Overheating

my 90 xj has been running hot for the last few days. over 210. then yesterday, it started idling over 1000 and the temp shot up to 260. the coolant tank on the passenger side boils and overflows. when it cools, i can drive it for a few minutes then it does the same thing again. when i start it when its cool, i can put about a gallon of fluid into it. it is not leaking anywhere but the tank. whats the deal.

It sounds like yourcooling system isn't maintaining pressurization because the pressure cap on the expansion bottle is faulty. Replace the cap and check the bottle itself to make sure it isn't cracked.

definitely the cap, I just went through this last month, get a new cap from the dealer and then start thinking about replacing the bottle, thermo, fan clutch etc if that doesnt do it.