes, I would suppose there is some stress there depending on degree of acceleration. The main bolt takes most of it, but I would make sure the area of the smaller bolts is sound. Just went through a complete overhaul of the area, but without the rust. My problem was the passenger door weatherstrip (cracked) leaking just above the striker. The door bottom would hold the water until it spilled over the sill & into the rear footwell. I'm sure others will join in with more expertise in this area of repair.
Hope it goes well, Paul.

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'84 300CD 290,000

It looks like the second picture is going to take a little welding to fix.

Keep peeling back the undercoat it looks like their is a lot more under their.

I really hate undercoating.

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1999 SL500
1969 280SE
2023 Ram 1500
2007 Tiara 3200

I'm not really sure what you mean by the last statement. Which direction do the forces point? I think the 2 side bolts see mostly mild horizontal forces, but then again there is that vertical stop there which implies they can see vertical forces as well. I think if there is any force on them it's mostly during cornering and not as much during acceleration, because during acceleration the rotational forces on the suspension would cancel each other out. I still have the unanswered question of whether the suspension mounts bolts (like the one on the left side of the 2nd picture) go through the chassis, or merely press against a rubber pad on the chassis.

I wish the rust was somewhere in the middle of the floorboard. This area will be tricky to repair.

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2004 VW Jetta TDI (manual)

Past MB's: '96 E300D, '83 240D, '82 300D, '87 300D, '87 420SEL

Anyone?

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2004 VW Jetta TDI (manual)

Past MB's: '96 E300D, '83 240D, '82 300D, '87 300D, '87 420SEL

Yuk I hate rust.

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1981 300D 147k
1998 VW Jetta Tdi 320k
2001 Dodge Ram 2500 141k
1979 300D 234k (sold)
1984 300D "Astor" 262k(sold)
Mercedes How-To and Repair Pictorials
I love the smell of diesel smoke in my hair

This was the floor from my W116 6.9. One would guess that the water came through the windscreen seal or through the vent. Seems to be a pretty common problem for these cars.
Excuse the huge picture

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With best regards

Al

Damn, that's some nasty rust! I was wondering where this thread went and it turned out it got moved from the diesel discussion to the rust repair section. Makes sense. Anyway I figured after a long time and finally getting the structural repairs completed last night I should update this thread.

As you can see on the attached pictures after painstakingly peeling off all the undercoating and interior insulation in that area (basically along the whole rear seat edge from left to right, including above the driveshaft) I used silver POR15 paint, followed by black POR15 patch (the thick paste stuff) as a second coat and to fill in minor holes, followed by the POR15 Epoxy Putty to fill in the larger holes. Later on I got some sheet metal from Lowes and with some more Epoxy as the base I bolted it in to provide reinforcement to the rear suspension trailing brackets should the original two bolts break loose. It's not a pretty repair and I'm still going to paint the Epoxy black from the bottom, but I think it's pretty functional. I was amazed how much rust I actually found. This is a southern car and in pristine condition, but look how much damage a little interior leak and old undercoating can do if left unchecked. Don't let these old Benzes rust away!

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2004 VW Jetta TDI (manual)

Past MB's: '96 E300D, '83 240D, '82 300D, '87 300D, '87 420SEL

On the W116's the vent that is being talked about is the winshield cowl vent, correct. I understand the windshield seal which I need to replace on mine ASAP or only drive on dry days.

Also when the floor at the acelerator pedal mount is that bad are there any specs for the size or just trial and error cut and paste ( weld ) so to speak.

I lucked out, mine ( W116 300SD ) is in really good shape. Just some small amount of rust in the spare tire well and the drivers side floor.

Rust, if you want to see rust work in a Fire Truck Refurb shop.

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The Stable.....

1979 300SD
2003 Siverrado ( D-Max )
1984 International S1603 7.3
1984 Kubota L135
2000 Kia Sephia ( Cummute Beast )