@jimvw57 you would think but air in cooling systems can be brutal to get rid of. The other possibility is a pin hole leak,you get a bit of coolant loss while driving then air sucked in as the engine cools, usually you will see a wet or damp spot where the leak is though.

 

No apparent leaks, unless it is where I can't see it.. coolant level stays constant. parked on a pretty steep incline and filled up when it was warm (up to temp) and it still did the same thing heading to work. Might have to find a steeper incline, or more of a high point for pass side of truck. I do have a bit of a pressure when I open the cap first thing in the morning, with engine stone cold so I'd think that would rule out a leak (maybe not)

i hate to resort to stop leak....

 

Avoid the stop leak at all costs, after a drive pop the hood look for coolant under hoses etc

 

Check the input/output to the heater box in the engine compartment. I recently had an issue where the clamps which hold the hoses onto the male fittings were old and allowing air to enter the system. Sounded like the typical water fall until the hoses heated up and swelled up. Had a small amount of coolant burn off on the exhaust manifold (turn into a white powder-like residue)

 

Only used it once and regretted it (back in the '70's) took forever to clean it out. no leaks that i could find. I did replace the upper rad hose a few months ago as it looked kind of bulged.

Ever seen one of these? wonder if it would help. They aren't cheap..
https://shell7.tdl.com/~jags/Pages/P...ightGlass.html

 

Interesting drive to work today....
outside temp is 85 degrees, i drove in with the AC on, just to see what would happen. Temp gauge in dash was at about 1/2 way, temp readout was 201. Once I started hitting hills, it went up to 215-221, gauge stayed at 1/2. suddenly it went to just before the red zone (on the dash gauge) and temp readout was at 221... I shut off the headlights, ad the gauge dropped a bit. Slowing down through a small town, the gauge went down to just above 1/2 and temp stayed at 26-219.. once I got on the highway, gauge went up to almost red zone, and temp readout was about 225-219. After 20 miles, i got off on exit to work, gauge went down to 1/2 and temp readout stayed at 217.

Starting to think my dash gauge has a nasty gremlin. I did try swapping the coolant cap with my other D1 to see if anything would change. Doesn't seem to make a difference.

 

I have to admit I ignore my dash gauge, 1 went from 176 to 215,back to 188.6 and that thing never moved

 

At this point you really need to pull the radiator and have it serviced. You are just chasing your tail. Sorry to be so direct, but you've chased all but the most important component of the system.

 

I did a search on radiators and found quite a few posts with almost exactly the same symptoms and the fix seems to be replacing the radiator. Luckily I have a spare D1 to steal a working radiator out of, and I know it's history for the last 7 years. I keep thinking I am overlooking something and this might be it.

I did check the temp of the radiator while it was running and did see a 20+ degree drop between the top and bottom of the radiator, but maybe that doesn't totally count. I'll be looking at extracting the radiator ...
When I have picked up donor radiators in the past, I have cut the cooler lines and spliced them with rubber hoses so as not to damage the threads on the radiator. Any thoughts??

 

Its supposed to have a temp drop across the radiator, that means its working.

I'd just spend $60-80 for a new Nissens off of ebay. I have and tons of other have had great success with them -- why muck around with something that i nearly a decade