Sounds like it is having problems once it's in closed loop operation. When you first start the motor the o2 sensor is not hot enough to proceduce data correctly, so they have an open loop mode that ignores the o2 sensor input until it get's warm enough. However they don't have a temperature sensor at the o2 sensor and use engine water temperature to determine when the o2 sensor should be hot enough. If the O2 sensor is really far off then the computer should set a code. If the O2 sensor is not off that far it can give bad readings that will make the engine run poorly and never set a code. I had this problem when I switched to headers, the thin walls of the header didn't hold enough heat to keep the o2 sensor hot enough. Are you running headers? Could also be a bad O2 sensor, wiring etc. The only difference between closed loop and open loop is the O2 sensor input. You really need to get a scanner so you can watch the sensor input and the computers reaction to it.
Good Luck, /forums/images/graemlins/thumb.gif
Mike
