You have a thermostatic first fast idle valve in the TB, plus the ICM, plus the manual adjust air bypass screw. The Fidle valve closes quite fast, before the temp gauge comes off the peg on mine. Possible to epoxy it shut, but tricky, as it shares the air port to the TB bore with the ICM. I've seen a very few go bad. I'd try a junkyard TB before I paid for a new one....

Not responsible for advice not taken...