Have a 2006 GMC Serria. Have been driving around with a check engine light for some time. It is a p0455 EVAP large leak detected. I have been trying to get a smoke tester to find the leak. I now also have a problem filling the tank. I get spit back while fueling. Have resorted to filling with a funnel to cut down on fill time. Do you think it could be a vent valve? Any suggestions?