At about 100,00 miles evap solenoid goes bad.it goes to canister so it has 3 hose's 2 large 1 small 1 large goes to solenoid the other the top of tank.take it loose any charcoal fall out?now with line still loose shake it does it sound like rice inside tubing?IF yes to these the canister has exploded inside from fuel entering from topping off the tank gm says funny how they happen together.anyways the fix is drop tank pull the line and clear with compressed air then install new canister and evap valve.easy hardest part is the dropping of the tank.

 

I have a new to me 07 burb with the same problem. Can’t seem to get gas to flow into the tank. I’ll try some of the suggested work arounds until I have to spend $$$ on it.

 

Drop tank down enough to get to vap line closest to rear.unhook other end at canister closest to front.take compressed air blow it out most likely full of charcoal.just shake line if it rattles charcoal.canister basically expodes internally.start unhooking canister end first if no charcoal no need to drop tank it's the valve under your spare tire about 30 bucks.liquid gas in canister causes charcoal canister to go bad.only made for vapors.if canister is pouring charcoal stuff a screen or steel wool in end at canister so you don't have it blow line out again when you can replace .the valve is known for going bad at around 100,000 miles.good luck it sucks taking 15 minutes to put gas in I know.