White Cap Obstruction in Filler Neck

[SamSmurf]

Hi, I have a 2021 Motorhome on a Chevy express van 3500 dually chassis with 6.6L engine and 30 gallon fuel tank

I've always had issues filling up the gas tank and the pump would shut off if the fuel pumped too fast. I have always needed to squeeze the pump just enough for a slower flow to not cause the fuel to splash back and turn off the pump.

Today I stick a borescope into the filler neck to see if there are any obstructions leading to the gas tank and I find this white cap with a small hole about a foot into the filler neck. This white cap doesn't look like a flap or a vent and it looks like all the fuel must go through the little hole which may be the cause of why the fuel pump stops so early while fueling.

First picture shows the white cap with a small hole inside the filler neck
Second picture shows the filler neck underneath the RV. The 11 inch mark is most likely where the white cap is located. Basically where the metal tube ends and where the black tube begins

I wanted to ask if any one knows what this white cap is? and if I take it out would it cause a check engine code to come?

[MountainManJoe]

Judging by the DIY hose clamps, the previous owner has installed something there. Possibly an anti-siphon device because he was getting his fuel stolen. It might be the cause of your fuelling up issue. Looks like you can remove it fairly easily to test that theory.

Another thing that causes premature pump shut off is obstructions in the EVAP system. That smaller hose next to the main fuel filler is a vent which is part of that system. If it's full of spider nests or charcoal pellets then that will cause you issues too. If you remove it from the tank you can probably clear it out with some compressed air or something.

[SamSmurf]

wow, a anti-siphon device makes alot of sense!!

Hopefully it is and it is easy to take out thank you!

[SamSmurf]

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Pulled it out of the filler neck and it was definitely the cause of my fueling issue.
I drove my motorhome to the nearest gas station and was able to fuel up to a full tank without the pump stopping. I wish I pulled this out so much sooner

I looked all around this "flap cap" for a part number and there isn't.
If any one knows what it's purpose is, please let me know. I don't think it is a anti siphon device because the flap seems large enough for a siphon tube to go through.
This Flap cap is spring loaded and the spring resistance increases the further back it is pushed open. 
Although the resistance is light with my finger, I would imagine the force from fueling only opens the flap a quarter way. The housing of this flap cap also reduces the filler neck tube even more leading to the premature pump shut off I was experiencing.
This flap cap was most likely installed by the motorhome's manufacturer (Gulfstream) and not Chevrolet  because the gas cap and filler neck is installed on the house section of the motorhome.

I hope this post helps anyone with a motorhome who is experiencing premature fuel pump shut off issues because I was about ready to buy and install a new charcoal canister and/or vent solenoid since the evap system is the leading cause of this fueling issue

[MountainManJoe]

[SamSmurf]

ohh wow. that is what it is.
So the point of this flapper valve is to prevent fuel from coming up the filler tube before the pump shut off.

I should be fine to run my RV without this flapper valve right? Honestly I rather run the risk of letting the gas squirt out due to a slow shut off from the pump rather than pumping a few dollars worth of gas a at a time.

[MountainManJoe]

I should be fine to run my RV without this flapper valve right?

I have no idea. Try it and see.
Maybe motorhome fuel tanks are different in some way? I can't see why they would install it unless they had a reason.

It looks like it should open easily to allow normal fuelling. Maybe yours has gotten stiff? Maybe you actually do have an EVAP obstruction. I can't tell from my end.