92 sentra warms up kind of slow, air coming from heater gradually gets warmer, then when temp guage gets the normal warmed up point (dead center between the C and H where it should be), the air coming from the heater is cool at best. Running the fan on higher speeds seems to make the air get colder. Put in a new thermostat several months ago, I dont think its that since the engine temp guage indicates all is well. Radiator leaks a little, I filled it up thinking that maybe a low amount of coolant would prevent the heater from doing its job. Still cool air. Could I have an air bubble or maybe the heater core is bad. Help, its getting real cold in Chicago these days. I dont want to have to donate old reliable just yet. thanks for any help.