Several things here:
1. 4psi is the maximum psi you should have.
2. Zero psi with the mechanical pump means it is defective and should be rebuilt or replaced.
3. 190 is not overheating (my truck stays at 180 most of the time but will creep to 190 or so in stopped rush hour traffic).
4. The FSM recommends a 160 thermostat but 180 isn't going to cause your engine to overheat.
5. What makes you think your heat riser flapper is stuck? Move it manually (when cold) to see if it is moving freely.
6. Is the spring intact on your flapper?
The T-stat does not govern the maximum operating temperature of your engine. It only allows the engine to warm up from cold faster by staying shut and keeping hot water in the engine. Once the engine reaches the temp stamped on the T stat, it opens up and water flows into the radiator. The engine will operate at whatever temperature is normal for it to operate. Modern cars usually run between 200 and 220 degrees.
