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Rawze

As far as the fuel mileage being low ... and oil turning black prematurely, thare can be a hundred plus things that can cause that. Do a proper oil sample and send it off to a lab. For all anyone knows, it could be getting coolant or fuel seepage into the oil, causing accelerated wear.. on not. It could just be a crappy delete program in the ecm causing excess soot, or missing block-plates still allowing egr gas into the intake, it could be a lot of other things too.

You also need to do a tune-up on it if it has not been done recently. That engine still needs a lot of its sensors to run properly.. even after a de-mandate. Replacing the IMAP and exhaust gas pressure sensors (and ensuring the feed-circuit is not blocked up for it) if they are more than a couple years old.. a proper turbo inspection .. pressurizing the engine up to 30 psi and checking for boost/CAC leaks etc., ... and doing a proper injector leak test, etc.etc.etc.

-- These engines do no heal themselves.. they do not have internal fix-it sauce inside them.. they need proper maintenance and regular care for them to run correctly and not give issues or have lower than normal fuel efficiency. - NOT doing proper and regular maintenance will cost someone big time by letting things go .. even a little bit.

- Also, How much boost does it make in a hard pull?.. What is the pyro temps in a hard pull?.. etc.?

The First and biggest priority however is to have the program reviewed to see if it is a trash delete program. That is one of the biggest plagues in the trucking industry with regards to de-mandated trucks. More then 90%+ of all de-mandate programs are extremely hard on the engine and absolutely will cause pre-mature failures like fretted liners, blown head gaskets, repeating turbo failures, etc. So start by Copying the program out of the ecm and e-mail it to me for a review. I will tell you if it is harmful to the engine or not... or if it is contributing towrds lower than normal fuel mileage, etc.