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slow gas fill?


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I've got an issue which was a nuisance and now is turning into a problem.

When I fill up at the pump, the gas pump auto shutoff kicks on early.  I used to be able to nurse an extra gallon or two slowly into the tank, but now it's like 5 gallons.  It's annoying because those last five gallons can take an extra 5-10 minutes.  It's also particularly frustrating because with a 42 gallon tank I need to make sure we are hitting the water with every last possible drop of fuel.

I'm not sure where or what to look at.  I have confirmed that the vent on the tank works, by blowing a little air into the tank vent and then seeing the resulting burp of fuel.  Or maybe that's indicative of it not working right?  (in any event, if I introduce air "down" the vent the wrong way, the air clearly makes it to the tank).

What do I do to fill the tank full in one shot?

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Same issue years ago and it was a clogged up tank vent full of buffing compound from the factory on a new boat, surprised! It could also be from spiders or a kink in the vent house.

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8 minutes ago, RyanB said:

Does it help if you open the fuel fill on the opposite side you are using?

haha it might if I had an opposite side (axis).

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The problem with "topping off" with the newer fuel systems is the possibility of damaging the fuel tank, vents, or other components.

The diurnal tank vent is designed to prevent the tank from being completely filled to allow for fuel expansion.  When fuel gets up to the valve on the tank it closes the vent and that is what makes the gas pump click off when filling.  Over filling the tank can eventually cause damage to the the diurnal valve, back flow valve on the fill hose, and the carbon canister (at least on Malibu boats, Axis does not have a carbon canister).  Damage to those components can cause fuel filling problems, venting issues, or fuel leaks.

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32 minutes ago, csleaver said:

The problem with "topping off" with the newer fuel systems is the possibility of damaging the fuel tank, vents, or other components.

The diurnal tank vent is designed to prevent the tank from being completely filled to allow for fuel expansion.  When fuel gets up to the valve on the tank it closes the vent and that is what makes the gas pump click off when filling.  Over filling the tank can eventually cause damage to the the diurnal valve, back flow valve on the fill hose, and the carbon canister (at least on Malibu boats, Axis does not have a carbon canister).  Damage to those components can cause fuel filling problems, venting issues, or fuel leaks.

I don’t have venting issues or leaks but I do have filling problems. I know my fill hose is clear. That points to diurnal valve?

Correct me if I’m wrong(?!?) but I don’t think an extra five gallons would be considered “topping off” in the context you are suggesting?

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The fuel fill cap is a positive locking design with a pressure check valve and there is a one way check valve on the filler hose to prevent fuel from spitting out when fueling the boat.  There is a diurnal vent valve on the top rear of the tank to close the tank vent when the fuel level gets high enough.  That provides room for fuel expansion, or ullage, and prevents expelling fuel from the vent when the tank is full.  A level, or grade, vent valve is near the front on the top of the tank to allow venting when the bow is raised during acceleration that would cause the diurnal vent valve to close due to high fuel level in the back of the tank.  All of it is mandated by the U.S. Coast Guard.

There can either be a carbon vapor canister inline on the vent hose to the deck vent to capture VOCs or a 1 PSI check valve, usually integrated into the fuel fill cap.  Malibu uses carbon canisters and Axis uses a pressure check valve.

Here are a couple explanations of how it works.

https://www.criboats.com/knowledge-base/im-unable-able-to-fill-my-gas-tank-all-the-way-why/

 

https://www.boats.com/on-the-water/modern-gasoline-fuel-systems-on-boats/

 

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If liquid fuel gets forced into a carbon canister from topping off or over filling it will clog the vent and very likely cause a problem with fueling the boat.  A canister with fuel in it can be removed and left in the sun for a few days.  After the liquid gas has evaporated the canister can be reinstalled.

Since the Axis does not use a carbon canister that is not likely causing a fuel filling issue, but on a Malibu it could be a problem.

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@csleaver, thanks for taking the time to write this up for us.  We really appreciate you taking the time to share your knowledge, and especially in such a complete way.  I had no idea that new boats had a charcoal canister.

My 1999 has always been pesky and slow at filling, but that is usually solved by starting slowly and letting the vent start flowing before increasing the nozzle flow.  I don't like to fill mine past about 3/4 tank because of spilling issues.  The amount of engineering applied to these things in the last 20 years is staggering.  I understand that the Coast Guard rules require it, so they can't just slap a tank and some hose in anymore, but the new ones seem to have fully addressed the issues.

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