I have experience with 240, non-turbo varieties. Air box flap may look different, but likely works the same.

There is a stiff spring that pushes the flap into "hot air" position against the pressure of the brass button you saw. That button is actually a rod that extends out in hot temps, retracts in cold temps.

That's the airbox thermostat. It's under $40USD aftermarket. When it fails, the rod stays in, the flap stays open, the AMM get a constant diet of hot air and eventually gets fried. Bummer.

Test the airbox t-stat by taking it out and putting it in your freezer. If the rod doesn't extend, it's dead. Replace it. In my case, it's easier to get a new one, put it in, and then test the old one to see if I have a good spare.

Emisions testing probably won't check if the system is working. They might, as in Texas, look to see if the pre-heat tube is properly connected.

Good Luck,

Bob

:>)