GM-GMC: 6.5 diesel Wont blow hot air and heating problems, gmc sierra, restricted flow

Question
Hey.I have a 1994 GMC Sierra Diesel 6.5 aspirated around 400k. the truck heater won't blow hot air when it is idling only at high rpm's. What could be wrong?

Answer
low fluid level or restricted flow of radatior fluid especcially to the heater core.

the heater core can also become clogged and they can be replaced or flushed out.

to flush them out disconnect the two hoses going into it and u can leave it in the truck or remove it completely and take a garden hose and run water thru it both hoses untill the water comes out clear both directions.

Hope this helps
Bruce