I just got finished replacing the heater core in my 2001 Blazer. I will list what mine did, and allow you to draw your own conclusion.
My heat basically stopped being warm during the summer months when it was not being used that much. Therefore I was not aware if the loss of heat was gradual or all at once.
I flushed my radiator system and got the worst looking black soot water from it. I tried to flush my heater core but it was clogged. After working the pressure on the intake and exhaust sides back and forth, the water began to flow. After about an hour of flushing the fluid flowing thru my heater core was clear again, and the heat worked great. I discovered that the inner lining of both heater hoses had dissolved and contaminated my system. I replaced the hoses and thought I had a great fix.
About 2 weeks later the heater core began to leak. I was now faced with a $1,000 shop visit, or replace the heater core myself. Make no mistake, changing this core is a heack of a job, and I would recommend knowing maintenace before attempting it.
Everything is working in fine order now, and I am not sure if any of my problems are related to yours. I did not have reduced air flow with my situation other than the normal flow reduction of routing the air across the heater core.
My advice on trouble shooting is to disconnect the heater hoses at the firewall, and after placing a water deflector over your distributor, flush the heater core. DO NOT overpressurize the core. Just a normal flush pressure to make sure the core is flowing.
If you have never changed your thermostat, change it. It is very cheap and easy to do. I find it alot easier on this model to refill your engine block thru the thermostat hole prior to installing it. It takes less to bleed the system and just makes the job quicker.
Now, if the heater core and thermostat are not your problem, then there are tests on the body compter that can be run to determine if you have a control problem. I am affraid that might have to be done by a certified shop.
Good luck and hope you get heat before the winter is over.
Monday, January 1st, 2007 AT 3:30 PM