I don't see an answer to this question. I am having the same problem with my 2013 264. I am thinking the issue is that the vent tube has water in it. If I follow the vent tube from the fill cap, it appears to go down and then come back up to a T-connector, making a "U" shape. I am wondering if when the water tank is over-filled if the vent tube gets water in it and the water sits in the bottom of the U. (sort of like a P-trap). I haven't been able to get the PVC vent line off yet to verify if this is the problem, as it is not the easiest thing to reach and disconnect. I can't really tell how air gets into this line and/or how water could but if this is the case it seems like a poor design and would happen every time someone over-filled the water tank, or used a fill hose that didn't have a steady flowing stream.