Is this sound normal? If I fill up and run the tank pretty far down, when I open the tank up, I get this massive vacuum hissing like sound, is this a good or bad thing?

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1996 E300 D
1992 300D
1990 Ford E-350 7.3 Diesel AKA "the Deep Fryer"

That's a bad thing. Your tank vent is likely obstructed.

I just noticed that for the first time the other day on my wagon when I ran it practically empty.

The sound only lasted a second or so, but it was surely noticeable. It's really not supposed to do that?

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-justin

1987 300TD, 1987 300TD
2008 R32, 2000 Passat Wagon

If the vacuum manages to match the positive pulling pressure of the lift pump your car will die from lack of fuel. Happened to more than one member before. Make sure the tank vent line is not obstructed. Until you get that done if car quits on the road loosen off the fuel cap.

Ok so how much of a task is it to get this corrected?

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1996 E300 D
1992 300D
1990 Ford E-350 7.3 Diesel AKA "the Deep Fryer"

You should be able to access the vent tube through the trunk, on the right side. You'll probably have to pull some of that trunk paneling away. I'm not familiar with the w124 sedan, but I can see my vent tube with ease when I remove the right side panel in the wagon.

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-justin

1987 300TD, 1987 300TD
2008 R32, 2000 Passat Wagon

Not sure on the 92 300D. But on the older W123's look underneath under the driver's side back door. About 10-12" in you should see a bell shaped piece there. Take it out, clean it. Reinstall and see how it goes.

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No matter how many times you explain it to me, I'm still right.....Surf-n-Turf

Thanks guys, when the temp gets out of the single digits, I will give this a look.

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1996 E300 D
1992 300D
1990 Ford E-350 7.3 Diesel AKA "the Deep Fryer"

That is why I suggested if the car quits or starts acting funny just loosen the fuel cap. If a guy does not have a heated garage to work in he would most likely deffer the problem till it's warmer out. Just human nature as the car still runs well until the problem starts to have a noticeable effect.
At least the tanks do not devlop enough vacuum to colapse them The engine quits before that much vacuum is developed. There is also the reverse problem to this that develops on some cars.
When you take the fuel cap off it blows it backwards towards you hand with some noticable force. I have seen it on a few japanese cars. This was before fuel injection was common. Complaint was excessive fuel consumption on a couple of them.
At least it is a very minor item to repair after you know you have it either way.