There are enzyme based products designed to get rid of odors. I would try one of those. They are used to get rid of pet urine odors and to put down drains to dissolve grease build up. The products absolutely work wonders on cat urine and I suspect it would work on diesel. You spray it on, add a little warm water, cover with plastic to keep it damp and let it have its way with the residue.
You can buy the product at maintenance supply stores. It is more expensive but you can buy it in pet stores where it is sold just to remove pet odors. I have bought it under the brand names of Microdyne and Liquid Alive. The stuff is a little creepy since it is described as enzyme producing bacteria in a state of suspended animination and they are activated by warm water.
If that fails, go to mrlandlord.com and look for the product called Odorxit. It is more expensive but the landlords at that site swear by it.

__________________
1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13