[777stickman]

Do you have rear a/c-heat?
When you talk about "heater control valve" where is located exactely?

If you have a rear system there is a "heater bypass valve" located in the engine compartment on the pass side forward of the firewall. Not sure what controls the vacumm to that but I'll look into it some more.

Heated engine water flows thru the front heater core all the time. The temp control door is what determines hot or cool air coming into the cabin. There is a "recirc door" but I cannot find the location of it in my "genuine '98 GM manual". It says that it's on the plenum.

Not sure why you can't get the temp control motor off. I looked at my '98 Sub and it looks very accessable (sp). If and when you do get it off just use a flat blade screwdriver to check the freedom of the door. If it moves free and the motor runs and is engaged with the door it should be good.

All this stuff ( recirc, temp, a/c and modes) go thru the dash control panel and there have been a lot of posts and fixes by replacing the panel. I'm like you, don't want to throw parts at problems without knowing that the money spent has a really good chance of fixing the problem (although I do have some rather expensive tx case parts as spares that did not fix a simple wiring problem)!!!

So post back and lets see if we can get you fixed up..............Steve