I don't know if this is the dumbest question ever, but I certainly feel like it for asking. I replaced the air filter on my '83 300DT. I didn't pay attention to how it came off, and now I can seem to get it back on! The threaded metal piece that comes in from the top of the housing certainly seems like it should mate to the plastic piece inside the housing, but they are the same circumference. Does this point aimlessly into the housing or should it be connected to something???? I briefly searched the forums but I have to drive to work shortly and I'm not sure if I will find the answer in time. Any help is greatly appreciated!!!

the clips go on the sides.. the breather hose goes onto the plastic receiver and the 10mm nut and washer go in the center on the shaft

My problem is the breather hose. Does the metal piece that is inside the house from the top connect to anything inside the housing itself?

I'm not sure I understand what you are referring to. The oil separator (inside the air filter housing) has a connection that protrudes through the cover and connects to the hose.

Yes it goes to the opening on the valve cover.

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83 SD

84 CD

Well, therein lies the dilemma. The two pieces don't mate together. The one that portrudes through the top of the housing is the same size as the opening inside on the separator. I don't get it.

Photo of housing in question Do these two mate to each other? How so?

These are pics of the top of the housing in my other pic.
First is underside.
You should be able to get the number from the second.
You may be able to remove the fitting, and replace with the proper grommet.

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83 SD

84 CD

Seems like you have some kind of mismatch there. That metal tube from the top must have just been dead ending in the air filter assembly when you took it apart. But, then wouldn't the pipe from below have been hitting the cover of the air filter????

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

It was definitely dead ending as far as I can see. That's why I was so confused. With the housing latched on, it doesn't hit the top. Bizarre. I just bought the car so who knows what went on. I was changing every fluid and filter I could think of. The oil was blacker than the inside of a coffin.

Completely normal. Your new oil will look the same in about 100 miles.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

In these cars, the oil turns like that as soon as it is added it seems.

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1988 Toyota Camry Wagon......CRUSHED
1988 Porsche 944.......SOLD
1984 300SD.....SOLD
1995 Honda Civic DX... Totaled 11/13/08
1991 Madza Miata 193K and counting.....Mercedes t3 Turbocharger in the works 10 pounds per RWHP FTW

Does anyone else wanna weigh in? Does your PCV valve come in from the top and connect directly to the oil separator or is there some sort of in between coupling piece?
*just ordered service manual from MBUSA due to lack of knowledge and abundance of frustration*

My vent host attaches directly to the nipple from the air/oil separator. The nipple extends slightly through a grommet in the housing lid. There is no pipe that is part of the lid. My guess is that your lid was jury rigged.

There is no "PCV" on your car, it is just a hose that runs from the valve cover vent to the oil separator. The nipple on the top of the separator "canister" normally extends through the filter housing cover and connects to the hose. I have no idea why you have an extra piece connected to the cover or why your separator connection doe not stick up high enough to protrude through the cover. It appears that someone "modified" it.