As a footnote, maybe the reason the coolant temp warning did not go off with the UG warning was that the sensor does not work if it contacts air instead of liquid, which happens when air gets into the coolant loop. At least this is now the AJV8 works on the disco 3. Glad you found the fix.

 

Believe the previous suggestion of doing a pressure test might indicate some pressure loss. It's common to have a "minor" HG leak without having any other visual clues or performance issues. Other minor seeps and leaks can easily go undetected and leave little or no evidence. If pressure test is good?


That's a good one to ponder considering the small amount replenished. The air mixing with liquid can cause pump cavitation and a low flow condition according to the experts. Others have claimed pockets of air move through the system and sometimes block flow?

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Two months ago I installed a new, genuine LR, grey soft-spring thermostat. Not long after my last post in this thread in February the truck overheated again despite the micro-top up procedure discussed earlier. When I say it overheated the temp on a short, 5-mile trip climbed up to around 212°F. Not devastating by any means, but unusual for my truck. Temps were back to normal for the drive home an hour and a half later that evening. That indicated to me that my original diagnosis was likely correct --- a stuck or sticking thermostat.

The truck is not a daily driver right now so I parked it and ordered the new thermostat. I decided to spend the additional $25 for the 180° unit. I waited until a nice spring-like day to install it. Installation was a breeze, partly due to me having replaced the original spring-type hose clamps with worm gear clamps the last time I changed the thermostat a few years ago. I even captured and reused the gallon or so of coolant that drained, filtering it twice through paper towels. The RAVE says to drain the cooling system and to remove the fan. I don’t know why; I didn’t need to do either step. Some coolant drains when the hoses are released from the old thermostat but it’s a lot less than from a full drain like I do when I’m flushing and refilling the system. And by the way, I had no problem bleeding the system, following the RAVE method. In fact I’ve never had a problem bleeding the cooling system.

Before the recent problems my temps were consistently 194°-195.8°, dipping very rarely to 188.6°. In the heat of summer in traffic it would occasionally climb to 201°. In the two months and (only) ~800 miles (city and highway) I've driven since changing to the 180° thermostat the cruising temp has been 188.6°, climbing to 194°-197.6° at stoplights and very rarely dipping to 186°.

I've long read all the forum material about the 180° thermostat but since my temps were already good I was also always skeptical its value in an otherwise healthy system. I think my experience bears that out. I doubt that the 5-6° difference in cruising temps is of much value. But I could be wrong.