Ok I got a 2001 Blazer Lt and am having heating issues. Here is the deal: when I am at normal temperature range (right below 210) the heater blows cold air at idle to around 2000rpm. However, when i am at 2000rpm (roughly) and above it will blow hot air.

Here is what I've done:
I checked the inlet hose to the heater core and it was hot, checked the outlet hose it was warm. SO... I did a backflush to the heater core to flush anything out, but I didn't see any junk come out. Just water mixed with anti-freeze and then just clear water. The water came out the same pressure as I put in. So i blew out the remaining water... filled up with anti-freeze again. Ran the engine again.. checked levels.. good. I am not sure what the cause of the problem may be beeing as I get hot air at over 2000rpm...

Any suggestions? Anything that I am overlooking?

 

Oh, also it is manual temperature control..

 

I would say air in the system or a defective heater door... Do you have any vacuum leaks? Does your HVAC system work on all modes?

 

Try back flushing the heater core and then if it still does not work try running a water hose over your thermostat housing with the rad. cap off for a few min.Keep your rpm around 1500-2000 and this should help purge the air out of the system.Just went through same thing on a 2001 Prism