My 90' XJ with a 4.0L started out with the typical cooling problems you often hear about. At idle it would start to heat up, but with the ac turned on and the auxillary fan running, it kept the temps down. After going to Camp Jeep back in July, I noticed that it started to force coolant out of the pressure bottle while still running and would just empty the bottle after shutdown. Just prior to Camp Jeep, I had replaced the pressure bottle and cap, the t-stat and the auxillary fan switch in the radiator. When the t-stat opened up, the water flowing into the bottle was boiling violently which led me to believe I may have a blown head gasket allowing CO to add pressure to the system. Well, after spending part of the holiday weekend replacing the head gasket (which did not appear to be blown), the fan clutch, the t-stat again, the water pump and all the hoses, the problem still exists. The radiator had been previously replaced due to a small leak. I started the Jeep up and after about 15 minutes of running at idle and higher revs, just after the t-stat opened up the pressure in the bottle was so great, it blew the cap right off the bottle. The temp had stayed at 210 and started rising to 235 about the time of the explosion. I am starting to think I may possibly have a crack in the head, but was wondering if anyone else had some thoughts on this or maybe experienced a similiar problem with their closed system before. The coolant seemed to be circulating through the system the way it is suppose to with the coolant filling the bottle when warmed up and drawing it back into the system upon cooling. The level in the bottle was where it was suppose to be on the notch, but after warming up, it will fill the bottle until it wasn't capable of holding anymore coolant, thus leading to the explosion of the cap this morning. Anyone have any ideas?