I have a 2001 Silverado 1500 5.3 LS. All that I can get is luke warm heat in the cab. I replaced the thermostat, flushed and filled the coolant, and checked the air cabin filters and still no heat. The hose going in is hot and the hose coming out is warm. It seems like there is just cold air blowing by the core. Any help would be greatly appreciated.

 

Disconnect your heater core hoses at the water pump. Back flush core. These cores have a problem w/holding crap.

 

Someone has a similar post just a couple of days ago on the same subject, I forget what vehicle it was on but the fix was to have to have the heater core flushed and that resolved the issue.
The input hose was hot and output was cold, yours sounds very similar...
I'd be willing to bet that someone mixed green anti-freeze with your pink or orange anti-freeze and now you have a nice cottage cheese mix clogging your heater core....

 

Thanks Guys!!! I flushed out the core and all seems to working normal now. I hope that it stays that way. I have also heard about some sort or air damper that closes when an electrode fails. No idea where it is. Hopefully the flush fixed it. Thanks again!!