Sounds like your delivery valve seals need to be replaced. Issue with OM60X engines and the switch to ULSD as I recall. Do a search. You'll need a special socket. I bought one but I think you can rent it here. One of the more gratifying fixes I've accomplished (190D om 601). Study the procedure and be aware of the potential complications beforehand. Plenty of info to be found on the site.

Kevin

1978 300D
1979 240D