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GM has had major vent valve failures since OBD2 started lol... The seal gets weak when the vent valve shuts to seal during EVAP purge. The trucks had more problems with dirt/dust/water getting into them so GM made us re-route the vent valve fresh air side. Best way to check is with a scantool and smoke machine to see if there are any leaks. On the newer cars 2005+ the gas cap has to be perfect! Make sure your filler neck is clean and not marred up. Clean the gas cap seal and check for any cracks.
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