So I bought my truck in late March, never turned the heat on until last night. Turned it on, and it's not even warm. The vents are blowing out a good amount of air. If I turn the heat all the way to hot, it's just a hair warmer than if I turn it all the way to cold.

The truck was at about 195 degrees, so it had fully warmed up. I've got a 180* thermostat, which I know effects the heat some, but it shouldn't take away all my heat. My car has 1 180* thermostat in it (factory 195) and the heat in it works fine.

Is my heater core bad? Is there anything I can do to diagnose this before replacing my heater core?

 

Backflush the heater core, and make SURE it is FULL. Try your heat again. It will probably be hot. (heater cores in these trucks like to trap air, and it is a real drag to get it out.)

 

I had not used heat for several years, and the blend door got stuck by the decaying foam seal... after many cycles of the a/c-heat switch, it came free, and dried up foam crumbs came bowing out the vents.

 

With the truck up to temp feel the heater hoses where they go in the cab and see if they are both hot to determine if it's clogged.

 

Yeah, I thought it might be clogged. The hose on the right was cooler than the one on the left. I ended up hooking the air compressor up to it which broke it loose. Ran lots of water through it (both ways), put some radiator flush in it and let it sit, then ran lots of through it again (both ways), filled it up with antifreeze and reconnected my hoses. Heat works now. Thanks!