I just bought a '96 3/4 ext cab 6.5l diesel. Engine, transmission and transfer case rebuilt; seems to run fine until I try to pull my fifth wheel (7400 lbs) and then it bogs down on every hill. MPG, wether I'm pulling the trailer or not is around 9mpg which seems low. I have taken it back to where I bought it and had it Sun tested and they said everything checks out. I've been told I should have no problem pulling this trailer. What could be wrong?

 

open the T valve where you drain the water out of the fuel while its running...if nothing comes out,you have a lift pump problem....has the filter been changed?