agreed with the comments made on this one.

I just got back from doing a 2,000 mile trip with my 99 XJ and even sitting in city traffic over Memorial Day weekend with no breeze at all, it was 90F+ out...I thought for sure she'd go **** up on me... but she never moved any higher than just a hair over the 210 mark. I was surprised to say the least... and went right back down to just under 210 after I got moving. And except for the towns/cities I went through, it was all highway... so I'd agree that I think the OP has something like that going on. Could be as easy as a t-stat change, but could very easily be the rad. Fortunately I've seen that those aren't too expensive to replace either.

 

Thanks for all the input and replies! I reckon I'm gonna keep an eye on things for the time being. It hasn't really been that hot since I replaced the fan clutch. Maybe I was just over reacting when it got past the 210 mark the other day going down the highway. Maybe do some test runs to see. But a radiator might not be a bad investment anyway.

 

These can be had for well under $100 if you shop around or search for A/Z coupons. Cheap insurance.
https://www.autozone.com/cooling-hea...107695_0_13752

 

I also think you should try the oil analysis, they can find indicators that coolant is mixing with the oil. And in many cases that may be the only sign that you have a leaking head gasket, except for the overheating.

Just like yours, my jeep overheats but only on the highway after 30-60 minutes of driving, depending on outside temps. I replaced the entire cooling system, aggressively flushed all the rust out, and then replaced the radiator again. Still didn't solve it. I did all the tests - compression, exhaust gases in coolant, cooling system pressure - none of them showed any problems. No obvious signs of a head gasket leak. I was dumbfounded.

I then send my oil sample into Blackstone labs and they found the coolant in the oil. I was in HGD (head gasket denial) for a few days until I finally accepted the truth, and I had the head off the next day. I took it to a machine shop yesterday and it was warped but the HG was not damage though you can see where the exhaust gases where getting past it from the discoloration.

In a few days I'll know for sure if it solves my problems. So right now it's just a theory, but my theory is that these engines are so badass they run great even with a warped head or blown head gasket. You'd never know until it starts to run a little hot. And when it does, you can, or at least I could, keep the temps under control by slowing down and turning on the heater. I did that well over a year, including a long trip to Colorado about a year ago. It did boil over a few times but it didn't seem to damage the engine at all. So most people might not ever detect it. There's many folks out there, myself included for a while, who say that these engines just like to run hot and the cooling system just isn't good enough. But maybe they have a tiny head gasket leak just like I did, and they'll never know it!

I'm not saying you for sure have a bad HG - the only thing worse than a bad HG is a misdiagnosed HG - but I think you should really check hard to make sure you're is not blown, including an oil analysis.

 

Is the clutch fan shroud on it? If not then that'll bring the temp down a bit. Mine was getting just under the red on hot days and during long trips immediately after breaking my shroud and coolant was boiling but car want actually exactly overheating then I installed a new shroud and now its right about 210 all the time. Haven't had the problem in 3-4 weeks.