My 84 300D was getting between 23 & 26 mpg. Lately it seems to have diminished to about 19-22 mpg. I replaced the fuel filters 6,000 miles ago and replaced the small primary filter 250 miles ago, still no help. What is the likely culprit? Valve job? Turbo/boost checkout?

Anxious to get some ideas and direction to troubleshoot.

Thanks,
John