cooling issues

1990 4.0 that belongs to a friend dumped coolant from the overflow and overheated. i figured could be bad cap but its a closed system or something and no cap. the prev owner did water pump and prob thermostat to try to stop a bad squealing sound. i looked at the truck today and half filled the overflow and started it. the upper hose never got hard but the overflow did have coolant coming out. not much but didn't let the temp get past 1/3 of the gauge. where to start not used to these older xj's any ideas?

search burping...may be just the solution, especially if water pump and t-stat have been replaced (squeeling is probably pump related). So you re-filled...started...and it didn't overheat? Then, what's the problem. I've been down this road, until it's burped properly, it will show hot...and boil through the overflow bottle...until it self-burps...then all is good.

i wasn't sure if the system was full because the truck had just dumped allot of freeze but i couldn't get it to fill up from the overflow so just ran it to diagnose and saw that it was dumping to the overflow without even vein that hot i didn't let the truck run that long i guess i will search burping