Figure this one out. All summer my empty mileage has been about 15 or so. I run PS Diesel Kleene every tank full. I don't see EGTs much above 800* or the boost gauge above about 5 psi so I am not gettin' on it to get going. I get about 14mpg pulling my two trailers (17,100lbs cgvw). When I do get into it it seems the power is down untill I see about 10 psi boost. Then it really pulls. Could I be having a problem with the diaphram in the AFC? Would that cause a mileage drop such as I am seeing? BTW, fuel filter is clean, pre filter clean too. I don't even see much smoke unless I put my foot to the floor. Any suggestions?
Tom

 

Your towing mileage seems normal. Just a shot in the dark, but maybe you're not building enough heat in the ol girl when running empty. Judging by your sig I'd say 18-19 mpg is more the ballpark your truck should be in.

g

 

Yea, 18 or so is what I used to get. I even broke 20 once on the highway. I forgot to mention, the truck has 179k miles on it and I adjusted the valves about 35k miles ago.