Hi,
I just noticed that when I run the heat after running this vehicle for quite some time no heat comes out of the vents. Any ideas on what could cause this and if so how can I fix it? I did check the two lines running out of the firewall and they both are hot to the touch. Any help would be greatly appreciated....

Welcome to AF, will it get hot if the engine is speeded up? Any sign of sludge in the coolant or radiator?

Time for a system flush, but be warned. I had similar issues and after flushing my system it took out the intake manifold gaskets, heater core and the rad. All very common issues on these trucks.

A little quick on the system flush without more info, temp doors, and actuators may still be in the mix.

i have a 2000 slt also and my problem was the heater core was clogged up

i had them back flush the heater core and then flushed the system

and to be honest i couldnt believe how much heat i have now

i had that done winter of 2008 and no problems with intake or anything else

just a idea good luck

Jeff

I don't mean to imply that a flush will definitely cause issues, I just know that it can and one should be prepared for it, especially with these trucks, lol.