It's also unlikely an engine would lose coolant without an external leak but not impossible. Stranger things have happened. I don't think cracked cast iron heads are especially rare. But I'm just guessing at something that may account for all the symptoms described. It's my understanding, while the O2 sensor is functioning while the engine is cold and you can monitor it, the computer does not use it's output until the engine warms up. As BJ said, it would not run well cold on the computers Normal temp program. OBD-II systems may be different, but we're dealing with OBD-I here.

BTW, I don't think anyone mentioned checking for troublecodes yet. That should be step #1 when diagnosing a Jeep with OBD. <img src="/forums/images/graemlins/cheers.gif" alt="" />