Sealing fuel tank filler

[HeliCooper]

I am testing the fuel tanks using the manometer method. I've had about a 3"/10 hr leak. I used soapy water to find a small leak around where I duct taped the fuel filler cap. I'd really like to seal it up so I can make sure I don't have a really small leak somewhere else. I haven't seen anything with the soapy water but I'd sleep so much better at night with a solid water level.

Long story short anyone have a good way to make the fuel filler cap air tight so I can be sure the tank doesn't have a small leak somewhere else.

Thanks

[Mel]

Cover the hole with plastic wrap and insert the cap. the plastic wrap makes a good seal.
Another method is to use plastic shipping tape instead of duct tape.

[Doug]

A temperature drop of the air inside the tank or an increase in atmospheric pressure will also look like a leak when using a 'sensitive' manometer.
Instead, try using an inflated balloon, if it deflates - you have a leak. The pressure will also aid with the soapy water treatment.

[wirejock]

Log your temps. You are closer than you might think.
Temp change is .810" total per degree.

[HeliCooper]

I have the fuel tank inside so the temperature has been remaining stable. I tried the plastic wrap idea before coming on here and it also leaked through that. I will give packing tape a try.

Thanks

[wirejock]

You could blow a balloon up so it fits but tie something to it so it can't pop and fall inside.

[rvbuilder2002]

A stable manometer pressure reading is not a good indicator of whether you have leaks or not (which is why it is not the procedure recommended by Van's).
A perfectly sealed leak free tank connected to a manometer is basically a barometer. It's reading will follow barometric pressure. Add just a few degrees change in environment temp and the reading can change even more.

Trying to get a perfect seal around the fuel cap is really a wast of time. All you need to confirm a well sealed tank as a slight amount of internal pressure and no evidence of soap bubbles at any rivets or rivets joints.
That is a lot more precise than trying to get a stable manometer reading.

Example.... what if during the period of time you were monitoring you gauge, the pressure was reducing because of a very slow leak and at the same time the barometric pressure was reducing? Your gauge would stay on the exact same reading (if the rates of change were the same).

[Cliff Goldstein]

I tried the soapy water and still could not identify leak. Used manometer but kept leaking down.
Finally took tank and put in hot tub . Goofy idea but found two leaks almost instantly . One in one of the fittings and one in an end baffle which I fixed through the fuel sender unit hole. But hot tub found the leaks in seconds!
I cold not think of a more effective way. Good luck