Have you checked for air bubbles in the return line yet? I suggested in an earlier post to put clear (opaque) fuel line to check for air bubbles. Just get some 5/16" poly line from just about any parts store, Wal-Mart, Farm Supply, etc. It is often blue or pink and you can easily see fuel passing through with a flashlight. Rev the engine with the linkages and check to see if you have air.

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2009 ML320 Bluetec
1985 300CD
1981 300TD

Past Mercedes
1979 300TD
1982 300TD
2000 E320 4Matic Wagon
1998 E430
1984 300SD
1980 300SD