My service engine soon light has come on and it says to check the gas cap which obviously is easy to figure out. I turned the car off and pulled the cap out checked it for signs of wear or damage but there was nothing visible. I put the cap back on started the car and sure enough the light and the check gas cap warning nevershut off.

It is a 99 Deville

 

Is it a factory cap? Is it on tight. Did it click a couple of times as you tightened it?

 

It is the factory cap and it did click a few times so i tried to wiggle it and it is in there pretty tight.

 

There could be a hairline crack in the cap that is not readily visable, happened on my vette.