Gas tank venting

[CowtownCobra]

I'm doing some work on my fuel filler and am looking at the tank vent line. Has a line from the top of the tank to the base of the filler, seems rather useless to me.

How are your gas tanks vented?

[Curt C.]

Same as yours. It is not useless. Allows displaced air to escape when filling the car up. It will also keep a full tank of gas from ruining your paint job on a hot summer day.

[CowtownCobra]

But how does the air get in to displace the fuel pumped out?

[Tim Brewer]

Where there is no fuel in your tank, there is air. When you put in fuel, you're displacing air. Without the vent tube, you would have a gurgling mess everytime you fueled up, and would take a long time as well.
Tim

[Curt C.]

Originally posted by CowtownCobra


But how does the air get in to displace the fuel pumped out?

I believe most gas caps have small holes to allow air back in as the fuel is slowy consumed but I am not sure. Good question.

[Tim Brewer]

Gas caps arn't air tight. If air couldn't get in, I believe that would be called vapor lock, and your car would die.
Tim

[Jack21]

You should have a vent tube on the tank that allows air in as fuel is consumed. Ideally, this should be a check valve so air only vents in. Since the early '70's, this inlet vent has connected to a carbon cannister to trap fuel vapors from venting to the atmosphere.

Without a vent, in the cap or tank, to let air in, a vacuum would form in the fuel tank and not allow the fuel pump to pump fuel to the carb. The engine would stall as if running out of fuel.

Heater control vacuum valves can be used to do this.

[392cobra]

Dave,
Our Le Mans fuel caps are vented.

unlatch the cap and you will see a disc that seals to the body part of the filler.That disc is spring loaded and held to the top part by a bolt.When it is latched shut the disc is moved towards the top and air can get around the bolt to vent.

So if you turn turtle gas will be spilling out the fuel fill too.