[Vwdiesel] why does diesel residue cover my vanagon tailgate?

[jon.sykes at ps.ge.com]

I did a 200 mile run in my '82 diesel vanagon the other day.  Severe
headwind one way, awesome tailwind coming back.  The rear of the vehicle is
covered in this blotchy, oily, diesel residue that will smudge but won't
really wipe off.  I was wondering what might be the cause of this (van
running rich, mechanical problem?) and whether I should worry about it.

Thanks in advance!

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