Had a call back (code was a miss-fire). GM replaced the head, at their cost. Ever since then, the tail pipe stays smutty. I've ran 3 tanks of 93 octane and tail pipe is still smutty. Talked with service adviser and he doesn't know. Called Hummer and they had a customer rep call me and thanked me for the inquire. Talking about being pissed off. They have someone call who doesn't know zip about an engine. I did notice the 2007 H3 models (same size engine 5cyl) didn't have a smutty tail pipe. I did look at 4 2007 H3's and clean tail pipes. All said they burned 87 octane. What do you think is the problem?