Pontiac Repair: 05 Pontiac Montana, pontiac montana, vent valve

Question
I have a 05 Pontiac Montana VAN with a v6 ENGINE.Driving home from work yesterday the message system in the dash told me to check the gas cap.I pulled over and although it seemed to be tight I took it off and refastened it.Everything seemed fine.No drivabilty issues.The next day the same thing happened and I repeated what I did before.This time however the yellow engine light came on and hasn't gone off.Do I just need to replace the gas cap and does this cause any issues for driving until I replace it?Thank you for your help.

Answer
Hey Walt:

If the cap seems to tighten fine and the rubber seal is still on the cap the nchances are it's NOT the cap. The engine computer detected a gross leeak in the emissions system so it turned on the check cap light first and the next drive cycle when the issues was still there it turned on the service engine soon lamp. Chance are it's a vent valve for the evap. system but it could be something else. Usually it's either the cap or the vent valve. If you live in the rust belt like I do it could even be the gast tank filler pipe the metal pipe rots out and causes a leak. Yes in only 5 years it can happen. If you don't live in the northeasy where they use rock salt on the road that is not going to be your issue. Good luck :)