I was just commenting this morning to my wife how great fuel mileage our '87 300D turbo is getting.I have been watching this scrupulously for 7 months.I recently did the Lubromoly "diesel purge".Great improvement since then.Also i have heard warmer weather improves diesel mileage as they are not blending it with kero.As of today my car has gone 157 miles on 1/4 tank.That's roughly 4.5 gals.No i didn't make a mistake somewhere.I fill it up..I keep track of the mileage.This is what it gets.Oh by the way...that is all country/around-town driving.No open road 70mph miles either.I am amazed and thrilled to have such a powerful,economical,luxurious,SAFE,vehicle to haul my ugly carcass around.She just turned 116k so I know I have many more years of enjoyment....Try the diesel purge after you get your problem fixed.I too think you have a leak somewhere.Check your return lines(braided on the injectors).
They can seep and the fuel evaporates on the hot engine.And check both lines to/from the tank.Good luck.

What's up Rick????