I was having problems with back pressure on the coolant reserve. If you started it cold it would quickly push coolant through the edge of the fill cap. So I replaced the thermostat with new one (OE specs). Now the coolant seems to flow fine (no back pressure), the coolant tank builds normal pressure but the heat doesn't work now (it heated up before). The gauge shows it going to normal operating temperature but no heat inside the passenger compartment. Heater hoses are warm, plenum seems to work fine. Coolant level is good and doesn't leak. Could that be a plugged heater core? Or something else? Does it hurt the engine if the heater core is plugged? Any thoughts are appreciated.

2000 F-350 7.3L Turbo Diesel Powerstroke

 

If both heater hoses are warm/hot, then coolant is flowing through the heater core. Some trucks do have a heater control valve in the coolant lines that diverts the flow of water around the core, that might be something to look at as well.