Fuel Leak at tank fittings with high octane fuel

[68 Sport Satellite]

I have a brand new replacement fuel tank that is built to OEM outside dimension specs and has both a send and return line. Been running this tank with no issues at all for the past year and very happy with it. It has an internal 40 psi fuel pump with baffle and sump (in possible prep for a future EFI conversion). Normally run 91 Octane fuel and never had a leak. While engine tuning last month, I topped of the tank to completely full with 2 gallons of 110 octane. Went into the closed 2 car garage only a half hour after parking it due to smelling fuel vapors and man oh man - fuel leaking from both inlet and return fittings at the tank. Not a lot, but just a weeping drip-drip-drip. I drove the car a mile the next day and the weeping drip was still there. Drove the car 100 miles the next day and now no leak. Topped off the tank with 91 like always and no more leak.

I'd love to just tighten the fittings, but with the hard lines I have I need to be careful not to damage anything.

Should I just leave it alone since I don't plan on running 110 any more? Can higher octane fuel just weep like that due to increased volatility compared to the 91 octane? Or should I address this as some loose fittings that will eventually leak even with the 91? Remember the tank has an internal 40 psi fuel pump. I have the under hood regulator dialed in at 6 psi. Tank and hard lines are aluminum. Fittings are brass NPT thread.

 

[khryslerkid]

I'm thinking it was a "pressure" thing that was taking place. Difference in temperature that expanded the fuel which caused the leaks that you don't normally have.
Just a guess. Do you have a vented cap?

 

[68 Sport Satellite]

Cap is original and labeled "non-Vented", however, it's 48 yrs old and I can't imagine the little rubber seal is still completely sealing the filler neck.

For tempertature difference, what do you mean by that? Are you saying it's related to the 110 octane being cooler or hotter than the 91 octane? Is there anything related to the differences in the 2 fuels that could explain this? Normally if I was starting with an empty tank I would put the 110 in first and then the 91 so the 110 gets mixed in better. This time I already had an almost full tank and just topped it off with a few gallons of 110 to see if my motor ping would go away.

 

[68 Sport Satellite]

bump

any other ideas?

 

[khryslerkid]

If you fill up your tank at the pump with ground temperature gas, say 55* to 60*, seal it up, then the outside air say 75* to 80*, warms things up expanding the gas, its going to push it out where it can. The new cars of today have solved this problem with different filler caps, return lines, ect.
Now if the higher octane fuel expands at a quicker rate or if it's thinner and can find it's way thru a loose connection easier, I don't know. My issue would be that there is a leak. Fuel line, sending unit gasket, filler neck hose or filler cap, it's a leak and we don't want that.
If you think it is the lines, see if you can tighten them a bit more. It could be the sending unit gasket leaking running gas down the line too. Sounds like it's only when the tank is topped off but I would want to follow thru and get it taken care of.

 

[poopdeckpappy]

Sending unit gasket! If it's aftermarket, you don't know about the quality of the rubber they are using for the seal. It's my best guess without looking myself. Empty it out, take it down, troubleshoot, fix! That's a serious safety issue!! Let us know of what you find?

 

[68 Sport Satellite]

yep, I need to take care of this one way or another. I've decided to add a few soft line connections in-between the tank and the hard lines and also tighten the hard lines to specified torque values. I came across a torque value spreadsheet on an FAA aircraft repair website and it lists the following:
https://www.faa.gov.regulation.pdf (Rigid Fluid Lines) - google it, the link doesn't work

TubingOD (in) Fitting size Aluminum tubing torque (in-lb) Steel tubing torque (in-lb)
5/16" -5 60-85 180-200
3/8" -6 75-125 270-300
1/2" -8 150-250 450-500

Quite a difference between the aluminum and steel torque values and among the above sizes. No torque wrench was used in installing my rigid lines, so they very well could be under or over-torqued.

Sorry, can't get the tabs to work correctly.