I have an 01 Trail Blazer and recently the temp gauge gets fairly hot while the heater blows cold air. Then after a few seconds theres a slight smell of anti-freeze and the heater blows warm air. I am guessing its a control door on the heater core. I checked the lines going into the core while running and one side is hot and the other is cold. I am just trying to narrow my search before I start to tear apart my truck. Also, I am curious to which is the best point of entry for fixing the problem

 

If the return hose from the heater core is cold, then you probably have a plugged core. Try using a garden hose to flush out the core.

 

I flushed the entire system and the core is fine. I work as a mechanic but I work on real american made Kenworth and Peterbuilt. I am just trying to narrow my search on the control door.

 

There is no valve between the engine and the heater core. As such, the core really does not have any impact on the engines operating temperature. If your engine is running hot, you need to get that problem fixed (thermostat). It is also possible that there is air in the system that is causing the system to air lock, prohibiting flow of coolant through the higher points (heater core). If the radiator cap burps off some pressure, it could cause the air bubble to move and allow for coolant to pass through the core again. When the truck cools, the air moves back into this location and the cycle starts over again.