The PO of my '87 XJ is a real POS, and managed to muck up virtually everything he touched in this thing. Most of my time has been spent fixing what that moron broke. Of course, he decided to convert the perfectly fine pressurized cooling system to an open one and did it very, very poorly. The temperature gauge never rises above around 160F while driving, and immediately starts overheating when idling at rest or stuck on the highway, so that I have to roll down the windows and crank up the heater (fun times!).

My question to you: I have an '89 XJ that I'm very familiar with, but I've never dealt with an open system conversion before. Where should I start troubleshooting? Thermostat? Is the electric auxiliary fan supposed to kick on at all in an open conversion, or should I install a dash switch for it? It's also worth mentioning that there's a small phantom leak somewhere in the system, but I haven't traced it down yet.

 

Check thermostat it probly never closes yes fan doesnt care what kinds cooling system its cooling its a fan.

Ive had ****ty results finding therrmoststs thst actually work correctly. Sometimes i gota buy 3 or 4 to get one that works.

 

I don't mean to be less-than-constructive but it seems foolish of you to criticize the PO so harshly for converting to a system you apparently don't understand how is supposed to work.

Perhaps they were dealing with an existing overheating problem and they gave up on the deal entirely which is why you have it now? Not uncommon at all for folks to convert from closed to open system.

Not uncommon for these jeeps to get air trapped in the system wreaking havoc on percieved running Temps.

Is the mechanical fan clutch is good working order? Is there a fan shroud for the mechanical fan? Is the electric fan actually turning on? Is the electric fan wired differently over stock via a switch or to constant 12v but is now dead? Your already looking in the right direction with thermostat, or even replacing the water pump.

 

First off, as Evan M says.....The thermostat is likely one of largest contributors to the the issue. Buy a Mopar one.
Did the PO put a new or used late model radiator in? How's the fan clutch? Is the radiator cap holding pressure?