Check the vaccum and check valves, and the coolant level. I am not a gasser expert and I am not sure but, IF your thermostat mounts horizontally, there is a small air vent tab that must be a 12 o'clock. If not, you could have an air bubble. And even air will heat the hoses. Make sure the water pump is pumping. These guys already gave you the advice to flush the core.

If all that is well, I'd then look to see if the temp control **** is the problem. I mean literally check to see if it's cracked, and not turning the temp control valve. Maybe the other end of the cable has come disconnected, or a blend door has malfuctioned.

Good luck.