Just bought a 2004 LS. When i go to fill up the tank with gas i cant put in more then .2 gal in at a time before the filler neck fills up and shuts off the pump. I believe this is a case of some vapor releif valve stuck shut or vapor line clogged shut rather then there actually being something in the filler neck to clog it. Any ideas where to begin looking to fix this? Its getting cold here in MI and im not about to spend half an hour standing in the cold filling up my tank.