Question about gas pumps and pressure

Factual Questions
1

In most cases, when I’m filling my tank with gas at the pump, I’ll hold the trigger down and the gas will run freely until the tank is full, at which time the trigger will depress (go slack) and stop the gas flow. I’ve heard it explained that back pressure causes this shut-off, and I’m willing to entertain other explanations, but my real question is this: Why does the pump sometimes shut off well before the tank is full, forcing me to tickle the handle repeatedly until I find the hair’s breadth amount of pressure sufficient to engage the pump without triggering the shut-off, only to have it pop back on me so I have to start the whole thing over again dozens of times before I decide that 1.5 gallons is enough to get me where I’m going?

I usually experience this phenomenon when I’m running late…hmmm.

2

The Master Speaks

3

Most likely, the nozzle is positioned in such a way that the gasoline can’t easily squirt out the end of it. The end of the gas pump nozzle is a tube, that’s going into another tube (the filler tube leading down to the gas tank). If you don’t line them up right, the nozzle ends up squirting against the side of the filler tube instead of straight down into it, and the back pressure shuts off the pump (as explained in Cecil’s article).

When that happens, just move the nozzle around a bit, and it should be ok. I found on one car I used to have that I had to back out the nozzle a bit or this would happen a lot.