Fuel leak after fill up. 68 chevelle.

Mike, easy solution for a 68. You say there's 2 vent lines out the rear of the tank? I ask because sometimes there's 1, 2 or even 3 depending on the tank. If you have 2 then plug one of them. The remaining vent should have a fuel hose coming out of it and zip tied high above the tank. Mine is zip tied to the fuel hardline way above the tank. Use a cheap paper style fuel filter in the vent fuel hose to reduce/eliminate fuel smell (the main purpose of this is to stop junk/dirt from getting into your tank as well). You don't need to use both vent lines to make this work...just use one and save yourself a headache.

The reason your tank is overflowing is because the 2 vent lines are creating a siphoning affect. On a full tank, the gas is slightly above the vent holes and one line is allowing air into the tank and the other line is allowing fuel to dump on the ground (pressure buildup in the tank does this). I think, from the factory, the 2 vent lines went to the same canister under the car and there was a line from the canister to vent. If you don't have the canister and both vent lines are open to the environment then you'll have issues like you described.

Use the non vented cap. Make sure your cap has a proper seal and the rubber isn't cracked.

I had the same problem and this solution fixed the issue.

Siv said:

The reason your tank is overflowing is because the 2 vent lines are creating a siphoning affect. On a full tank, the gas is slightly above the vent holes and one line is allowing air into the tank and the other line is allowing fuel to dump on the ground (pressure buildup in the tank does this). I think, from the factory, the 2 vent lines went to the same canister under the car and there was a line from the canister to vent. If you don't have the canister and both vent lines are open to the environment then you'll have issues like you described.

Click to expand...

Actually, the vents are placed in an expansion chamber that should be higher than the fuel level, unless the car is down in the front.


The large vent is s short tube that ends about 2" inside the tank. The smaller tube runs at a diagonal from the right front to the left rear, to provide venting if the large vent is covered. Also notice how the large tube is crimped almost shut, to help prevent fuel from flowing out.




The hoses attach to a plastic reservoir that is mounted to the underbody higher than the fuel tank. The original design has a piece of fuel-resistant foam stuck into the upper tube to keep debris from entering the reservoir, and to soak up any fuel that tries to go out of the top. Ground Up sells this reservoir as part #XTV-689, for $39.95.

[url]http://www.ss396.com/mm5/graphics/00000009/XTV-689.jpg[/url]


Unfortunately, this idea works better in theory than reality. If the car is parked downhill, or leaning to the right, with a full or almost full tank of gas, it will drip out, and it drips on to the right tailpipe. My solution for the location of the drip is to run another hose from the top tube of the reservoir down the frame rail, exiting through a hole in the frame. This seems to have helped the dripping problem, too.



Cars of this era were expected to smell of fuel when parked. All of them had tanks vented to the atmosphere, and that hydrocarbon leakage was a large contributor to smog and air pollution. That's also why houses built in this era had detached garages or carports, so the smell couldn't go into the house.
I hope this helps you understand what's happening, and may provide a fix for you.
BillL