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In general I have found leakage at the filler cap a consequence of overfilling the tank when the fuel is particularly foamy. A big blob of foam can burp out now and then, causing the filler cap cavity to get a little wet with Diesel fuel. Fuel leaking out the vent line is an indication the previous owner regularly overfilled the tank.
I do that too, but it is not a good practice if you fill it to the point where liquid fuel is visible and then park it. When the fuel leaves the underground tank it is usually around 60*F. During the day it will get warmed to the ambient air temperature, which in the Winter is usually not a problem. In the summer it will cause the fuel to expand significantly, and it will force itself out the vent lines.
This makes the vent line rubber junction fittings get soft and slimy with Diesel fuel, and can lead to vent line leaks or vent line plugging. There is a little tank on the passenger side outer wall of the trunk, behind the wheel well, I believe, that is intended to address the occasional burp of foam, separating the liquid from the vapors and air, and letting the liquid run back to the tank. If you fill it with fuel the fuel has no place to go but out that little fitting.
The fact that the fitting is wet is a good indication the cap and the filler neck are not damaged. The filler neck, I believe, is a solid pipe, but its connection to the body in the little cavity enclosed by the body fuel filler flap is rubber to allow the tank to move independently of that connection to avoid overstressing the filler neck as the car body flexes going over bumps and taking corners, etc.
Hope this helps, and good luck fixing that leak. 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)
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