Sorry to hear that. If you look around the post, this is not an uncommon problem - some, have had terrible luck with the dealer. Sounds like one of the VSV (vacuum switching valves) is stuck or malfunctioning. The canister purge valve, pressure switching valve, and canister vent valves are all VSVs.
The EVAP canister vent valve and purge valves are located under the hood, connected to the rearmost exterior of the air box (I'm ****uming that your vechile is US spec 1998-2002 model years). If it is the fuel tank pressure valve/sensor or pressure switching valve - they ar epart of the charcoal canister you got to drop under the car and remove the charcoal canister from right above the crossmember and between the tank and spare tire well.
You can check the valves yourself - I did on my 2002 Corolla and saved well over $350, ended up being a $8 valve gasket and dirty electrical contacts. But I would highly recommend buying a shop manual - the procedure is not hard, but very specific and time consuming.
If your not 100% messing with - then take it to the dealer. You don't want to start fooling around under there and accidently knock off some hoses or snap off some electrical connectors - really easy to do.
Hope this help. Good Luck.
