My 2008 grand prix just started giving me a message "Check the gas cap". This just started after I bought some octane booster which I have used many times in the past. For some reason, no matter how many times I tighten it, it still comes back. The funny thing is it never comes on until my way home from work, never on my way there. I read up on this and saw that this message doesn't always refer to a bad cap. Has anyone out there successfully resolved this?