Dear Tom and Ray:
I have a 1996 Subaru Legacy L wagon. Four mechanics in and around Boston can't figure out how (twice now) in excess of three gallons of water has gotten into my gas tank. The first time, it stalled out on the highway. After lots of attempts to fix it, we took a gas sample. It was water! After both instances, I siphoned out the water and gas, blew out lines, changed a vile-looking fuel filter and drained the fuel rail.
I had the state investigate the gas station I use. Twice, reports came back negative for water. Both times, the problem arose after excess wet/stormy weather. Is there a vent? Is the water maybe coming off the tires? Is it getting flung in? I have a locked gas cap, so I've ruled out vandalism. Oh, the filler neck has lots of rust in it. Pebbles have made it into the tank, also. It does not smell like gasoline vapors outside the car. Any ideas?
TOM: Yeah. Your filler neck is rusted out. We replace a ton of those. Why? Because they rust away, like yours did.
RAY: I don't think all of the water is getting in there from one drive in a bad rainstorm. I think the water is accumulating over many weeks or months, due to a combination of splashing rain and naturally occurring moisture in the air that condenses once it gets inside the tank.
TOM: And then a really rainy or snowy day puts it over the top, and your engine can no longer burn the gasoline because there's too much water in it.
RAY: The fact that you've found pebbles in the tank indicates that clearly, stuff is getting kicked up from the road and is coming in through holes in the filler neck. And if pebbles can get in, certainly water can, too.
TOM: I don't know why you can't smell the gasoline. You should be able to. And your Check Engine light should be on, since if water can get in, gasoline vapors can get out, and that's an emissions violation. Of course, you wouldn't notice that if, as we suspect, the light's been on continuously since 2004.
RAY: So, at the very least, you need a filler neck, Sarah. You should get one soon, before they stop making them for this car. And while you're there, have your mechanic check the integrity of the fuel system -- and the rest of the car -- and make sure the filler neck isn't the only thing that's rusted out. Good luck.