97 JGC Laredo gas tank fill problem

I have a 97 JGC Laredo 6 cyl with a gas tank problem. I am unable to fill the tank without the filler popping continuously. Service stations tell me I have air in the tank??? A mechanic told me that the tank passed the smoke test meaning the vent lines are ok. What I did today was to pull back the rubber boot on the gas nozzle and I saw that gas was coming back up the fuel tank fill tube causing the filler to pop. Does anyone have this same problem? Does this mean the gas tank vent lines are clogged? Thanks.

Thats the gas take vent that is on the filler pipe. I would have to say either its cloged or it has some how came undone.