Heater only blows cold air

I would have guessed actuator. How did you check it? Here is another thread on the subject that you may find useful. https://www.cargurus.com/Cars/Discussion- t14310_ds538830

I had this exact problem with my 91 gmc sierra 4.2 L. I try to keep Alldata subscriptions for my vehicles and followed the diagnostic flow chart to no avail. I noticed that the inlet hose to the heater core was hot and outlet hose and heater core was cold. Pulled the heater core and found it clogged with a grainy/flakey brown substance that I'm not sure what it was, but I'm assuming it was black pepper. I flushed it out thoroughly and reinstalled it and now I now have heat. Hope this helps.