if it is a 5.4 the early 3 valve 5.4s had a problem with injectors leaking and while running will cause an excessive rich condition they also have extended coverage for this problem you very well could have an injector leaking.

you could run a balloon test but have fun getting to the fitting under the turbos.

running bio diesel will also lower fuel mileage as it contains less energy to release as the same amount of regular diesel, you also might look into using a cetane boost like the motorcraft cetane boost.

whats aftermarket tuner makes a good economy tune for getting better mpg's on the highway?

pavehawk the crashed landed next to our work shop on base

engine and trans out of the rg31mk5e mrap everything is quick disconnect engine and trans can be removed in one hour by myself.

what i repaired at my last job