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#3311682 - 03/14/14 11:17 AM Re: Gas coming out of tank while filling [Re: Tom NJ]
hsd Offline


Registered: 09/05/09
Posts: 235
Loc: NE
The charcoal canister can not cause overflow during filling as it only functions when the system is sealed up (gas cap is installed).

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#3311978 - 03/14/14 04:14 PM Re: Gas coming out of tank while filling [Re: Tom NJ]
1 FMF Offline


Registered: 08/12/02
Posts: 1633
Loc: CT
when i had the back of my 2006 gmc sierra apart, i found that there's a valve in the filler neck of the fuel tank that's spring loaded and stays shut unless a force acts from outside to push it inward, such as fuel when filling. the purpose of it is to prevent fuel from coming out in case of a rollover. i believe all gas tanks have this, and also makes it nearly impossible to siphon gas out of the tank because it will block any hose from making it through. my guess is that valve is sticking or not opening all the way preventing any significant volume of fuel from entering the tank quickly. otherwise everything else is sealed, the evap canister and purge valves are always closed unless commanded open by the pcm. your vent for when filling the tank is a separate small hose that runs alongside the big fill hose and goes to the filler neck thing up on the body where your gas cap is. if you look where you insert the gas pump into that fill neck, there will be a separate small hole there, that's the vent which works when filling the tank because the gas cap is off. but when the gas cap is on then the fuel system is completely sealed.


Edited by 1 FMF (03/14/14 04:16 PM)

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#3311989 - 03/14/14 04:22 PM Re: Gas coming out of tank while filling [Re: Tom NJ]
1 FMF Offline


Registered: 08/12/02
Posts: 1633
Loc: CT
also gas with ethanol is quite a solvent, also attracts water, and over time can dry out that plastic valve and make it stick or corrode/rust the metal spring. using some kind of oil, like marvel mystery oil, which you can run with little problem in the fuel, shoot that down the fill neck every now and then to coat that anti-siphon valve and it may get better. otherwise if it's integrated into the plastic tank then you're looking at a new fuel tank.
but don't dump a pint of mmo down the fill neck just before filling up with gas and wash all the mmo down, that'll defeat the purpose.

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