Fuel filler hose return line function?

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Breather. Consider when lots of fuel is trying to go down the filler hose - air has to escape from the tank to allow the fuel to get into the tank.... the breather provides a return line for that air to escape.

cheers,
george.
 
Then your tank would be vented to the atmosphere even with the cap on. Not cool, likely code, evap loss, etc.
 
The tank should also vent through a pressure release valve into a charcoal canister which catches fumes. Shouldn't it? Keeps it sealed but able to vent if required.
 
BruntForce77 said:
I'm confused. Isn't the cap vented anyway? If not then its a sealed system. Wouldn't that create a vacuum in the tank?
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no, the cap is not vented.

Haven't you seen the threads about pressurized tanks?

I had the "big whoosh of gasoline vapors when i remove the gas cap" issue, and cleared it up with that autozone vapor canister.
 
There have been people who have experienced boiling gas, gas pouring out when they take off the cap, and even ruptured tanks.

Likely due to a combination of modern gasoline mixtures and clogged vapor canisters.

Duralast VC-120. Way cheap at autozone. Fixed me right up.
 
Just read through a 6 page thread on the subject.. I now believe i understand. The charcoal canister relives pressure and filters the vapors when pressure builds to a certain point in the tank during operation. The breather on the filler hose simply allows the air/fumes that are displaced during filling to escape since the pressure is not great enough to open the valve on the cc. Theoretically, you COULD vent the filler breather line into the atmosphere, but that would reduce the pressure in the system during operation. This is a bad thing because the pressure raises the boiling point of the fuel, so open venting the system would burn more fuel, and may increase the likelihood of your fuel boiling... That about right?
 
pressure lowers the boiling point.

open venting would vent more fuel into the atmosphere, which is bad for the atmosphere, and will fail emissions inspection in any area that has emissions inspections.
 
Are you sure? If pressure lowers the boiling point, which means more pressure equals faster boiling, then it wouldnt make sense that the tanks start boiling rapidly when the caps are removed?... And I seem to remember from science class that boiling point and pressure are directly related.
 
I'm probably wrong about the boiling point.

If your gas tank doesn't vent when you remove the cap, either your vapor canister is still good, or it's venting somewhere else.
 
Timpanogos Slim said:
I'm probably wrong about the boiling point.

If your gas tank doesn't vent when you remove the cap, either your vapor canister is still good, or it's venting somewhere else.
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Mine vents sometimes, just a second or two. Not really a problem. I actually started this thread because I am figuring out where I'm going to relocate my filler cap/neck, and I was looking into flush mounted caps. Most of them don't have breather tubes though, so I was curious to see what the breather tube was for since I was debating removing it, or leaving it open... Now I know. I think I ll look for a flush mount filler door/neck with a built in breather tube. If not ill have to figure out a way to use the stock stuff.
 
Flush mounted cap simply needs a fitting for the breather pipe to flow into. Preferably at the 12 o'clock position. It's to allow for faster filling of gas. Otherwise you could cap off the breather pipe.
 
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