dmaxallitech, I work at a Chevrolet dealership in Albuquerque, NM. We have a very compitent ASE/Master diesel tech in our shop, however, we have a gremlin truck that no one can seem to figure out. Customer has an 03 Ex-Cab SB 4x4 d/a that seems to run just "OK" without a load and gets only 13-14 mpg but drops dramatically to 6 mpg with a small 4000 lb trailer. Truck has about 6100 miles on it. Customer traded in an 02 dually crew cab d/a that got 17-18 mpg empty and around 12-13 mpg pulling the same trailer. Our technician shows no codes and everything shows normal on the tech 2. He has even driven it about 150 miles himself for the customer to try to figure something out, but to no avail. Customer also complains of an intermitent "slip" in the transmission, but with no duplication once it's in the shop. Any thoughts? Customer is very concerned...kind of thinking about going back to a gasser at this point. Thanks for any insight that you may have!