Small Evap Leak

Your symptoms suggest an evap canister problem. The original vent is badly designed. You have 2 choices. You can buy a new one(reviewed design) at your Nissan dealership (which will cost around 100-200$). Or you can try to clean/lubricate it like described here: http://www.b15sentra.net/forums/showthread.php?t=142973 I replaced mine 2 years ago on my Sentra 2006. At that time, the car was 5 yo. I would not go into such expanses for a 12 yo car. Either way, do the job yourself, it's easy and you will save the labor cost. The original vent looks like this: http://www.b15u.com/attachment.php?attachmentid=38398&d=1351801333 The new one has a much more sturdy design: http://i.ebayimg.com/00/s/MTYwMFgxNDgz/z/tNEAAMXQGQRSAXRd/$%28KGrHqN,!ocFHlLL5OdqBS!%28Rc%295fw~~60_35.JPG?set_id=880000500F