Hey fellas my eng temp is running a lil on the warm side but the heater blows cold air. I checked to see if I had air in the system, when car reached operating temp the coolant around the radiator neck bubbled some but never "burped" over. When I would give it some gas like 3-4k rpm the fluid would drop and then raise back up. What do yall think is causing the heater to blow cold, heater core, thermostat stuck open?