I have noticed this is a popular subject. I have looked through the other threads first.

1985 K5 Blazer with 305. Heat barely works, I live in Maine and the heat will keep the cab bearable but no real hot air but the winter has been pretty mild so far.

I replaced the thermostat twice and the temp seems to be running normal now at 190-195. I flushed the heater core, both hoses are warm. I switched the intake hose from the radiator to the manafold, even though the hose is really to big and seem to be made for the radiator. The interior heater door does switch from hot to cold (pulgged vaccum lines back up and it all works now), and there is a difference in temp, just not very warm. I can feel where the heater core is below the dash and it is very warm.

Any other ideas????????

 

This should be in the full size blazer section. but have you burped your cooling system? make sure all the air bubbles are out?

 

The blender door could be getting stuck (instead of going full heat it may be stuck at a mix of hot/cold). I only suggest this because you say the heater core is very warm... that means its likely a problem between the core and the vents.

That being said, I don't know much about the full-sizes. Hopefully a mod can move this post to the full-size section and one of those guys with have some more in-depth information. But if you have a chilton/haynes manual you could look up the blend door and see if it's easy to get a look at. Sometimes you can look at it while activating it, and see where it is getting stuck.