Not sure I'm following you....when the temp reaches a certain point, the t-stat opens and temp drops. Not sure why you're saying that's undetectable, but I'm probably just not understanding you.
Once you get your old thermostat off, or if you already bought a new one, you can put it in a pot of water on the stove and you'll see how it really works. It is supposed to be *fully* open at the temperature threshold spec. In fact, it slowly opens and closes in response to the coolant temperature.
In a properly sized and funtioning cooling system, the car should come up to the operating temp and stay there at cruise. You won't see the gauge move because the thermostat achieves the ideal opening amount and the system reaches stability. If the thermostat is working properly, you should not be able to detect when the thermostat opens and closes EDIT: I don't mean to say completely opening and closing here as the thermostat probably isn't going to completely close; you shouldn't be detecting the variance of the opening size. That would indicate a problem if you see rapid changes not associated with your fan. While idling with no air flow to cool the system, the temperature will start to rise until the fan comes on and that is normal.
Cooler is not better. When I modify a cooling system, I'm usually looking at the oil temp and targeting a cruising water temp that maintains the oil at the desired temp. Then I just need sufficient capacity for the dynamic heat loads that I put into those systems.
If I had no thermostat at all in the system, the water might not spend enough time in the radiator core to cool sufficiently and could cause the car to get hotter and hotter in thermal runaway. You can accomplish the same thing with a thermostat that is always fully open. I've never had a need to use a thermostat under 180 deg. Thermodynamics dictates that the hotter something is, the faster it cools. So ideally I want the water as hot as possible, but not so hot that my oil temps get too high.
FYI Evans makes a coolant that doesn't need pressure because it won't boil off. That coolant can get as hot as you need it to and will never boil over.
In this particular case with my car.....it's been sitting in the garage idling or I'm varying the RPMs slightly with throttle modulation.....and the temp slowly climbs....I usually shut it off once it hits 212ºF, but will occassionally let it get up to 220 or so. For a few weeks recently, it acted normally.....with the t-stat opening and temps dropping to normal here in Phoenix (190-205º is pretty normal).....but it's been getting worse with the sticking and overheating issue.
The fan should definitely be coming on by 212 deg. Have you checked that? Sitting in the car, I can feel the fans when I put my hand by the driver's side mirror.
If the thermostat is stuck fully closed, then I would expect the temp at the radiator to be cool. If that is the case, then replace the thermostat with a similar spec unit. I wouldn't go cooler unless you have thought it all through.
I do have an Airlift for bleeding the system....I'll give your recommendation a shot. Question though....do I drain the system completely and then try to find the hissing noise?
Yes, but it sounds like you are reasonably sure that you don't have leaks, so I wouldn't go there. BTW, to change the Tstat, I usually don't drain the system at all. I just change it quick
If you plan on working on your car a lot, it pays to the nth degree to get a clam hinge. It really does only take me about 7 minutes to flip up the clam and then it's the easiest car I've ever worked on.