Oddity...

[bocefas]

This has happened to me 3 times in a row now, I'm 42 years old and have never experienced anything like it. I stopped last night on my way home from work to fill up with gas...I let the truck run (yeah I know you're not supposed to, blah blah, blah)...anyway...just as the pump was getting ready to automatically shut off, the truck's idle stumbled and it almost stalled out. As soon as the pump clicked off, the idle returned to normal. The first time it happened I thought I imagined it, but it has done it both times I've filled up since then. WTF, over?

 

[LearjetMinako]

Sounds like the engine was sucking up extra fuel through the EVAP system.

 

[RonD]

Because engine is running I would suspect the Fuel tanks EVAP system is the cause.

EVAP system connection is at the top of the tank, it's job is to keep gas fumes from polluting the air, so with engine running, "normally" the gas cap would be on so EVAP system keeps a slight negative pressure in the tank, this pulls any fumes through a charcoal canister.
With cap off EVAP might be putting full vacuum on the gas tank line since it is not getting any back pressure, the engine idle change is probably a purge valve slamming shut when fuel gets in the filler neck, sealing the tank, and a vacuum(back pressure) is created instantly.
The full vacuum to the EVAP would cause the computer to close the IAC(idle air control) valve to keep idle the same with the large vacuum "leak"(open gas cap).
When vacuum "leak" stops suddenly the IAC valve is close so idle drops like a stone until computer can open up the IAC valve to get idle back up.

I would check OBD code history, I bet there will be a few EVAP codes
I doubt it will hurt anything,...........but I wouldn't run an engine while filling up for the other obvious reasons

 

[adsm08]

Sounds like you need to shut the truck off while you fuel up. There is a reason you are supposed to shut the truck off while fueling, and it isn't because a state law maker though it would be a fun idea to make you shut the truck off. You probably sit thee and click the pump a bunch of times after it shuts off too.

Also, pretty sure that is by order of state fire marshal, so you could actually be ticketed for that if a cop pulls up while you are doing that.

 

[Mark_88]

yeah, it's against the law in Canada too...as is smoking within 3 meters...

But it does sound like the evap system to me too...when the fuel is getting near the top of the tank that probably forces a huge fart bubble up to the evap that then sends it over to the intake and because it's methane in gas fart bubbles the engine passes out briefly...

 

[wildbill23c]

Emissions system will do that. Diesel guys fill up all the time with their trucks running LOL.

 

[RonD]

diesel fuel is less likely to have vapors that will explode

Out side of a bad battery or bad starter, leaving a gasoline engine running while fueling has no upside, only a down side.
In cold or very cold weather the residual heat in an engine that was just shut off will continue to hold or spread it's heat if engine was just started.
In fairness on very cold days the fuel vapor is minimal, but not 0, but any heat feels warm so still no advantage to leaving the engine running.

 

[bocefas]

I figured it had to be something concerning the EVAP system, but in my 26+ years of vehicle ownership I have had my fair share of cars, trucks, SUV's whatever, including other late model Ford products (1995 Bronco, 1990 F250) that never acted this way. I always let whatever it is run when I stop for gas, and 99.9% of the time I fill the tank. I guess it is just one of the Maaaz-duh's little quirks.