I have a 1996 blazer with the 4.3. When I have the heat (red)on there is very little airflow but when I turn the dial to cool (blue) there is a ton of airflow. The heater hose from the block to the heater core is hot and the one to the water pump is warm. My temp gauge is a little under half and the radiator is full. I think it is the blend door but do not want to waste money on guessing. If it is the blend door do I have to remove the dash and where is the door at?

 

I believe blend door is behind glove box fairly easy to get to.

 

Does anyone think that is the problem or should i go a different route?

 

you may want to clean out the heater core its easy to do, as you could have a blockage
this has happened to me.. Drain the rad disconnect the hoses going to the firewall add some CLR let it sit 10 to 15 min flush,Add the Clr to the outlets at the firewall not the rad
flush refill with coolant should blow strong and hot

 

Heater core blockage wont restrict air flow (but aflush is never a bad idea), check blend door. It takes like 3 or 4 minutes to be able to see and remove it. Just remove the bar that holds glove box from falling out ( little piece in left hand corner when open). Then it will flip down and you can see blend door actuator. See if it moves and if vacuum leak is present (with truck running using dash switch). If it doesn't move check to see if door is sticky or stuck, pull on arm attached to actuator to see if it moves and is still attached. If no luck check for vacuum on the small line to actuator. Basically if its your problem pull actuator in or out by hand and vents should blow right if trucks running.