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Fuel Tank Caps Leaking

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iamtheari

Well Known Member
I have done the leak testing on one of my fuel tanks and I am satisfied that no rivets, seams, or fittings leak. However, my balloon doesn't stay inflated all day. It held well for a couple hours last night but in the morning it was in sad shape. I re-inflated the balloon and went to work, then came home tonight to a deflated balloon again.

Soapy water sprayed on the tank shows only one spot where any bubbles form. The edge of the fuel cap. It is a very slow leak.

I feel pretty confident that the tank is fine. But I wanted to check here for some reassurance. How long should that balloon stay inflated when I test the tanks? Is a very slow air leak through the fuel cap any cause for concern?
 
My balloon stayed inflated until I removed it after a couple of days. Barometric pressure will affect it some. I'd put packing tape over the fuel cap to see if that keeps the balloon inflated. A fuel cap leak should be easy to fix
 
Like you, I had a cap leak, and even packing tape over the cap was unsuccessful in stopping it completely. The caps are relatively easy to adjust and correct the leak - and they will seal very well.

I also was very unhappy (unsuccessful) using balloons. I instead set up a manometer-like set up, with a long loop of clear rubber tubing with water. I believe about 30? of water accomplished the needed pressure. It was amazing how well the fuel tank worked as a barometer with the pressure ?level? moving down (oh,oh, I have a leak) and then up as local pressure changed.
 
The balloon is only there to establish positive pressure while you check for leaks using the soapy water method. It is not expected to stay inflated for any particular length of time. Many exotic methods have been devised, but are mostly reinventing the wheel. Use the search feature to unearth some lively discussions on this topic.
Cheers,
DaveH
 
daveyator said:
The balloon is only there to establish positive pressure while you check for leaks using the soapy water method. It is not expected to stay inflated for any particular length of time. Many exotic methods have been devised, but are mostly reinventing the wheel. Use the search feature to unearth some lively discussions on this topic.
Cheers,
DaveH
Click to expand...

Correct

The plans recommendation to use the balloon is just to establish a safe testing pressure and avoid accidentally damaging the tank (it somewhat acts as a simple pressure regulator). Deciding a tank is leak free because the balloon appeared to stay inflated has fooled more than one RV builder.
 
I also saw a very small leak only at the fuel cap when balloon testing my first tank with soap solution. I'm not too worried about that and consider my tank leak test successful.
 
Thanks for the replies. I tried packing tape, to no avail. So I grabbed the little container of Seal Lube that I had from fixing the primer in the Cub a couple years ago, adjusted and lubed the gas cap, and tried again. The balloon had no measurable shrinkage after 12 hours. (I had a few marks that had at various times been 1" apart. I picked a row of 3 marks and measured them at 1-1/8" apart, marked them accordingly, and measured again after 1, 2, and 12 hours.)

Gas tank #1, complete, with a little help from my friends. Have a great week out there in RV land.
 
I sealed the fuel cap with Vaseline then taped the edge shut with Gorilla tape. The rubber o-ring is there to keep the fuel from sloshing away and not to seal it airtight. There was a tiny weep leak from the fuel cap using this method but it kept the ballon inflated for a long time. Also, the tiny leak helped me to determine what the air leak looked like with soapy water. If you see similar bubble trails at the rivet heads then the leak will look the same.
 
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