Ok, So I'm going to try to describe my situation as best as I can for an "accurate" diagnosis. I have a 2001 blazer 4.3 and I'm having heat issues, which I hear are normal with these POS's.

My problems started about 2 month's ago. I noticed that the hot air in the passenger compartment got cooler all of a sudden. I didn't think too much of it until it started to get really cold. I also noticed that the temp. reading on the gauges was just shy of 210, maybe 180. I read that it's usually not the thermostat that causes this, but I decided to replace it anyway just in case. That worked for about 3 hrs. with the heat that I was used to. However, now were back to square 1. I've noticed that the only time I can get the same good heat is when I'm driving and even that is hit or miss. I've checked coolant levels, and checked to see if I was gettin the same flow of air through the vents. This is all normal. I had my radiator replaced in July and am wondering if this has anything to do with my current problem. I have also checked the heater core inlet and outlet hoses and I'm not exactly sure what I should be finding there. I'm thinking maybe my problem now is the core and that fact that it might be clogged.

Am I even close.....

 

I would say that if you get heat some times, the core is working properly and that there is air in the system, hindering flow.

Did you try to flush the core when you had the system drained to replace the t-stat?

 

I know that the core was flushed in July when they replaced the radiator and completely flushed the system.

Im going to try to get the air out today when I get home by elevating the truck and taking the radiator cap off.

When they replaced the thermostat, they only replaced it. They did not drain any coolant or flush anything.