The winter is not a great time for it, but an FIAV Delete will probly solve the issue.
I actually tried purchasing those fancy FIAV delete plates, but my throttle body looks different:
I ended up sealing one of the FIAV ports with JB-Weld instead, and keeping the ISC port open. I was really hoping that would be the end of it, but it looks like the ISC is not cooperating for some reason.
The idle speed control is a stepper motor. If plugging the passage lowers the speed either the stepper motor has failed or the drivers on the circuit board in the ECU are bad.
So since my last posting, I had the throttle body rebuilt (TPS and ISC tested to spec), and the ECU checked out by an ECU shop (they could not find any problems). My idle is still high ~2800 with temp gauge in the middle after all this high idling. Before I shut off the engine, I unplugged the ISC harness to "capture" the state of the ISC, shut off the engine, removed the ISC and saw the pintle is not extended at all, as if the engine was cold. So it's either the stepper motor isn't working right (but specced OK??), or the engine thinks it's cold?? The ISC vibrates when the key is switched on (also the harness to the ECU tested around 0.1 ohm resistance for all wires, so it looks like the ECU can talk to the ISC no problem).
Old Bad ISC, manually extended
New tested-good ISC, unplugged harness while warm engine running
Tomorrow I am planning to install the old ISC (manually extended) into the throttle body, and connect the harness to the tested-good ISC motor hanging free outside the throttle body, so I can see if it does anything at all when I rev the engine.
Not sure what else this could be, I am thinking maybe test with a known good ECU would be the last final step?