Join Date: Jan 2010
Location: Fountain Valley California
Sounds like your in for an extended T/S condition here. Since you feel that the purge valve an gas cap are not the problem, your left with the possibility of a faulty tank pressure sensor, bad EVAP canister, and or its vent valve, leak in the purge line from the engine back to the tank, or that same line is plugged. Any of which can prevent the correct level of vacuum getting back to the tank in a timely manner. Or the vacuum is unable to hold at the required level once the supply from the engine (thru the purge valve) has timed out an shut down the vacuum source. You may have to settle for the GM service department on this, an have them run the complete EVAP system test to locate the problem(s).