John,

That is quite a drop in fuel economy, even with winterized fuel mpg should not drop that much. Are you using the same station and pump park the car in the same position???

Charles has some good advice. I have never attempted to adjust the start of delivery so cannot speak to that, but the valve adjustment is a good idea (recommended every 15K for the 617 diesels). The valve adjustment is not a difficult task just time consuming and you will imporve your technique each time you complete one. I was seeing 25-26mpg all highway at 70-75mph, two weeks ago I adjusted the valves all were tight, and replaced the air filter and now I'm getting 27-28mpg. I have also only replaced the air filter as routine maintenance and seen a 1mpg improvement. Check your tire inflation as well, FWIW I run Michelin MXV4s at 32psi front and back.

Good Luck,

Mark

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1987 300DT 127K
1987 300TDT 283K (Sold)
1999 528iTa 75K
1991 300D 2.5T 214K - Dead Tranny
1984 300DT 375K (Sold)