I have not actually gotten any responses yet, but I think I have figured most of it out. I think this XJ had some small crack in the head gasket when I bought it. That'd explain why the radiator cap popped off the first (hot) day. We only drive it on the weekends and there have been only a handful of warm days. I have had continued problems with the radiator cap, and with the oil pressure and temperature gauges, which I have read can be caused by a blown head gasket. Today when trying to test things I ran the engine with the cap off it spewed everywhere from the radiator, which I have read is a surefire blown gasket. My only concern now is how to tell a blown gasket which is surprisingly cheap to fix in Mexico from a cracked head or block. I've also realized I made things all much worse yesterday by refilling at the reservoir and not at the radiator when the cap flew off. I know it's my fault, but I've never owned a car before. It's a lesson I won't soon forget.

Hello everyone,
I've been looking over stuff on the forum and elsewhere but haven't quite found an answer to my problems.
If I've posted in the wrong area or describe things poorly I apologise, I'm new to the forum and honestly to owning a car.
So I've owned this 99 XJ for 6 months now, it's an old car but running well until now, and I'm learning to maintain it, bolt back the parts as they fall off. It's 2 WD has 250k miles on it. I've been trying to go over it all and make sure it's in good shape, new oil, filter, fluids flushed the radiator got a little scanner so I can check for codes. It has been running well, with only one fault, #1698. I've been taking it to a couple of mechanics trying to resolve the code without success, but as far as I could tell it hasn't been effecting the car much. The oil pressure and temperature gauges have been weird at times, shutting off or bouncing around sometimes check gauges will flash briefly, but then they'll go back and they're normally at 40 and 210.
The first day I drove it something went wrong and the radiator cap popped loose and boiled over. A while later that would happen again, and I thought I'd resolved by that by getting a new cap.
Today while going up the mountain the radiator cap blew off, antifreeze steaming everywhere. I pulled over, crimped the cap a bit so it would get a tight seal again, then tried to start the Jeep. It ran fine, then there was a sort of chattering noise. I thought it was the replacement grill I just installed so I pulled over, unscrewed that and kept going. The gauges were both being responsive and at 40 and 210. Then the chatter became serious knocking and clicking and the engine died. I let it cool down, started it up it ran fine for 15 minutes then died. I let it cool down longer, it ran 20 minutes, got me home then died again. I had thought that the radiator cap blowing off let air in my system so I have run it with the cap off and it was a bit better after, but still not fixed.

Any help would be greatly appreciated.
Sincerely,
Peter