The most common evap problem on these GM vehicle is a clogged vent solenoid. Spiders make nests in the vent tube. So the canister gets filled with gas and that prevents you from filling the tank. GM has several TSBs on the problem and has released a new vent, purge solenoid and new vent tube.

On some GM models, its behind the front wheelwell. I doubt it has anything to do with not refueling however.

Actually it could be just about anywhere. Google a schematic of the evaporative control system your particular vehicle.

The canister is a cylindrically shaped black plastic tube with a bunch of rubber hoses tapped into it located under the vehicle near the gas tank toward the rear.

http://www.justanswer.com/chevy/4frlb-19...

I have a gm 3.1 liter engine in a 1998 Chevrolet malibu and my evap system is failing, causing me to not be able to re fuel the car. I need to check the hoses and possibly replace the canister, but I don't know where it is.