i recetly picked up an 88 cherokee 4.0 and it ran fine when i got it but i started wheelin it and now it overheats then it idles really high the hotter the higher it gets
btw i think it might help if i mention i take it in alot of water and mud
and when it overheat it blows presure out of the cap

 

Cooling system flush, new t-stat (OEM 190-195 deg), new cap and posibly new reservoir. It runs a closed system, so if you can afford to switch get a rad for a later model XJ with an Open cooling system, new overflow tank and new upper rad hose.
Def check for coolant in oil and any other problems you may have. Also, check water pump, mech fan/clutch and sensors and your electric fan.