If you can solve this mystery it will probably solve a series of similar mysteries that have appeared on the board in the past. People have reported high oil consumption and clouds of smoke with no explicable cause. I have speculated myself that it has something to do with that check valve. Have you poked anything down the pipe to see if you can feel any oddities with it, particularly with the engine running?

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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