Background
I rebuilt the engine less than a year ago to address the spike in engine temps especially during summer days. Idle temps and city temps ranges from 193 to 198. It’s when I get over 50 or so is when the temps slowly climbs over 204 plus. Even higher with AC on. I have caught myself using 87 octane once or twice on 400 miles plus travels with a trailer (Air temps was 60 degrees and engine temps didn’t go over 210 at times). I tried to bleed the air over and over many times and get a steady coolant flow.
• Less than a year new radiator (Silla #7868A) 2 rows.
• Less than a year new water pump.
• In-line system with a 180 thermostat.
• Conventional green (Flush many times) and now Prestone Dex-Cool (No issues).
• Clutch fan is great working condition. Aux electric fan works.

 

If all cooling system in perfect shape & fairly new. Then,
What did U exactly do, when rebuilding engine?
Did U deck the block? Did U mill heads?
I am asking cause u might have milled too much & raised comp/ratio
Cause if u did , it will explain temp going higher when getting closer to maximum efficiency of engine, running constantly, (50 mph & more) 2000 to 4000 rpm range.

But city driving , the rpm goes up & down , so engine doesnt stay at same comp/ ratio all the time & it doesn't heat up.
This is only correct if everything else is in percfect order.
If cats are not flowing good or clogged, can also have same symptoms, in higher FWY speeds,

 

I had machine shop here in Vegas do pressure test the engine and machined the heads and engine short block rebuild with parts I supplied while it was tore down. I'm leaning towards the cats too. During the past winter months. I lived within 2 miles of work and was doing morning short drives to the gym, then home, and than work in 25 to 46 degree mornings. same with short drives home.

 

There is nothing wrong with the temps you have mentioned. Any temp at or below 198 at long idle or in town driving is great, much under 110 on the road with or with out the a/c on is great.
Have you done a test on your viscus clutch to verify how well it is working, how about your cooling fan, is it still working?

 

Those temps don't look too terrible, but if you think it should be running cooler, you could check to make sure there isn't any debris stuck in the radiator or A/C condenser. Often, they look good if you're looking down from the top, but when you look directly through the the cooling fins, they are full of lots of fine debris. They make a cleaning tool for them that you hook up to a hose and you can put high pressure water through them from the back without removing them from the vehicle. They work really good.

 

The clutched fan works great and A/C condenser is less than a year too. Ran water thought them both.
• 100° weather city/ off road 195°-204°
• 100° weather city/ off road with AC 207°-215° and slowly climbing
• 100° weather HWY 195°-204°
• 100° weather HWY with AC 211°-215° and slowly climbing

 

Those temps are higher than I just experienced in similar conditions with a genuine grey LR 180° stat. That being said the stock thermostat let it run up to 225° and apparently the factory thought it was ok.

What brand of 180° thermostat are you running? There are a lot of reports on here of Motorad 180's running higher temps.

You mention you bled it a lot... Which procedure did you use? Cold engine, Elevated overflow tank, then open bleed screw? You can still have air in the system and get a steady stream out the bleed hole if you don't elevate the reservoir or if you attempt bleed while engine is hot. (for new readers if you already know this).

 

I did an in line thermostat and bleed screw mod about a year ago. My bleeding process been all over the place. I drove up on ramps and flushed and refilled on cold. I always rise the tank. Heater on high. Everything.

 

If radiator shroud not missing, with inline 180* t-stat as you mentioned in first post ,
You should run cooler in hwy speeds, But you say temp goes up to 215, that means your combination of components in cooling system can't bring temp down,
Because after 205* , t-stat doesn't close anymore, but system can't keep up, & temp raises to 215*, (too much, for any engine with 180* inline t-stat)!
If you are positive there is no air in system, then check radiator & water pump, ( I know you wrote in beginning , both are new, But it's their job to circulate coolant , & cool it down),
If both good, then what about inside the Engine Block cooling jacket, Maybe lots of crystal deposits, from previous Dexcool? That also can prevent good water circulation & same symptoms.
It's hard to pin point, exact problem, since you have so many new cooling components, & inline system, it really should run much cooler.
(Also don't rull out faulty t-stat, but I doubt it, since runs cooler in city)
Good luck finding problem source.

 

The biggest coolant change I had was when I put in the grey 180 degree thermostat.
Not the motorrad. But the Genuine Land Rover grey cased thermostat.
Made an improvement.

If you don't hear the water swoosh sound when you take off from a red light then you are probably OK in the bleed department.

I bleed my coolant by raising the coolant tank way up.
Takling off the cap on the coolant tank.
Then run the engine at around 1800 for about 8 minutes.
Then idle down the engine.
Replace the cap.
Clip the coolant tank back in.

That method is how a Discovery I bleeds naturally as the coolant tank is at the highest point on a Discovery I.

Works for a D2 if you just raise the coolant tank and run the engine fast.

I never used the bleed screw.
Bleed screw is just trouble and bogus.