Please let us know when you finally figure this out. That hole appears to be an opening into the bell housing (contains the flywheel and the connection to the transmission - clutch if manual or torque converter if automatic). The only fluid that should be present would be brake fluid (if hydraulic clutch - manual transmission) or ATF (auto transmission fluid). I cannot see how diesel fuel could be present in that area.

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Respectfully,
/s/
M. Dillon
'87 124.193 (300TD) "White Whale", ~392k miles, 3.5l IP fitted
'95 124.131 (E300) "Sapphire", 380k miles
'73 Balboa 20 "Sanctification"
Charleston SC