I think it's very possible that a similar obstruction you observed within the thermostat is happening somewhere else. It's very possible that a piece of gasket is stuck within hose somewhere or blocking something else like heater core.
I tool off heater hoses and back-flushed the core. It seemed to flow really well. Flushed out everything I could when the water pump was off as well. Not sure I have any other thoughts on what to do. Guess I'm just going to start driving it and see what happens unless someone has a different thought.
I haven't yet. I'm not sure I totally understand where a second could be. At the back of the top hose here?
The theory of just giving the car a drive wasn't a good idea. drove a 4 mile trip and back home. Coolant system was under severe pressure. Barely touched the overfill on the reservoir tank to see if anything had come and I guess I touched the line from the radiator to reservoir. Plastic busted apart and blew hot coolant all over my arm. Thankful that my face wasn't in front of it trying to see coolant on the wheel well.
That's a Thor so slightly different. You have a steel pipe attached to the front of the inlet manifold that the hose attaches to. The earlier GEMS has an alloy stub that looks just like a thermostat housing. On odd occasions people have been known to fit a second thermostat in there.
Pressurising within a few miles is almost always due to air trapped in the cooling system.