bisric,

I know the older cars (W123 and earlier) have a venting system that is prone to being clogged if you overfill the tank a lot. However, the foaming of Diesel fuel at fill up is not handled by this system. Depending on how you fill the tank, the rate of foam making can very quickly fill a quarter of the tank with foam. I typically turn the nozzle on the lowest rate of filling, which lets the foam return to liquid as the tank is being filled.

The main function of the venting system is to allow air back into the tank as you use the Diesel fuel up. When it clogs you cannot, or at least have to work really hard to, get the fuel cap off. Eventually the car will stop running and your tank indicator will begin to read full when the reserve light is on.

The 350SD may not have a venting system as EPA regulations on evaporative emissions may have applied to the 1991 models. Someone with a 350SD or SDL will likely pipe up on this subject. I think the system on my E300D TurboDiesel is sealed and either lets air in or lets vapors out when the cap is removed.

The determination of mileage requires a consistent filling technique, or, the data must be gathered over numerous fill-ups. I fill up at a few stations, and always at the same pumps at those stations.

As for what is in your tank, if it turned black chances are it was alive, and running the tank near empty then filling it up again, will not stop the stuff (algae) from growing again. You need to put some algae killing additive in there and replace the prefilter (the in-line one) pretty frequently until the problem is gone. There is also a screen like filter in the tank that may need to be cleaned.

So, I would not be surprised if your car does not have a simple vent, and the burping of foam out the filler openning is not unusual. Slow down the flow rate out the pump nozzle significantly and you should not have the foam burping out on your hands or the car paint. Good luck and I hope this helps. Jim

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Own:
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles

Owned:
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)