ISX poor fuel economy

[Rawze]

is it the original engine in the truck?.. or a replacement. Your post was not very clear. Was it the engine purchased used?.. or the whole truck?

and if the engine was replaced, what are the ser# (the ESN#) of the old engine and new engine? .. and what program does it have in it?. These are all very important things that need to be verified if the engine was replaced.