OK, I'm thinking it's the thermostat...

Here's what I came up with:

While I was doing this, the outdoor temp went from 18*F to 28*F.

• 11:02:30am - Started the truck. It ran smooth at 1500rpm.
• 11:03:03am - The idle kicked down to just above 1k rpm. No "thump".
• 11:13:02am - The temp gauge climbed to the halfway mark and stayed there. It was still blowing ambient air and the hoses were cool to the touch.

I let it sit idling for a while to see if anything changed. The upper radiator hose and supply hose for the heater core started feeling warmish, but not warm enough - it wasn't enough to open the valve on the heater core supply line to let the warm water through.

While it was idling, I got in behind the glove box to check things out - the door to the right of the AC box seems to be working fine - I worked the control from AC to Off to Heat and that door opened and closed just fine with no issues. I couldn't see the blend door - it seems to be buried - but while working the temp control from hot to cold and back, I could hear the door open and close - it didn't sound unusual (no weird noises) and didn't appear to be sticking from what I could hear. All controls work normally - no issues to report.

• 11:42:01am - Went for a drive to warm the truck up. I also had some errands to do while I was out.
• 11:43:48am - Warm air started coming out of the defrost.
• 11:53:51am - Temp in the cab started getting comfortable.
• 11:58:40am - Temp in the cab started getting rather toasty.
• 12:03:20pm - While stopped at a store, I checked the hoses and they all felt normal - the upper radiator hose was warm but not excessively hot - same for the heater supply hose and the temp on either side of that valve was the same (what triggers that valve to open? Temp?). The return line from the heater core was slightly cooler than the supply line indicating to me that the heater core is fine.

After getting back home, I let it sit at an idle for a bit and the air coming out of the ducts started cooling back down to just above ambient. Controls had not changed.

Coolant level is fine.

This is all telling me that the thermostat is stuck open but is still providing just enough restriction to allow the water to warm up for the heater while the truck is being driven around - just not at an idle...

Sound about right to you guys?