Several things come to mind. First is the O2 Sensor. You can get a faulty one and it throws off your readings causing the vehicle to run too lean or rich. Second, is one that I have experienced before on another v6 Mitsubishi in the past. Owner had a friend do the timing belt on his car, but afterward he couldn't get it to smog for the life of him, so he sold it at a loss. Turned out the front cam was off by 2 teeth. Car couldn't run right because of that. I fixed the timing and it worked just fine after. Last might be ECU, but lets not go there just yet.