Ok, '98 GT..... just installed a Pypes o/r X and a Mac flowpath catback yesterday. Before I fired the car to check for leaks, I installed the UPR Mils I had bought. When I finally started it, the CEL light came on... and my buddy cleared the light with his tuner. Last night, car sounded awesome.... but was lacking a lot of low end torque (to be expected).... but today, the car is acting completely different.... still no CEL, but the m'fer is loading up hardcore under any hard acceleration, not just WOT. It backfires and sputters and loads up and runs like a retard and I cant for the life of me figure out why. Another buddy told me his ran the same way and all he needed was a tune-up; i.e. new plugs and wires.... possibility even after it ran fine last night? WTF!......help?