What is this w123 under body part?
 

This is how my new 123 looks. Actually the two bolts on this part are about loose due to rust.

What is this? (It's near the back seat floor under the car-one on each side)

Is it important.

Why do the floors rot on these things?

more pics
 

Also, how do I know when the car will fall apart on me? I would hate to drive down the road with the car folding in half:D. What should I look for?

Anyways, here are some pics.
thanks!!!
Nick

Last... I'm betting there was a leak somewhere that's not draining. What can be done to keep water from getting in and if any does get in, gets OUT!

It's caused by the undercoating. After a couple decades it begins to crack.
Then water, and road salt, gets in and under the crack, migrates and stays wet for long periods.

From the looks of your pics, it won't be too terribly long. I fear the rust!! :eek:

What is there to look for structurally?

What's the hose that's been cut?

It may be a drain

What's the metal part with the two bolts near the opening of the hole? That's what I'm wondering.

That is one of the rear subframe mounts. My 240D had some rust in the floors in the same area. If yours is like mine it rusted from the inside out due to a water leak into the cabin. The lip where the subframe mounts was solid on my car so I cut out the rusty floor and welded in a patch from a junkyard car. It's as solid as new now.

Looks like you got some floor pan tab repairing to do and some rocker panel work

I would go through the whole car make sure you lick all the rust. These cars rust at the spot welds too because you cant seal the spot welds. The problem with the undercoating is it traps water, road salt and it has no where to go besides stay in place. these cars you need to catch the rust before its too late. The too late stage is when it is completely blown out and sticks out like a sore thumb.

I would first examine the car to see if it is a localised condition from water sitting in that floor area or a more general overall condition. If it is a general overall condition then the car has limited time. You will not find enough good metal to weld to.

Impossible to get good welds on oxidized metal if the oxidation is more than surface rust. A screwdriver test is valid. If the screwdriver pushes through the metal at many locations the iron oxide is being held together by the undercoating. The steel has basically returned to it's native state.

Localised condition is another story. Usually fixable with good results since you still have lots of metal nearby as good as new to weld to. . To be fair rust is rust but presents differently in different areas of north america to complicate things further.

Thats some pretty bad rust there......fix it if you can, otherwise just drive it till the body dies and transplant the motor/trans into a new home.

Wow - looks like that car's from Michigan! Start looking for a better body for a transplant!

Doesn't the car in your sig have like 414k on it? :D Should update that. ;)