I had a truck that would start sputtering and puffing smoke when towing my tractor on my utility trailer but only when pulling a hill (this before I bought my 5er). But ran fine not towing. Drove to the dealer with tractor in tow, told them the problem, tech connected his diagnostic device (OBDII or whatever they had back then), got in truck with me, and down the highway we went. First hill and he said he got it and back to shop we went. He replaced a sensor (forgot which one now, been 15 years) and no more problems. Just need an accommodating mechanic.
