Fuel cap leak

[littlecricket]

Just wondering if anybody has had a fuel cap leak? My friend’s did and stained his fabric... Just wondering how prevalent they leak and of any preventative measures in the future. Thanks.


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[DesertFox4]

A couple things can cause that big of a leak. First the gasket may be damaged or aged and no longer seals well.
That is a somewhat common issue after sometime in service. Years ago a solution was discovered that takes care of this issue. Rubber gaskets are used with the stock caps. They lose elasticity over time and don’t seal well. I’ll attach photo of a permanent solution. It is a blue gasket available through Mercedes dealers. The photo has the part number on the package. This gasket is somewhat thicker than the stock gasket and you may need to adjust the tabs on the caps due to the thicker blue gaskets. This blue gasket does not seem to age and lose shape or elasticity or be affected by fuels. These gaskets have solved many cap leaks over the last 18 years or so. I installed a set in my first Kitfox 16 years ago and they are still in service with no leaks. I hope they are still available from Mercedes under part number 140-471-00-79-493
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Secondly you can J.B. Weld shut the underside of the crimped rim of the cap and the four tiny “vent” holes found on the rim of the cap. Not the ram pressure tube hole in the center of the cap that is there to help pressurize the fuel system in flight. Either one or both of these fixs should end the fuel leaking during flight. Likely the first fix will take care of the problem. Hope this helps. I do these modifications to my caps and have never had cap leaks.

[jiott]

I agree; have those gaskets on my Kitfox also.

One other possibility-it could be overflow from the vent tube caused by:

• Parking on uneven surface with full tanks, possibly in hot weather.
• Flying with full tanks and ball not centered or one wing low. A friend of mine was following me one day as I flew carelessly with ball not centered and full tanks, and he observed a steady stream of full coming out of my vent tube. Quite a stain on my wing when I got home.

[Dave S]

Jim & All,

Very much agree that a person can get fuel to exit the cap pitot even though the cap gasket is perfect.

I discovered a couple additional ways to dump fuel out the pitot.

1) Maybe this won't happen with a tailwheel taxiing; however, on a trigear if a person makes a tight turn at low taxi speed with full tanks, it is easy to squirt a bit out the pitot due to the centrifugal force on the fuel in the tank.

2) On a maximum slip with full tanks - ditto. Wondered why I smelled fuel once following the slip - The stain on top of the wing pointed directly to the tail access cover - even a few stains on the TAC evident. The natural air flow back from the tail cone guarantees the odor will be pulled forward.

Oh well....

[jiott]

You're right on Dave; I have had that happen to me several times in a slip.

[littlecricket]

Yeah, it was weird. First leg full tanks no leak. Second leg 2/3 down to a 1/4 by the end, but more yanking and banking it leaked. Pretty bummed, 10 hours on the plane and now have stains on the Oratex wings. Thanks for all the info, I appreciate it.

[Shadowrider]

Sorry about the stains. Anything you can do? Did you wax the wings with Oratex wax? Wondering if that helps at all? What color are your wings?

[efwd]

I do use Decalin.
I have had the fuel flow from the vent in large qty as well and I have not had fuel stain the silver Oratex on the wing nor has it stained the white Polytone paint. I thought for sure the white was going to show but it wiped off with alcohol as well.

[SwoffDaddy]

I do use Decalin.
I have had the fuel flow from the vent in large qty as well and I have not had fuel stain the silver Oratex on the wing nor has it stained the white Polytone paint. I thought for sure the white was going to show but it wiped off with alcohol as well.

could the stain be coming from the decalin/lead exiting the exhaust pipe. That is what I was told mine stain is.

[sonex293]

We're using SuperFlite paint system and E0 UL Automotive 93 AKI Gas has stained our Paint. The SuperFlite folks told us that they haven't seen staining with 100LL but didn't have any experience with E0 UL fuels.

Wish we had some type of check valve that allowed air in and out, but kept the fuel in. We get fuel splashing out even with 3/4 full tanks while taxiing around.

==
Michael