Get a bright light and look at the fuel lines for any (even tiny) bubbles in the clear plastic lines after cranking (with failed start) and then while its running.

Note which lines have air and report back.

Anything gunky enough to have clogged your return fuel supply would have clogged the two fuell filters long ago.

__________________
Terry Allison
N. Calif. & Boca Chica, Panama

09' E320 Bluetec 77k (USA)
09' Hyundai Santa Fe Diesel 48k (S.A.)