When I was Driving Truck, I had a 89 Freightliner that started running bad, but when the guy came over from the shop, the comp said there was no codes, but when he tried to rev the engine up it would sputter like crazy, he thought it was the fuel pedal (the 3 wire type) and changed it out, still sputtered like crazy, AND still no codes, I asked him if the computer could be messed up and just didn't know it was messed up, he change the comp and the truck ran fine, in fact it ran better, and got 1.5 miles better fuel mileage, pulled a hill better, and so forth.
Could it be that there is something wrong with the trucks comp and it just doesn't know it, and do you have another comp to try on it?????
I hope you get it figured out.
Thanks Robert. I guess it could be the ECM. It's the only one I have so I don't have another to test... I thought the same thing too. It still runs really good just sucky MPGs
Robert