I have noticed the air from the vents when it is on heat is not very strong coming out of the vents above the radio and the side vents. If is switch it to cool air it comes out very strong, switch back to heat the volumn decreases. Any help would be good. How about the acuator blending door, not closing or opening all the way. I dont herar a vacumn leak, put in new thermostat, heater core not blocked.

 

Definitely sounds like a door not opening all the way to allow all of the air from the fan to go through the heater core. Is the volume of air different or is the sound just muted?

 

It"s not the sound. it" for sure the volumn. I turn it to cold and it has volumn put it on hot and very little volumn. The vents at the radio have more than the vents on each side. Oh, it is on a 96 gmc Jimmy. Where should I start checking for the door maybe not closing?

 

I have a 96 S10 Blazer 4x4 that had the same problem, what i did was i changed the thermostat, and took the blower motor and the resistor out, then i blew out both sides of the condenser, you will get a lot of debris down in there and some oil seepage that will restrict the air flow, also, check your squirl cage on your blower motor to make sure no fins are missing, ( a few can make a big difference).... Plus, while you have the resistor out, look down inside and you can see if the door is moving fully! Let me know!