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Testing gas tank but cap leaks

ChrisMallory

Active Member
All,

Looking for some help on how to pressure test my tank. The locking gas cap leaks, so I can't get the tank to hold pressure. It's not my proseal job around the filler, it is the rubber seal on the cap and the key hole.

I can't find any other leaks with soap and bubbles, but that's as far as I can go.

Any suggestions on how to temporarily seal it up so I can pressure test? Duct tape didn't work.

Thanks
 
All,

Looking for some help on how to pressure test my tank. The locking gas cap leaks, so I can't get the tank to hold pressure. It's not my proseal job around the filler, it is the rubber seal on the cap and the key hole.

I can't find any other leaks with soap and bubbles, but that's as far as I can go.

Any suggestions on how to temporarily seal it up so I can pressure test? Duct tape didn't work.

Thanks

Fill it with gas and let it sit for a day or so?
 
Clear Packing Tape Will Seal The Cap

Chris,

Put a couple of strips of clear packing tape over the cap. I extended about an inch or two beyond the cap.
 
You have tightened up the nut on the underside of the cap to make it seal appropriately, haven’t you?!
 
Just me

I had to use fuel safe grease on the gas caps to keep them from leaking during my (air) pressure test of the tanks. YMMV
 
Dont have to fill it all the way

Like previous poster mentioned put some gas in it, i put just enough to completely cover the baffle seams when the tank was sitting upright. Maybe a gallon. Then you can stand it on end to check those seams too. Fuel will leak out of the root rib pickup so plug that. Lay it down too. Any leaks will be readily visible. Good Luck.
 
but isn't filling it with gas just a static test at "nominal" static head loading?

From a practical sense that makes sense that it verifies against leaks when the thing is sitting in the hangar.... but I would have thought the idea is to test it at some higher pressure so that you verify that the seal is not right on that ragged edge of failure.... testing your factor of safety.

For one reason.... because if the seal is on that ragged edge, what happens as the wing flexes when in motion?

also, dynamic pressure increases from sloshing around in the tank, etc. wouldn't be increasing the pressure much over the static head pressure...but seems like it would a bit.
 
All,

Looking for some help on how to pressure test my tank. The locking gas cap leaks, so I can't get the tank to hold pressure. It's not my proseal job around the filler, it is the rubber seal on the cap and the key hole.

I can't find any other leaks with soap and bubbles, but that's as far as I can go.

Any suggestions on how to temporarily seal it up so I can pressure test? Duct tape didn't work.

Thanks

Check the seal for debris or nicks, then consider a fuel safe lubricant as mentioned above. Also, as someone else mentioned, you can adjust the seating pressure by tightening the nut on the underside of the cap.
 
but isn't filling it with gas just a static test at "nominal" static head loading?

From a practical sense that makes sense that it verifies against leaks when the thing is sitting in the hangar.... but I would have thought the idea is to test it at some higher pressure so that you verify that the seal is not right on that ragged edge of failure.... testing your factor of safety.

For one reason.... because if the seal is on that ragged edge, what happens as the wing flexes when in motion?

also, dynamic pressure increases from sloshing around in the tank, etc. wouldn't be increasing the pressure much over the static head pressure...but seems like it would a bit.


I did the calculations for this back wehn I tested my tanks. I can't remember the actual numbers, but I was able to calculate an air pressure equivalent to a full tank at 3.8g load and went maybe 10% over that.
 
I had the same issue, tried tape and tightening the nut, but I couldn’t get it to be airtight. So I did the test Vans suggest, soap and water, and left it at that.
 
High quality aluminum HVAC tape over the gas caps works very well. No creases and rub it down good.
 
Chris,

The suggestions for sealing the cap are good ideas. I wondering, are you using something to create a reserve pressure source?
The long time method was to connect a balloon to either the fuel line or vent fitting. This does two things, prevents over pressure and provides a reserve air supply that gives time to find leaks.
 
Done

Thanks for the suggestions. I tried clear tape, it worked ok, but still let a little leak out, so I filler her up with water and waited. Held water with no leaking. The floor is dry and the level hasn't changed. I'm going to call it good and move on.

Cheers,

Chris
 
A caution on filling with gas. If you put in a gallon, and then seal up all the outlets, you can get surprised by quite a lot of excess gas vapor which will find the weakest part of your tank. Remember, the tanks will never really be sealed once installed. There will always be a vent line open - hopefully. A latex balloon's worth of pressure is enough to check for existing leaks. More pressure may create them. It's a fine line.
 
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