I made the conversion to a 300tdi setup in my d-90 this summer. For the first few months I was averaging around 25mpg.. Into the start of winter my MPG fell to about 20, I assumed this was due to the low sulfer fuels?? over the last two tanks I have averaged about 16mpg... I haven't noticed and drop in power, no smoke differances, no leaks.. etc
Any suggestions on what to look for? How about and tdi tuning gurus in the denver area?
There are 3 fuel return lines on top of the engine running from injector to injector. If any of those come off you will dump large quantities of diesel on the pavement and the mileage will suffer greatly.