I converted my 1988 Comanche MJ from a closed cooling system to an open system. Closed systems are on all RENIX XJ's and MJ's (1987 to 1990). It now seems weird to look under the hood and see a rad cap!
It's a pretty easy job. The main part I needed was a rad from a 1991+ XJ but I replaced the water pump, fan clutch, thermostat and housing, all of the hoses, etc. while I was at it. Most guys get an over flow tank from a later Jeep, another vehicle or some of the generic ones that are available. I converted the original MJ pressure tank, which is a weak point in a closed system, to operate as an over flow tank so it still looks like an original system. I also got rid of the original heater control valve so it now operates using the dashboard controls (like a TJ).