I'm a (retired) refrigeration tech. I break everything down into events. You have 1 air flow, 2 surface area of the heat exchanger, 3 engine heat, 4 coolant flow. I'm sure there are some other factors also.
1. You can use brute force (bigger fans) to force more air through the heat exchanger. Or vent the engine bay better.
2. Add a three row radiator or tweek the system a little. like replacing as much rubber tubing as possible with copper. Finned copper, aluminum or stainless would be even better.
3. Run high octane fuel and/or richen up the motor a little or retard ignition.
4. Run a 180 thermostat or a high flow water pump. Or try a better coolant, to stay on thread.
1. the engine bay is full in the 4.0 and air doesn't flow through there well. Bigger or better fans are an option. But to my way of thinking at 30 MPH or above the fans mostly get in the way of air flow, they really don't help much and mostly just make turbulence. There has to be a better way to vent without installing hood vents that let the rain in. Venting at the rear of the hood is unlikely to be efficient, there is positive pressure in front of the windshield, especially at speed.
2, A three row radiator is an option, but space and clearance are a real factor without some serious modifications. And adding another row will not increase cooling by 33%, more likely half that because the differential between the amount of heat and the ambient air temp. over the entire heat exchange area is also a factor.
3. No real way to adjust timing, though you could richen it up by fooling the engine temperature ECU sensor.
4. High flow pump or 180 thermostat may work out for you (it is a stock Jeep offering as a severe service option. But like a high volume pump it may also backfire, either no change or it can get worse. The amount of time the coolant stays in the radiator (and the volume) also has a bearing on how much heat it can shed. Actually two ways, it does take time to shed the heat and it really only sheds heat in the outer portion of the flow stream. Or in other words the more turbulence you have inside the radiator (heat exchanger) the better it is going to function and shed heat.
It is a system, a series of events, a closed loop, a pizz poor, primitive system that you can tweek so far out of the envelope it ceases to function as designed.
The most bang for the buck IMO is likely to vent the engine bay, figuring out how to do it without making an opening the water gets into. The best way to vent at speed is too make an open 90 degrees to the air flow, like blowing across the top of a coke bottle.
I do a lot of slow speed driving cruising through my hunting lease. The aux fan comes on and I've noticed when I walk around in front of the Jeep that I can actually feel some hot air coming out of the mechanical fan side of the radiator. The air flow through the engine bay seems to be seriously constipated
.
An Aux tranny cooler is going to help get some heat out of the radiator, but can cause issues in extreme cold. The tranny cooler inside the radiator may also help to heat up the fluid in extreme cold.
Something nobody ever talks about, mainly because the oil pump in the 4.0 is kind of marginal IMO, is an engine oil cooler. If you can find one with a 180 thermostat and maybe install a high volume oil pump at the same time, it would likely help with the heat problems most XJ's have. Where I live there are no speed limits on the interstate (autobahn), all my freeway fliers have had an engine oil cooler installed. Synthetic oil and an oil cooler, with a thermostat works well. I keep my XJ's under a 100 MPH, so I've never really needed an engine oil cooler, it rarely gets that hot here anyway.
Sorry about the rant, I've given it a lot of thought.
