Can the fuel tank vent be used as a return line? (Page 1/1)
Mr. Farknocker AUG 06, 03:46 PM
I have a question about the fuel tank vent line on an 87' GT.

I blocked off the fuel tank vent line and removed its related equipment over 20 years ago when I converted over to a SBC set up. I realized that that it was not the best thing to do because the internal pressure of the tank changes when the temperature changes but I've been able to address the problem by loosening the gas cap to allow the gas tank to expand and contract as needed. Restoring the vent line is on my to-do list although its somewhat way down on the list.

I have a Walbro 255lph fuel pump pushing fuel to a carburetor through fuel pressure regulator with a return that's hooked up to the fuel tank return line to bleed off excess fuel being pushed by the pump. I'm planning to install a vapor separator fuel filter near the carburetor to alleviate a vapor lock problem that has plagued me for years. The filter has an in and out nipple and a third nipple that vents vapor accumulating in the fuel line due to heat. The third nipple is supposed to be connected via a 1/4" fuel line to the fuel return line so that excess vapor/fuel can be returned to the tank. The 3rd nipple is ported with a 60/1000th" orifice to restrict the flow of gas and to preserve fuel pressure in the line so that the fuel in the line gets to the carburetor. Since the fuel tank return line is already dedicated to returning fuel from the pressure regulator, I was wondering whether the (unused) fuel vent line can be used as an auxiliary fuel return line.

I recently removed my fuel tank and cleaned the fuel gauge sending unit and got a good look at the end of the lines inside of the tank. The return line appears to run to the bottom of the tank and the vent line seems to dead-end at the top of the fuel tank with some sort of black cap/device attached to the end of it.





What's the purpose of that black cap/device and can I use the vent line to return excess (albeit, very little) vapor/fuel to the tank while leaving the gas cap loose to allow the tank to breath?

Is there another port (aside from the return line) which I can use to return excess fuel/vapor to the tank?

TIA

~F

[This message has been edited by Mr. Farknocker (edited 08-06-2019).]

fierofool AUG 06, 05:15 PM
The black capped device is the vapor vent line. The cap is to prevent raw fuel from being pushed out into the vapor tank as it sloshes around. The long tube that goes all the way to the bottom is the liquid fuel return from the fuel rail. Returns excess fuel to the bottom of the tank rather than spraying it down and causing vapors.
AsaBergman AUG 06, 05:28 PM
The purpose of the black cap is a safety device for if the car flips over to stop/slow the draining of fuel out that line. Who knows if it actually does anything though.
fierofool AUG 06, 06:16 PM
Having just removed the cap from one a moment ago, I agree that it may serve a dual purpose. There is a small ball of some type inside a housing. It may cover the vapor vent pipe if the car was turned upside down. Primary purpose is to vent vapors with a secondary to prevent spillage in the rare event the Fiero lands upside down.

The two units I tested will flow air in both directions when the unit is upright as installed in the tank, but will not flow air in either direction if the unit is turned upside down.

The tank is a closed system. The vapors have to be vented during expansion of the tank content and this line facilitates the venting without letting liquid fuel escape.

[This message has been edited by fierofool (edited 08-06-2019).]

Mr. Farknocker AUG 06, 07:08 PM
Thanks guys. Good to know. I wonder if the check ball also sits on a float which closes the vent when the tank is filled to capacity. I don't think it would be a problem if it didn't but wouldn't be surprised if it did.

Also the sealing of the vent line when the car is upside down is a good reason why I should "re-activate" the vent line. A loose fuel filler cap would allow fuel to leak out of the tank if the car ends up upside down.

[This message has been edited by Mr. Farknocker (edited 08-06-2019).]

fierofool AUG 06, 08:14 PM
There is nothing in the fuel filler line to prevent fuel from leaking from a loose filler cap.

With the car sitting level or nose up, you can't fill the tank all the way to the very top. The small line from the top rear end of the tank will put gasoline back up to the filler and shut off the nozzle. Now, if the car was then moved nose uphill, fuel could reach that vent pipe and if the ball float didn't work, fuel can go all the way to the recovery tank or the vapor recovery canister.