Running bad all the time, losing coolant, strange deposit on #1 plug. Hmmmmm No it's not a common "Jeep thing" but strange things happen to all makes. I'm thinking a cracked head could leak coolant into the intake valve area, above the valve, so the engine could indeed "suck coolant". cracks open and close some with temp changes and properties would change as the crack grew with time.

The O2 sensor should not affect the way the engine runs cold, it's out of the loop so to speak. I would talk about the fuel system but that would have nothing to do with coolant loss.

If what is said is true, coolant loss with no external leaks, I'll guess cracked head. <img src="/forums/images/graemlins/notooth.gif" alt="" />