So uhm, not sure how this has happened, unless a troupe of British fairies came in the night and fixed it... But I haven't driven my XJ in about a month, as she overheated and leaked a nice puddle of coolant. After gathering the most likely replacement parts, I topped off with distilled water and performed a pressure test at 15psi. To my surprise there were no leaks after 20 minutes, so I pumped the pressure up to 20psi. Still no leaks! I then drove slowly for about 10 miles with the AC off. All good aside from the LOW COOLANT warning. Then I drove at 'jaguar speed' for another 10 miles. Low coolant light now turns off, but came back within a minute. After this, I put the AC at a comfortable level and drove quite aggressively in Dynamic Mode the rest of the way home. No leaks, no overheating, AC blows cold.

Can someone please help me understand what is happening?
Before the leak occurred, I had just changed injectors, and thus removed the overflow tank. Do you think I didn't fully burp the coolant when I topped it off, air stayed trapped, caused me to overheat, and the leak was from the overflow cap letting off pressure?! Is that logical or possible? There was coolant nearby the overflow after the original leak, after all.

As far as the low coolant warning staying on, I suspect if the above is true, then perhaps the sensor in the overflow is broken, luckily I have a new tank sitting here. Any advice would be deeply appreciated, as I currently feel like I'm trapped in an elaborate practical joke lol

 

I'm no expert, BUT
If the pressure in the coolant system is high, the coolant will come out even without a leak, just like if you overfill the system. Maybe that's your issue, especially if you know you overheated.

Also, please let us know if you catch one of those fairies. I hear they have a high market value.

 

The float in the coolant tank can become saturated and sink, this leads to a low coolant caption.

The only way to fix that is with a new tank.

wombat

 

This happened to mine- I replaced the tank when the car was about 8 years old. This would only trip the sensor (low coolant), but would not explain the puddle of coolant.

 

So it's possible that the overheat caused high pressure and thus the venting of coolant? Only thing I can think of since there's no leaks during pressure testing.

 

Same here. Bad sensor float at 19,000 miles. 5 year old V6 petrol. But no overheating or leaks.