On my 04 Suburban, I was having a devil of a time filling up, it would pump 2 dollars then shut off, I tried pulling out early and letting more air our from the tank and nothing would change. At first I was blaming it on the stations hose gun, some are more sensitive than others, but it would not do it and then do it on the same pump and nozzle. It drove me and my war department nuts when the time came to fill it. I did change the vent the purge canister and solenoids. I even spent some bucks and got a GM filler tube as mine was rusty anyway. The new one did not come with the overflow tube but it was fine to work. There is no valve inside this tube so elsewere I went. It turned out that inside the tank filler neck, the plastic part that protrudes before you slide the rubber fill tube over has a flapper inside. This flapper is for rollover protection. What was happening was this flapper was staying closed about 90 to 95 percent of the time and when it came time to fill up, the fuel backed up into the tube and stopped the gun, but it did not do it all the time. One of the good ole boys who was a gm mechanic told me to force this flapper into the open position and viola, problem was solved. No more fuel back up and war department was happy again.
