When you fill up a car with gas, the pump knows when to stop pumping gas. How does this system work? Is it by suction or detection of liquid or fumes? Is there something in the car that signals to the nozzle, or is it entirely the pump itself?
I browsed google and cartalk.com but still no clear answer, so I turn to the Dope.
there is a pressure sensor in the nozzle. as the tank fills to near the top the air pressure increases and that is detected by the sensor shutting off the nozzle.
If only there was some guy with a newspaper column who could answer these questions!
http://www.straightdope.com/columns/read/230/how-does-a-gas-pump-know-to-shut-itself-off
Not AFAIK - it’s due to the level of petrol/gas in the tank reaching the tip of the nozzle. Cecil’s column linked just above backs that up.
Thanks, howye!
I thought I’d skimmed a thread here recently (IMHO, perhaps) about issues stemming from something faulty inside the car causing an overflow of gas, which is why the question stuck with me, but I don’t see it in my browsing history. Ah well, question resolved.
Missed the edit window but here’s a pic to illustrate it. The blue path leads to the “sensing” hole near the tip of the nozzle. Once that is covered, it causes a partial vacuum to form which shuts off the flow.
I think you can just about see the hole on the bottom of the nozzle here. The pic on this page seems to show a slightly different layout, with the hole right in the end of the nozzle.
That’s very interesting, thanks. Now you’ve got me wondering why some nozzles have functioning “hold open” latches and others don’t.
One of the gas stations I went to regularly a few years ago did that - they took all the little “stay-cocked” ridges off of their pumps.
I asked the manager why they did that, and he said it was to reduce the number of people leaving their vehicles to come inside and shop while their gas pumped, because there were problems with overflow of gas, or people waiting in lines for people inside to leave, or people stealing gas from the active pump since there wasn’t anyone near it.
So - liability.
The latches were also made illegal in California in October. Lots of CA regulations trickle down to the other 49, so I wouldn’t be suprised if we start seeing fewer and fewer of them on new pumps. I think I’ve heard about some proactive towns banning them elsewhere too.
Really? All of the stations I use still have them.
It’s only certain types that are illegal, I believe. I’ve seen signs related to this at a gas station that took their off, and I’ve seen functioning ones at other stations recently. Most likely it’s stubborn/frugal station owners not wanting to dish out for the new catches.
The vapor sensor isn’t 100% infallible. In most states now, holding the handle cannot be done with a device, switch, lock, lever, etc. Also, it’s an issue of user error. Let’s say the gas tank is full and the nozzle shuts off. The user thinks it’s not full, and locks the lever again. The tank is now merrily overflowing, but the handle is in a locked position. Or, in the previous scenario, the handle locks accidentally.
Vapor sensor failure most often occurs when you are filling up a gas can or when the vapor sensor is under liquid. In these situations, it can be very dangerous to lock the handle.
On top of all this, gasoline is not a safe substance. It can irritate eyes and skin, and even if it doesn’t feel wet, evaporating gas is very, very flammable. For example, there’s numerous stories of people who spilled gas on their pants, lit a cigarette, and set themselves on fire. The vapors from the pants caught fire even though the source of fire was relatively far away.
On some of the ‘stay cocked’ equipped nozzles, if you shut the lever on the pump off with out clicking the nozzle, you can give the next customer a gasoline douche.
Not fun, it happened to me. Lucky I wasn’t immolated.
Oh, I see, so according to Musicat’s link it’s only a certain model of gas pump (although a popular one) that have latches the fire marshall didn’t like. Funny how all the headlines are written as if it were all pumps!
I was down there a couple of months before the deadline and about half the stations I stopped at in California didn’t have the catches. It was noteworthy enough that I asked the guy at one station and he said they had to be gone by October. It’s annoying because, as one of the few people who still checks their oil, I’m used to just keeping an eye on the nozzle while I’m futzing under the hood.
Ok guys, so what would be the reason for a pump not shutting off automatically?
Because on two different occasions, at two different gas stations, I’ve spilled gas all over the ground because I had it latched and thought they were supposed to stop when it got full. What’s the deal?
User error most likely. The vapor sensor needs air pressure. If you didn’t put the nozzle in far enough, air pressure can’t increase.
Makes sense, but I swear I have it in as deep as it’ll go.
And the gas pump is in as far as it’ll go into my car as well.
Maybe you need a longer hose, er…nozzle, er…I don’t seem to have that problem. 
Not only is this not how the shutoff works, neither does the equilibrium air pressure in the tank increase. The tank is open to the atmosphere. It is not pressurized.
Again, this is not how the shutoff works. The air pressure is not increasing. As others have stated, the sensing port in the nozzle needs to be covered by liquid to activate the shutoff. So if the nozzle is not put in far enough, it never gets covered by liquid.
