heater issue 95 LWB

[ScottH95lwb]

Now that it is cold it is time to do something about my heater. When I first turn on the truck and the engine gets warm my heater blows hot air, after just a few minutes the air turns cold and it stays that way. No fun. What is up? Is it my heater core or is it the other valve/ thermostat thing? I thougt I was smelling some antifreeze in the cab a while back, however, that seems to have stopped. Your input is much appreciated.
Scott

[phoenix]

start by replacing the thremostat

[xrjinga]

Same thing here!!!

I've got the same thing going on here with my 89, I replaced the t-stat, made sure the burp hole was at 12 o'clock, bled it out as far as I can tell of air. Temp did go up a little hotter than before with the old t-stat, but now it stays at center or to the left, and as soon as I start driving cool air is all that comes out! Could there be deeper issues here? I've owned the truck now for 9 months, and the PO serviced it when it should have been. Thanks in advance for the next steps to take.

Jim