I've searched around a bit about this problem, but each case is slightly different. As I've heard, the 300D gets around 25 mpg city and 30+ mpg highway. Mine gets 15-17 mpg city, 21 highway.
So, things I've checked:
-Filters, screens
-ALDA
What's next? Should I check the timing chain stretch (it has never been replaced)? Should I have the injectors pop tested? I can live with low mileage but it has been getting progressively worse. The PO two owners ago (owned the car for 8 years) kept a log of mileage that I found in the glovebox. He was averaging 29 mpg at the beginning of his ownership. At the end it was averaging just 25. I'm inclined to think timing chain but that will be expensive to fix- one of the many problems with the low income high school job .

BTW- it burns no oil

__________________
Andrew

1989 Volvo 745- 202K