Certainly a possibility.

New observations today. As I mentioned yesterday, I did see t-stat temps briefly climb within 10-30 F differential from ECT.

Well today, both ECT and t-stat temp hit the 190-199 F mark simultaneously with about a 1 degree differential, multiple times, but only at idle, once driving, t-stat temp immediately starts dropping and ECT starts climbing.

So perhaps there's more to it. Also, can't confirm if it's true, but it seems when both sensors are close to each other, the upper radiator hose's definitely NOT rock hard, but once the differential increases, the hose's rock hard again and it does not soften up when cooling down.

 

The thing that is troubling is that there must be some volume of air being added to the cooling system when it’s running. If you start with a cold engine, release the cap on the reservoir so that it’s at ambient pressure, then run the car and let it cool back down, it should be back to ambient pressure. If it has pressure in the reservoir, then additional air must be getting into the system while it is running, right? Is my logic flawed? If it were just trapped air moving from one place to another (e.g., air from radiator moving into reservoir), the pressure still should be the same.

I would test for combustion gases in the coolant.

 

Yes, that's what's happens, no pressure, matching ambient, pressure builds up from heat but then never dissipates when cold if the cap is never opened. Never had this problem before replacing the radiator 2 months ago. Have spoken with 2 different techs and they insist the pressure is normal when hot, but should be soft when it returns to ambient as it started out before heat cycling. Makes sense to me. The both believed either air pockets or clogs but that doesn't make sense to me. I can do the combustion test but as I've said, I do NOT see bubbles in the expansion tank if I just let the engine run and heat up stationary and I am not losing any coolant, cold level is always the same.