Chrysler Repair: Low Heat in 1993 Chrysler Lebaron V6 3L, air vent valve, 1993 chrysler lebaron

Question
I spent hours reading other answers. Still not sure. I bought the car as-is months ago. Runs good, no overheating, no leaks. However, the heat is just slightly warm. The door closes over the heater core. The thermostat replaced. The hoses into the core both warm. I cannot find any heater valve in the hoses. (I don't think this car has one.) I flushed water through the heater core through the attached hoses and it flowed.  The only things I notice is that I can drive for an hour and the radiator cap does not have any pressure. Replaced cap. Same problem. But I see no leaks. Fluids never drop. No overheating. The last thing is the temp gauge will show total cold for hours and then jump up to the middle as if a loose electrical connection. Replaced sensor, does the same thing. I am clueless. Possibly a clog inside the engine slowing where the fluid comes out to the core? Air bubbles in the system? But it does stay constantly luke-warm heat, not cool then warm. The only thing I did NOT do is a total cleaning/flush. Concerned that the chemicals that clean it might clog the core. Thanks for any help.

Answer
Hi Mike,
What was the temp opening point on the new thermostat? It should have been 195F. If you get a 180 or so thermostat that will make the cooling system run at too low a temp to pressurize and to bring the coolant to a level to produce good heat. Is you radiator fan running all the time? (it shouldn't) is another possibility. It is also possible that the thermostat isn't seated properly:  there is a casting "bridge" in the thermostat well and you want to be sure that the flange of the thermostat is aligned with the bridge when you set it in place; I believe the small air vent valve will be facing the front end of the engine if it is properly positioned.
Consider those possibilities and get back to me with questions or results.
Roland