This is an interesting one.
It's a habit that I would guess 95% of us were taught as new drivers. For some reason, when you're getting gas and the pump automatically clicks off, it's seemingly become second nature for most of us to go ahead and give the pump a few more pulls, just to top off the tank and make sure we're getting every little bit that we can. You know, while we're there we might as well, right?
The pump is designed with a little mechanical device that senses when liquid gasoline rises to the top of the tank and gets to the nozzle of the pump. When the nozzle of the pump touches gasoline, this device cuts the pressure that keeps the nozzle open or the handle depressed and stops the flow of fuel. This, as simple as it sounds, means the tank is full.
So when gasoline is this high and you top it off with those few extra clicks, it often doesn't go into the tank as intended. More often than not, even though we can't see it, this fuel will overflow through hoses or vents and get into areas that were only designed for fuel vapor and never intended to have liquid raw fuel in them. This can contaminate components of the Evaporative Emissions Control System and potentially cost you much more money than the time that extra click or two at the pump saved you!
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