For what its worth when mine was running those kind of temps, certain folks indicated my radiator was sludged up especially since I still run dex cool. So, I threw a new radiator on and it made no difference in temps and I also tore apart the old one to check out the sludge and it was sludge free. 115k miles all with dex cool and no sludge. I then changed the fan/clutch unit and still no change also threw in a new thermostat and still no change. It wasnt untill I switched to the gray thermostat that my temps dropped. Bottom line your temps are normal for the standard thermostat. With the gray thermostat I have never gotten back into the 200s no matter how hot it was out (over 100deg out this summer) and how long it was stop and go idle. My normal cruising temp is 188-190deg with 194-197 in stop and go traffic.

 

Outside temps have zero to do with engine temps at idle, it can be -15*F and your truck will still get over 200*F if you let it idle long enough, thats just the way it is.
Gasoline engines are very ineffiecent, they create more heat than usable energy and as well all know heat is wasted energy. (thats why diesels get twice the MPG and run colder than a gasoline engine)
Your engine temps are fine.
No it would not hurt to change the fan clutch, but otherwise you are fine.
Radiators dont just clog up at "X" miles or "Y" years of age.
They clog up from not being flushed and having old worn out engine coolant or just plain old water in them.
You're temps are fine.

 

What temp is that thermostat, and is it the "soft spring" type I've heard mention of? I have a Disco 1, so the thermostat is completely different, but I'd be curious to try and match the specs. Hopefully with the same result.

 

The more tests you can do on site, the less you have to depend on our long distance x-ray vision and Jedi mind tricks. Agree with Spike that radiator is the less likely on this one, fan clutch easy to check. Don't change it if mechanical spin test is good.

 

For the D1, pop in a 180F stat for about $10. I tried a 160, too cold; no stat, even colder. I run 180-183 at modest speed around town in the 90's, 187 at 70mph on I-95. But we get below 30F three days a year.

 

Ok, I've installed the 180 deg stat already. Also replaced the coolant tank cap, water pump, fan clutch & serpentine belt, and flushed the radiator. The truck will level off around 195, (local or highway,) but then very sloowly creep up to around 210 - 215 over the course of an hour or two.

In the near future I plan to find a boneyard radiator & have it rodded, possibly re-cored. Any other thoughts as to why the temp would gradually increase like this?

 

You can't re-core a DII radiator, plastic tanks.
You can buy a brand new one for around $200.

 

The radiator lower tubes are full of calcium, enough so that heat just can't be gotten rid of fast enough. You have 3/4 radiator, or some fraction. If you can find a small indy rad shop, one or two guys, farm edge of town (not Alpharetta), may can have your own one rodded out for under $100 carry in. I did. Look for a shop with bus and tractor radiators. Your teacup Rover is not much of a challenge. I tried white vinegar, commercial flush product, brick washing acid (muriatic acid) in just the radiator, but nothing did the job like the rad shop. If hiking about the boneyard be sure the tranny cooler and oil cooler sides are not restricted.

 

Sorry, wrong section! I have a DI.

So it is likely the radiator. Any recommendations on a good shop north/east of Atlanta?

 

I last lived there when Indian Trail was dirt from the expressway to Lilburn.... so places have changed. Here's pix of calcium build up.