Tank Assembly problems of the 2006 Nissan Sentra

When I try to start my 2006 Nissan Sentra ser it repeatly turns over even when there is a full tank of gas. I have stop and start it over and over to get to to key up. At first it was here and there now it is everyday everytime I try to turn on the car. I called about the issue and was told I may need a tune up, my car just hit 50,000 miles. I have read over the manufactur book given at purchase to see if they is any information available about this. I do have an extended warranty on my car, but right now I just trying to figure out what is going on. I found this website and saw others are having the same issues.

I have a Nissan 2006 Sentra 1. 8. When refueling, gas pours back out of the gas tank, even when on empty. Nissan has published a service bulletin about a vent control valve replacement (using a new style vent control valve). I contacted Nissan and the parts & labor will total approximately $350 for a serious safety issue such as this; any spark would surely ignite the driver and probably auto; they are unwilling to send me the part at this point & I still can't use the vehicle- this has been happening for approximately 3 weeks.