Last week I looked in the airbox and saw the flapper assembly was half way between, allowing air from both hot and cold to mix. I did notice that mileage was bad, but attributed it to a lot of other problems that I have since fixed. I replaced the T'stat ( took about an hour it was so hard to get in- LOL) and now I'm sure that my mileage has gone way up - will check at the next fill-up. My AMM is still going strong , but I wouldn't be surprised if it fails at anytime. The old T'stat was stuck with the metal bar half way extended. Think the Volvo design on this is really bad as the steel bar has to be all the way extended to close the hot air flap. Failure mode is when the T'stat bar can't extend all the way out allowing the hot air flapper to open. Should have been designed to fail in such a way that the cold air port remains open, not the hot air port. Bad design when Volvo would rather meet emissions leaving you stranded when your AMM fails. ( BTW the last time my AMM failed, the limp home mode didn't work- LOL ?).

Rather than pulling the air box thermostat out (and getting a ratio of hot & cold air that is unknown at best, but in any case hotter than the ideal temperature at operating temperature), I'm curious why you didn't just disconnect the hot air hose at the exhaust manifold? That would provide the ideal situation (all cold) when the car warms up, and would ensure you didn't fry your air mass meter.

If all you drove were short trips to the corner market, I could understand dealing with it the way you have. But assuming you're usually driving for a few minutes or more most of the time, I would recommend just taking out the hot air supply completely until you've had time to fix the air box thermostat.