# Purpose of side-tube on modern gas tank filler hose? **URL:** \ **Category:** General Discussion\ **Created:** [May 26, 2024, 11:53pm UTC](https://community.cartalk.com/t/purpose-of-side-tube-on-modern-gas-tank-filler-hose/194986 "2024-05-26T23:53:25Z")\ **Posts on this page:** 4\ **Page:** 1
**Author:** ![George\_San\_Jose1](https://avatars.discourse-cdn.com/v4/letter/g/e5b9ba/32.png) [@George\_San\_Jose1](https://community.cartalk.com/u/George_San_Jose1)\ **Post date:** [May 26, 2024, 11:53pm UTC](https://community.cartalk.com/t/purpose-of-side-tube-on-modern-gas-tank-filler-hose/194986/1 "2024-05-26T23:53:25Z")
Just a curiosity. What is the purpose of that little side-hose in a gas tank filler tube? Before the invention of modern evap systems I can see its purpose, to allow the air in the gas tank to escape when refilling. But wouldn’t the evap’s venting system preclude the use of that side-tube in modern vehicles? The air in the tank gets pushed into the charcoal canister during refiling. Yet the side-tube remains??? Any theories? > **[Amazon.com: BVVINC 4C3Z9034C Rear Fuel Tank Filler Neck Fits for Ford F250 F350...](https://www.amazon.com/BVVINC-4C3Z9034C-Replace-8C3Z9034L-F81Z9034SA/dp/B0CXX86YYW)** > > 【Fits for】: Fit perfectly for FN782 fuel filler neck fit for Ford F250 Super Duty 3/4 Ton Pickup 2003-2004, fits for F350 Super Duty 1 Ton Pickup 1999-2010, fits for F450 Super Duty 1 1/2 Ton Truck 1999-2010, fits for F550 Super Duty 1999-2010 ---
**Author:** ![George\_San\_Jose1](https://avatars.discourse-cdn.com/v4/letter/g/e5b9ba/32.png) [@George\_San\_Jose1](https://community.cartalk.com/u/George_San_Jose1)\ **Post date:** [May 27, 2024, 12:20am UTC](https://community.cartalk.com/t/purpose-of-side-tube-on-modern-gas-tank-filler-hose/194986/2 "2024-05-27T00:20:37Z")
I’ve come up w/a theory to explain the need for the side-tube. There’s air in the filler tube before beginning refilling. Some of the air gets pushed back into the tank by the fuel as refilling starts, but the side-tube gives the rest of the air another way to escape, back down the side tube to the tank’s airspace. So the purpose is to make it easier to refill the gas tank without the pump shutting off automatically from air pressure building up during refueling. None of the fuel-polluted air escapes to the atmosphere, it still gets pushed to the charcoal canister. ---
**Author:** ![davesmopar](https://sea1.discourse-cdn.com/cartalk/user_avatar/community.cartalk.com/davesmopar/32/41750_2.png) [@davesmopar](https://community.cartalk.com/u/davesmopar)\ **Post date:** [May 27, 2024, 12:48am UTC](https://community.cartalk.com/t/purpose-of-side-tube-on-modern-gas-tank-filler-hose/194986/3 "2024-05-27T00:48:30Z")
Simple terms air behind water…You would get a lot of splash back or the handle would keep clicking off… ---
**Author:** ![George\_San\_Jose1](https://avatars.discourse-cdn.com/v4/letter/g/e5b9ba/32.png) [@George\_San\_Jose1](https://community.cartalk.com/u/George_San_Jose1)\ **Post date:** [May 28, 2024, 9:28pm UTC](https://community.cartalk.com/t/purpose-of-side-tube-on-modern-gas-tank-filler-hose/194986/4 "2024-05-28T21:28:56Z")
That sounds correct. I got to thinking that part of the reason must be b/c on modern-evap equipped cars the main filler tube’s far end is usually near the tank’s bottom & therefore submerged. So it is going to be hard for the air in the filler tube to move to the fuel tank’s airspace above the gasoline level. The little tube must provide an easy path for the air in the filler tube to move directly to the fuel tank’s airspace, doesn’t have to be pushed to the bottom of the tank and bubble up.