I have a 98 blazer and when the heat is on it blows cold air. I check the everything and the coolant is full, both hoses are warm/hot, the heater core is warm as well, I guessing there is a door or something stuck that wont let the harm air come through the vents, it there a way to rig the door open?

 

There is a blend door that controls the flow of cold and hot air. There might be a problem with the actuator that controls this door as I just had this problem with my 2000 blazer. The blazer would get up to temperature and still not blow any hot air even though it would turn colder when I turn the cold air on. In my case I had to replace the blend door actuator because there was a cracked gear in the actuator.

The bad news is that this actuator sits on top on the heater core box under the dash and much of the dash needs to be taken apart in order to even access it. My advice is to go ahead and replace it if you take the time to get access to this blend door control rather than rig it to stay open. I think I paid about 40-50 dollars for the actuator.

 

I get this happening a lot. replace ur thermostat with a premium grade one not a cheap one. then run the truck with the rad cap off and burb any air pockets that have found their way into the cooling system

 

Is the blend door actuator accessible by dropping the glove box lid down on a '98?

EARZ86: Check this: open the glove box, remove all contents and then lift up on the part that stops the lid travel. This would then allow the lid to open further, actually all the way to towards the floor.

Then you can access the blend door actuator. Do a search on the forum and you will find pictures of what to look for and other information.