How Do Gasoline Pump Nozzles Know When to Turn Off?


What I would like to know, really, is why they all (this is intentional, methinks) turn off too soon and leave 1.5 - 2 gallons of empty space in the tank. I always compensate for this by doing my best to defeat the system by pulling the nozzle out as far as it will go and still depress the flap in the entry hole and then use the fine motor control in my fingers so as to trickle in enough gas to actually top off my tank, by not allowing it to shut off (and re-starting a trickle every time it does). I know, John, just part of the friction designed into modern life.

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I think this is because gasoline splashed back up the filler pipe momentarily blocks the end of the venturi tube, triggering the cut-off. The cut-off mechanism has to be pretty sensitive to stop before a gusher of gasoline comes out when the tank fills up, and I suspect just a little splash is enough to trigger it. This seems dependent on the geometry of the filler tube running to the tank. With my earlier Honda Civic, I would almost never get a premature cut-off, but with the Toyota Starlet it happens almost every time and I need to do the manual top-off procedure as you do to get a full tank.


Yes. In my own empirical efforts to more easily top off, I have, as I said, taken to withdrawing the nozzle to varying degrees; I also rotate it in an attempt to aim it squarely down to filler tube so as to limit splashes. All this trial and error with little effect in terms of ease.


Get no chance to try early withdrawal and topping off here in OR. We aren’t allowed to fill our tanks here.


At my normal gas station, there are a couple of the pumps I avoid due to premature cutoff. The venturi tube is spiral wound around the outside of the nozzle to the tip, and I suspect those have been dented. Which restricts them and makes the whole mechanism run a bit closer to tripping.


What I find so cool about the pump handles is .
They are bullet and idiot proof . Put something vital in the hands of every idiot in the village and expect it to keep working flawlessly . Mission Accomplished . An unappreciated example of great engineering.


What’s more, something that transfers energy to the vehicle at a rate of 5 megawatts. I find those electric vehicle chargers rated at 250 (Tesla) to 400 (SAE J1772) kilowatts scary, but the humble gas pump is delivering power at a rate 12.5 to 20 times greater and, although “blowups happen”, they are very rare.