A FE book I read recently said you can lose 30 gallons of fuel per year through a bad gasket on your gas cap. Evaporation, it seems, is the culprit. I doubt their figure, but don't doubt it's possible to lose a lot over time that way.
That only work out to 0.082 gal/day. While I believe this is somewhat high, it is within the realm of possibility on a yearly basis. You could probably exceed that amount on hot 100+ degree days where the car sits in the sun all day.
You have to disassemble it as much as possible. Somewhere inside it is the solenoid itself, which should plunge at least a centimeter (not sure exactly how much but it'll be obvious by looking). If it just clicks and moves a half millimeter then it's dead.
Can you post a photo of yours, or which one from http://www.google.com/images?q=chevrolet+purge+solenoid it looks like? Some of those appear impossible to disassemble. You could probably feel the inertia of it as it engages if you hold it in your hand while you test it, instead of merely the audible click if it's messed up.
it's the first one in your link...not able to be disassembled. i did go ahead and buy a new one.
funny thing, it doesn't look the same and it wont fit in engine compartment space where the old one goes. checked the auto part site(advance), it says i have the correct part. checked napa, same thing. checked autozone, its different, looks like our's.
so, i tested both to compare. both have a very strong audible "click." gonna return it, and maybe check the valve and vacuum lines at the charcoal canister next.
It might have been redesigned. My vent solenoid was, although I had to specifically seek the redesigned one else I would have gotten the old design.
I guess if the clicks feel the same then the solenoid was electrically and mechanically operating well. Could be a leak inside it. Not sure how to test that...maybe try blowing through each nipple (lol) with it engaged and disengaged (if you don't mind putting your mouth where fuel vapors go)?