Today was the first time I’ve ever overflowed the gastank filling up any of my cars, ever. It was also the first time I ever filled up one of my cars in a foreign country (Quebec, Canada). It can’t be a coincidence. But afterwards I took a closer look at the gas tank opening and could have sworn that there was a metal insert flap at the opening. Can anyone confirm how theirs appears compared to mine? Thanks

 

There is no metal insert flap at the fuel opening on my 2018 GS 450h (image below). The fuel opening is only slightly larger than the nozzle's diameter. The necessary venting has always been adequate to handle fast-flowing gas delivery up until the pump handle automatically shuts off.

I've never had any overflow problem with the gas tank when refueling, even when delivering gas at full flow speed the pump's nozzle always shuts off before fast-flowing gas backs up on its way to the tank.



I'm guessing you happened to use a gas pump that had a defective pump nozzle, possibly with a damaged or worn pump handle vacuum chamber diaphragm, or maybe the gas station had its pumping pressure too high, out of the approved range.


Typically, as the gas level rises in the car’s gas tank and filler pipe, the distance between the dispenser nozzle tip and the fuel grows smaller. A small pipe called a venturi tube or vacuum tube (see image below) runs alongside, inside the gas nozzle spout to a small opening or sensing port near the end of the handle spout. When the end of the venturi/vacuum tube becomes submerged with the rising gas, it chokes off the air pressure that allows the pump handle to hold the nozzle valve open and shuts down the gas flow.

 

Thank you very much for your wonderfully insightful post, much appreciated. I know that my 1992 Cadillac Seville has an insert, I work on it often and have looked there many times. But it’s also 25yrs older and domestic production.

No codes were thrown on the 15mile drive back to the hotel, so I figured it was the pump. The pumps are very similar to US pumps from what I observed but this particular handle and noozle did seem like an older design that I remember from the early 2000’s.

 

The mechanism and design of the gas pump handle is such a brilliant solution without the use of any electronics or sensors by implementing the venturi effect. Its also very cool in that the actual product or commodity thats being pumped, is also the key source in the functionality of the mechanism to work in the first place. If this was a german design, it would have double the parts and double the complicity 😆. The old saying "Keep It Simple Stupid" really shows itself here, its simple in the sense that it could have been way more complex then what it is.

 

The next time you visit a gas station, before beginning to pump fuel - turn the nozzle spout upside-down where you can see the underside of the spout. You'll see a small hole or slot about an inch from the spout's end about the size of a pencil eraser. That's the venturi tube's sensing port near the end of the spout. When the sensing port gets submerged in the rising gas as you fill-up your gas tank, it chokes off the air pressure that allows the pump handle vacuum chamber diaphragm to hold the nozzle valve open and shuts down the gas flow. Look for that little hole on the underside of the spout near the end.

 

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