no heat! its getting cold!

I have an 87 gn. iv had this car for 4 years now and im very aware of the fact these cars run hot! at all times if i have the vent on hot, even if the fan is off i feel heat seaping in. i never actually put it on heater, because even if its 30 below and i turn the vent on hot, and the fan on, it feels like a sauna. last week was a little cold in the morning, so i turned the vent on hot. it blew cold. i put it on heater, it blew cold. this is strange because it NEVER blew cold before, even if i wanted it to. i really dont want to take it back to another machanic, because i found that 90% of the time i get my car back worse than it was before i brought it in, and im a few hundred $ in the hole. does anyone know what this is, could be? what should i check for etc. iv never worked on this system before, and dont know much about it.

check your valve that controlls coolant flow to the heater core. When they are bad they can do stuff like that. Also you might want to check underdash with all the cables and vacume pots that open and close flaps and whatnot. I had to do both when mine did the same thing.

I agree with Highpsi87. Sounds like the heater control valve. Just incase you are not familiar with this piece, it is located on the passenger side on the upper heater hose that goes into the heater core. If this thing slips internally, the temp. will not change. :2cents: