I had my delco battery go dead too. The dealership said that they don't warrenty batteries and gave me some shaft job about having after market stereo and other crap.. I'm like.. umm.. it was in for 6 months.. why is it not working now.. and what about the remote wire from the deck smartie... if there's no siglan.. there's no demand on the amp.
Anyways.. 100 bucks latter and bought another battery. As far as the fuel cap.. I had mine through up a light .. I cehcked the cap and it was fine.. however it was venting at the time...

Oh and about dealers.. don't ask them if they have preformance parts.. or ask what you can do to make it faster.. You'll get the standard GM "Don't do anything it'll void your warrenty.."