Fuel Tank Vent Question

[EngineerDoug]

Hello all,

I have a question about stock fuel tank venting. My tank is a repro '68 installed in a B body. I have both the external vent lines connected - the short one that runs up to the shock mount crossmember area, and the the long one that runs up into the trunk and then loops up and down into the frame rail.

I am experiencing the common issue of fuel smell once the car is parked in garage. It happens with a full tank, particularly on a warm day. The fuel is being forced out the vent line that exits into the frame rail. It's not just vapor, which leads me to believe the line inside the tank is below the fuel level.

I understand that the two tank vent lines inside the tank are different - one is short and one long. I need to look inside with a borescope to confirm which is which, but my basic question is, why did the factory set it up this way? Did they anticipate that the car might be parked on an incline? This arrangement would ensure that one of the lines would be above the fuel level, I would think.

But what about when the car is parked on a level surface? The behavior I am seeing suggests that both internal vent lines are below the fuel level.

And why would the factory setup include a very short line outside the tank, and a very long one outside as well? I guess I am just trying to understand the logic of it, as the fuel puking is a problem with an attached garage.

Thanks.

 

[dfrazz]

Already a discussion on this here - Garage stench after parking car - does non-ethanol help?

 

[EngineerDoug]

I will check it out; one thing that is on the way is a charcoal filter vapor canister. My plan is to put it inline with the trunk tank vent line.

 

[EngineerDoug]

An update - I spoke with the folks at Vapor Trapper and they suggested the following:

1) Tee both vent ports on the tank together, and run the combined line to their charcoal canister.
2) Use a non-vented cap with this scheme.

They also believe the reason the factory went with two tank vents (one fore and one aft) was so that at least one vent would be above the fuel level even if the car was parked uphill or downhill.

We'll see how this works.

 

[It’s just me]

An update - I spoke with the folks at Vapor Trapper and they suggested the following:

1) Tee both vent ports on the tank together, and run the combined line to their charcoal canister.
2) Use a non-vented cap with this scheme.

They also believe the reason the factory went with two tank vents (one fore and one aft) was so that at least one vent would be above the fuel level even if the car was parked uphill or downhill.

We'll see how this works.

I have done that set up, and it works well. I used the tanks inc. canister though. It was a bit cheaper but doesn’t look as nice. Since I hid it under the rear package tray, looks didn’t matter.