Excuse the length but need to give some background. Alright heres the story, I put my 355 into my Suburban in place of my old tired 350. Now the motor has an RV camshaft, something like 204 208 or 210 duration. Really mild, low lift. 76cc heads with flat top pistons, so not lots of compression(8.5 or so). I'm running a throttle body I modified("ported" a little) and a 1" open TB spacer. I have headers that dump into 2.5" pipes, straight back to flowmaster 40s then they 90* out the sides before the tires. I put a new fuel filter in when I replaced the motor. Alright now, here is the issues. My TBI motor runs like it is carburated. When its cold, if I jump in and go it doesn't like me very much. It will go but I have to rev the piss out of it before I shift. If I lug the motor at all it spits and sputters back at me. The timing is spot on, I checked that a bunch of times before I left for school. Now when its warm, and I shift it will pop between gears. A very slight backfire. At speed on the highway, say cruising at 65, it will miss once in a while. Barely noticeable, except on a 5 hour drive across PA(just cause of the time to listen for it). When I first got the truck running the fuel pump made a nasty bit of noise. Can't even describe it, each time it turned on to prime it would just sound bad. I drove it a little and that stopped. Now all I get is the normal fuel pump noise. I am getting crap for gas mileage. Maybe like 8-10 around town and like 8 on the highway at best. Thats not even beating on it(much). On the highway I cruise at 65 which with the 245/75s, 3.42s, and 465 thats about 2500 RPMs. I think I should be doing better on gas, even if I get throttle happy sometimes. What do you guys think? Could an O2 sensor cause all this? The sensor is new but it still a single wire, and it is installed in the collector(actually in the collector) at the end of my long tube header. Think a heated would fix this? Seems to me like its running rich. Inside of exhaust pipes are a little black, smells of gas(no cats mind you) and crappy mileage. And as far as I know when a motor runs rich it will actually fool you and make you think its running lean(popping and missing at RPM). But I have no idea why it would run rich if I've only done things to improve airflow and not fuel. Any opinions? Computer tune is stock. I have yet to pull a plug and check color.