Both 87 4.0 Jeeps I have owned didn't have thermostats in them. I went to replace the thermostat in my old Jeep and there wasn't one, and it had been running fine so I left it out. The one I'm trying to fix currently was overheating, so I bought a new thermostat for it, but it doesn't have one either. Should I go ahead and put the thermostat in, or leave it out? I have replaced the water pump and I'm about to replace the radiator and remove the clutch fan to convert it to electric.

 

in my opinion yes you should. it helps the engine run at its designed temp.

 

Put a t-stat in.......at least a 180.......git 'er done.