After years of owning XJ's, I'm going to finally weigh in on cooling issues. These are my experiences and opinions, and I'm not intending to ruffle any feathers, so I'm not calling anyone out on their expertise. There are far many better and smarter mechanics on this and other forums than me.
The fact of the matter is, that XJ's seem to have inherent cooling issues. There's too much evidence of that to deny it. I've probably read every thread, post, article on the Internet about XJ cooling problems. I've talked with radiator shops and a number of other people about these issues, as well.
I've come to the conclusion that no one really knows what the Eff the problem is with XJ's. Period. One fix works for one person, but not another. Some people, (myself included) do some repairs and it works, while others, (myself included) try a number of things and just put up with the problem. Seems to me hit or miss. I suspect the root problem to some of these problems is a clogged passage somewhere due to corrosion. That's the only thing I can think of. Could also be head gasket issues that people can't diagnose properly. Regardless, no one has ever said that "culprit Z" is the issue and you need to fix that and the cooling issues are fixed. It's always try this, that, and the other thing, usually with mixed results. Here's my experiences with 4 XJ's that I still currently own.
On my 97, 160,000 miles, you couldn't get that thing to overheat if you tried. Have an American Eagle 2 row aluminum radiator, but everything else is stock. Stays just below 210 all day long even in the So Cal desert. Mostly a trail rig, but I can drive it all day long on the street or highway, short or long trips, doesn't matter.
99 2 door, 200,000 miles (current build). Had some overheating issues early on. Looked like PO neglected the cooling system. Changed out the radiator to a Mopar heavy duty, Hesco water pump, new stat, hoses, etc. Old radiator was clogged, but everything else looked okay, just old and dirty. Flushed the system and now she runs cool as could be.
My sons stock 99, 140,000 miles, his daily driver, has mild cooling issues, but nothing too major. Looks like PO used some stop leak. We've been flushing out the system, but I will probably change out the radiator and other components as part of regular maintenance.
My 95 trail rig, 81,000 miles, has been the typical XJ cooling headache. Started with temp creep on the highway, and in town. So after putting on the lift kit, lockers, gears, 31's, I decided to attack the cooling system. I ended up changing everything, and I do mean everything. I tried every fix that I read about or was recommended to me. I tried 3 different radiators, CSF, stock, and one other. I finally had the biggest, stupidest radiator custom made to get the most fluid capacity. Re-routed the trans to it's own cooler. Tried Hesco and stock water pump, high flow T-Stat housing, deleted the heater control valve, new muffler and cat, ZJ fan clutch and stock fan clutch. I've checked for exhaust manifold cracks. Checked the radiator fluid, checked the compression, checked for fluid in the oil, blah, blah, blah. No culprit anywhere that I could find. So I just put up with it. I just cycle my fan override switch on the trails, and that keeps the temps where I want them. That's how I choose to manage the problem for now.
That's been my experience with XJ's. I can't figure it out, and frankly, doesn't seem like anyone else has. Seems like your cooling system works, or it doesn't. All the dozens of vehicles I've owned over the years have never had a cooling issue. If they did, it was an obvious fix, and I've owned a variety of vehicles.
So my advice, throw some parts at it, do what you have to do to make it work for you. I've even read where people have done engine re-builds over this issue. But I have no evidence of anyone saying "this" is the root problem. Yes it can sometimes be obvious and changing a few parts work, or it can be a major headache. Everyone seems to have different results. It's just baffling how so many people can have these problems without a clear solution. I'm at a loss to figure it out.
JMHO