I noticed that I was getting a bit of water in my trunk near the fuel filler neck. Decided to get a new fuel filler neck seal. The old one looks to be in decent condition, but on removing it I discovered the back of the old seal was either coated in tar of some sort, or had turned back into tar from diesel exposure. I am thinking probably the later? Anyone recommend something to clean this gunk off?

AFAIK replacement is the only option. I noticed years ago that biodiesel softens/dissolves the rubber used to make the seal. In consequence, after replacing the seal and buying a few spares, I created a "diaper" from a piece of rubber sheeting (the stuff used to line backyard garden ponds) and placed it over the filler neck and its seal. The diaper is also attacked by biodiesel but is easy and inexpensive to change occasionally.

Jeremy

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"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970

Yeah, i was replacing it anyway. I thought maybe that was a sealer that someone added. I ended up using a label remover and a whole bunch of shop towels to clean it off, and got the new one installed.