My car will not defrost and blows out cold air. There is coolant in the radiator on 1999 Buick Century

and I just recently changed the thermostat in it. The temperature gauge is right where it should be so it is not overheating. When I took the radiator cap off it was full of gunk. I am wondering if I should replace the cap? What could be wrong with it.

Asked by for the 1999 Buick Century
1 answer 2 comments
you possibly have a plugged heater core. when the car is all warm turn your heater on and then feel both heater hoses at the firewall going to heater core if they should both be hot.if one is cold and one is hot you may have a plugged heater core. does the heater work ok?
It blows out warm air only when I have it on Max. How would I unplug the heater core if it is plugged.
you can back flush it thru the heater core after you remove the hoses.