2000 Discovery II, new AB long block in April of '21. Truck is a daily driver and in great shape, no rust, paint great, interior fine, everything works. It is running an inline thermostat (180°) from Extinct.

Started to notice odd temperature fluctuations on the Scan gage II on my dash. It would rise up to maybe 200° then drop to 172° while just driving along 40 mph in regular suburbia traffic. Took it to my mechanic, who suggested we put in a new temperature sensor, got one from AB, still the same, but now higher temps. When reading the temps with his infrared gun, it reads 190° to maybe 195°, while the scan gage is reading 210-230°. And today it's 24° outside. He's installed new Hayden 2991 clutch and a Dorman 620-112 fan. The cooling system is running standard green coolant. Extinct sent me a new thermostat, but no change still the same up and down. We pulled the thermostat as per Extinct's advice, and ran w/o one and on a 20s° day it was running 150-160s° no thermostat. Put in another new thermostat, with the bleed hole, same thing. Climbed from 140° leaving my mechanic's shop driving 3.6 miles at 40 mph, and it's up to 225-230° by the time I get home.

Radiator was new in April of 2013. Was converted from Dexcool at that time. My mechanic is very knowledgeable with British cars, being a restoration shop for MGs, Triumphs, etc, so he and his team know their way around all things "British". And we did run the test to detect exhaust gases in the coolant. There were no gases detected. So in that regard, not a problem.

Where to look? As the truck is a daily driver, with my schedule, I really need the truck.

Your thoughts and advice as always are appreciated.

 

How does it act idling? Age of water pump? Age of rad might make sense to replace just as a precaution and if system is drained I'd throw a flowkooler water pump in for piece of mind. Have one in all 3 of my rovers and very pleased with them.

 

Temperature fluctuations seem to be pretty common.
I also have a 2000 model and in the summer I was also seeing pretty wild temp fluctuations. At the time I still had the LR thermostat in it.

There are 3 things I don't like about LR's design which I think cause a lot of this issue.
1- The upside down U shaped lower radiator hose can trap a certain amount of air in it causing flow problems. Granted, this should not be a great issue if your water pump is in good shape and the flow is enough to flush out the air. But if the pump is getting weak or there is another restriction somewhere, it may not be able to fully clear it out.
2- Starting in '99 with the D2, LR decided to prevent the electric fan from kicking in until the computer registered a temperature of something well over 200 degrees.
This brings us to the issue of temperature guage. (And I'm not talking about the 'dummy light' guage on the dash!)
3- The temperature reading that is registered on the computer which you can monitor on your ScanGuage is NOT the highest temperature in the engine! This is because of where LR decided to put the temp sensor. You can measure the highest temperature with an infrared gun at the water outlet at the top of the engine. It will always read higher there than the computer reads.

I believe these things are what causes these engines to overheat themselves. Computers are only as good as the information they are given and the solutions they are allowed!

So I decided to make some changes. I replaced the thermostat with a 180 degree one in a Jaguar bypass housing. (This modification is written up on this website). I liked this mod because I was able to accomplish it without replacing or adding any hoses. I put a tee in the bottom hose at the top of the upside down U so I eliminated the airlock possibility and because this setup is a bypass thermostat, I get full circulation even before the thermostat opens.
I can report that my temps now sit at around 170-172 degrees constant. It is winter though and before the winter arrived I would still see the temps climbing if it was hot out and the engine was idling. I blame this on insufficient water flow, insufficient air flow with the mechanical fan, and the stupid LR idea of preventing the electric fan from kicking in until the engine is grossly overheated!
When we get back to summer weather I hope to find a way to convince the computer to turn the fan on at a lower temperature and/or put a toggle switch in to manually turn it on. Also may try the engine fan and hub upgrade but my issue was mainly when idling so getting the electric fan coming on earlier would be my first choice.
When it comes time to replace the water pump I will DEFINATELY be going for the highest flow pump I can get but I'm not going to change that until I have to!

 

I've had had trouble finding an accurate temp sensor for the Bosch engines. I trust the IR gun more than what it says on the OBD. Some are high, others low. Genuine seems to go best but they are $50+. It is what it is with replacement parts these days.

As for the wide variation, that sounds like an air leak... or worse. Any exhaust in the coolant?

 

Spoke to Carl this morning, sorry to hear it is still acting up. I have seen this symptom once before, in that instance it was about a year between verifying it ran fine without thermostat element and where the cracked block was so bad it would not run without overheating. I hope I am wrong this time. As a next step I recommend a hot pressure test of the entire cooling system, if it is cracked it should be detectable hot. If it is cracked, you can probably drive it a year or so without the thermostat element installed. It will run a little cold for a while but if it is cracked the damage is already done. Hopefully you have a warranty.