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What does it mean when your gas tank whistles at you?

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Dear Tom and Ray:


My beautiful '86 Chrysler Fifth Avenue is in excellent condition. It runs well and gets good gas mileage. But I get a scare when I gas up. When the gas cap is removed, I hear a burst or air that comes from the gas tank. What is happening? Is this dangerous?
Betty

TOM: It's supposed to do this, Betty. Modern fuel systems build up pressure. And when you unscrew the gas cap, you're simply letting that pressure escape.

RAY: In the old days--before the word "environment" came into modern usage-- the pressurized gasoline vapors used to escape right into the air. But that turned out to be a terrible source of pollution, as gasoline vapors escaped constantly--even when the car was parked.

TOM: So now, fuel tank ventilation systems are much more complicated. Gas vapors are held in the tank under pressure. In order to escape, they have to get through a charcoal canister, pass through a purge valve, and even say the secret word! This system creates a little "whoosh" when you remove the gas cap, but it's much better for the environment.

RAY: The only way it could be at all dangerous, Betty, is if you take off the gas cap when the tank is full. But why would you do that?? Most people wait until their tanks are at least partially empty before filling them up again. If you did remove the cap when the tank was completely full, the pressure might splash a little bit of gasoline onto your skirt. But that's the worst that could happen. So don't wear your Sunday best to the gas station, don't fill up the tank when it's already full, and don't worry about a thing, Betty.

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