I had a host of purge and vent issues on my 2004 Denali 6.0. I had the famous gas cap loose code. I found the purge vent valve you speak of (near or rear of the gas tank) was sticking open due to dusty conditions. I fully cleaned the valve (compressed air) and used 12volts to verify function before reinstall. The MIL went out after two starts. It seemed grit was preventing full closure. I subsequently replaced it (O'Reilly) for about $20. This was not the end of the problem as later had P0496 and replaced the purge valve on top of intake. That, plus the vent valve, plus the gas cap solved the unsecured gas cap issue. Good luck.