thanks mbeige for chiming in. i see now that this issue is the basis of the bergsma "123 and 126 Chassis Diesel Fuel Trunk Smell Fix Kit" ($20). first, i will order a stock of new filler aprons at $2 a pop and try to keep biodiesel off them.

as far as the cap gasket, nothing appears to be wrong with it. i'd replace it immediately if i could find it for less than $16, nearly the entire price of the OEM cap. i did find a meyle cap for $8. what's so special about that plain blue rubber gasket?

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'77 240D, 504H, OM617.952, etc.