What he said above, except, on 850's it is infact a thermostat. On 98 s70s they switched to vacuum. The 'piston' or whatever you would call it, expands with heat and will apply pressure to open the flap. when its cold the flap is closed and you get heated air off the exhaust manifold.
If you live in the arctic or very cold climate, then i would say buy a new one.
shows you the dowels method, the simplest. The best method is to take the bottom of an 850 turbo airbox. The next best solution is to get a dremel or whatever to remove the flap, close it off etc...but the bottom of an 850 turbo airbox is just oh so much cleaner lol...
If you should need or want to replace it, look on FCP or wherever, as they are around 25$.
The dowels method will hold you over for a long time with no issues, but, is by no means the best route, just the quick fast fix that'll last years
you gotta put in mattsvolvosittee since for some reason the forum is still filtering it lol..but if you go there you will see pictures of the dowels