That sounds like you may have air in the system or the coolant temp sensor is malfunctioning. Either that or there could be a problem with the temp sensor wiring. I would try bleeding the system first.

 

Well this thread took off!!

Thanks for all the replies and info guys. I may just flush the whole system when I install the new OEM stat and be sure to purge the air out of the system. Great info!

 

One way to ensure the entire system fills is to pull a vacuum on it and make sure you don't suck in any air when you refill it. I usually fill a five gallon bucket with 50/50 mix.
I've never done this on a Rover. So I'm not sure if you can get an adapter for the reservoir cap? I know it works great on difficult systems though. Just a thought.

 

So I am back home, working on the Disco today. I got the power steering pump replaced, but the reason for this thread being high temps...interesting find, but may be normal. The top coolant hose is readily apparent holding nothing but air. I can squeeze it wall to wall, and can feel no liquid. Is that normal? Or is that a dead giveaway of air in the system.

Engine is cold, haven't started it yet today.

 

Success!!!

Drained the system, replaced the thermostat with the LR grey 180 stat, filled it back up with water only (will drain that tonight), bled the system and let it idle for 20 minutes in the garage. Temps rose as one would expect, topped at 199 at minute 14, then dropped to 198 at minute 18.

I still got the tapping from the sleeve once the temp reached 189, but I'm less concerned about than I was the temps (yes, the tapping is of concern, so don't read that as "oh well"...I will keep an eye on it and troubleshoot as needed).

Of note here, I do believe there was a LOT of air in the system which explains the drastic temp swings and max temp I saw last week. Also, the stat that came out of the Disco...a 92C!! That's a 198F stat!! No wonder it was hot!

So, letting it cool of now, and just noticed that it looks like I cracked the T at the bleed screw. DOH!! Well that's easy enough to replace. I think I got the screw in a bit cockeyed and cracked it.

Success for today.

 

Drained and refilled, bled with 50/50 (green), temps in the garage on a 10 minute idle were 189. Took it for a test drive, highest I saw on the 10 minute town/highway drive were 185, settling on 183. Climbed to 189 back in the driveway.

Tapping was better...not constant, and came and went during stops. Very satisfied with today's work!

 

Thank you for posting your results. Often people forget to come back and do that when they fix something.

 

Most welcome! I don't like coming across threads that have no solution or ending either, so I always come back and follow up.

And on the cracked bleed T, I dropped some Krazy glue on to the crack, put some fuel lube on the threads, and there is no coolant escaping, no bubbles. I'll watch that, but for a temporary fix on that, it works for now.

 

The bleeder doesn't have much pressure on it. So it should hold up for a little while. I would definitely get a replacement as soon as possible though. That would really suck to ruin an engine because of a little bleeder screw!

 

Plan to soon, but haven't found it online anywhere yet. Found the T, but no bleed T.