With all the new emissions horse **** EPA does to our trucks like egr and pcv you may have a clogged intake manifold from all the hot sout the gets routed from your exhaust back into the combustion chamber. I'm not sure if that can cause a lack in fuel millage or not but it very well could be a clogged intake manifold.

 

I to am having a similar problem with my 07 Classic 3500 Dually Duramax. I shoe horses for a living and recently started using this truck as my shoeing rig. I travel alot of miles up and down the east coast shoeing show horses. My truck currently has 164,000 + miles and is completely stock. I have an aluminum constructed diamond plate shoeing body (similar to a slide in camper) that holds all my shoeing supplies tools etc. In a recent run from KYback home to Ga my truck gave me quit a scare. As I entered a steep upgrade my truck made a hard shutter and I quickly began to lose power. I had it matted and could only do 32 mph @ 2000 rpms. I limped the truck to the top where I could pull over and shut it off. After making several phone calls with none being any help, I attempted to run the truck after a 45 min rest i started the truck and drove off, it ran fine felt like it had a loss in power and fuel mileage was in the toilet. I made it the rest of the way home with no change and the check engine light stayed on. Later diagnostics were checked revealing a code for vane position sensor. So after replacing that truck ran fine for a day and half. Now light is back on power is down and fuel mileage sucks again. All service was recently done on truck IE oil and filter changes and fuel filter changes. Any insight or advice will be greatly appreciated.

Truly frustrated

 

bombed turbo?? does the truck smoke at all now?

 

try looking at the egr error performance and see if the egr is staying open while driving