gas fumes from my 97 buick lesabre
 

hi guys ! i have a 1997 Buick Lesabre and when i shut the engine off i smell gas fumes towards the back ! there is a pressure valve on the tank, would it be leaking fumes from there ? i had it on the rack and no gas leaks could be found ! where do you suppose it could be coming from ? like i said , could it be the pressure valve on the tank ? i already replaced the gas cap ! thanks guys !