thermostat ?

is it ok to run the jeep 4.0 with out the thermostat 1990 xj

NO! Small emergency run maybe. Bad things: It will run too cold and always be in rich mode, washing the cylinder wall with gas, contaminating the oil, fouling the plugs, fouling out the catalytic convertor. Also causing hot spots and warping in the head. Not a good thing to do.

No! Running without the thermostat is a bad idea for numerous reasons. Why are you considering it?

Your engine is designed to run best when equipped with a 195 degree OEM thermostat.

If you have cooling system symptoms, post them here for comment so we can help you get to the "root cause" of them.