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Fuel cap leak

rleffler

rleffler

Well Known Member
In testing my first fuel tank, it appears that I have a leak. The only leak I can find in the fuel cap. Air bubbles were coming up from under the tab. I moved the tab to different positions to ensure that the leak moved with the tab and to rule out the leak was coming between the flange and skin. The leak did move. So I decided to increase the tension by ?? turn. That helped eliminating the leak around the seal, but it does appear that I still have a leak coming from the center post. It also appears to be the same with both fuel caps.

I?ll give Van?s a call in the morning, but I was curious as if anyone else has run into anything similar and have some advice to share.

Thanks,

Bob
 
Had the exact same problem on both tanks. If I recall I undid the nut and disassembled the cap and applied a little hard grease on the small o-ring around the center pin,solved the problem. I had tried with plastic and all other sort of temporary cures with no luck. I even tried a balloon, didn't get much presure and the ballon burst. I was being careful not to get a quick decompression.
Ron
 
latex glove

I can't remember whose idea it is, but I used it. Put the cap inside a latex glove and then put it in the tank and lock it down, no leaks. There are pictures somewhere on the forums, I just can't remember where I saw it.
 
I had the same problem and also solved it with a generous amount of Fuel Lube.

My haunting fear though is that there is no way to tell when it starts leaking again and I am parked outside in the moisture. I would really like to find a perminant solution that I can feel works 100% of the time. However other things have taken priority of my time.

Any great solutions out there.

Gary Specketer
40274 Flying (used to be)
 
My Cessna 120 has vented fuel caps, Cessna drilled two holes in the top of each cap. You can actually see light through them. Even with a "leak" this massive a normal rain shower or washing doesn't seem to get water is the tanks. I would think the tiny seep we are talking about would not matter.
 
I used a combination of "generous amounts of fuel lube" and a latex glove to temporary solve the fuel cap leak.

I did find a secondary leak in one tank. I think I can add a little more sealant and fix that problem. I believe I can get to the inside and outside of the joint without much difficulty. The other tank has a small leak that I've yet to discover the location.

Frustrating!!!!!

Hopefully my fuselage kit will arrive on time at 9:30am tomorrow and that will take my mind off the tanks for a few days while I inventory the new parts.
 
Rain does get in there...

As I found out. Even though I had a good seal on my caps, I found that I always got a little water out of the tank drains after a rain. I now am hangared, so I didn't follow up with more investigation. Keep an eye on this if you are tied out. One thing I did was drill the cap roll pin to 1/8" and replaced the roll pins, and tightened them real good. Now they are a bit of a pain to remove, so some additional adjustment may be in order. I am building another 9 now, and may indeed investigate optional cap and flange installations. Have any of our friends out there used fuel lube as a cap O ring lubricant? I was reluctant to use it, as it is tough to get off of your hands, and didn't want it anywhere near anything that I handled often. As for tank testing, I slapped a piece of duct tape over the whole cap and flange when I tested them. Worked great!!

HTH,
Chris
 
Fuel Cap Help

Yeah, I know this sounds simple right? That's what I thought at first. It started one day after flying to an aux field to get cheaper AVGAS. After takeoff I noticed a little fuel streaming from the right fuel cap. After I burned off a little fuel it stopped. In the hangar later I took the fuel cap off, loosened the stop nut on the bottom and tightened the locking washer under the gasket and retightened the stopnut. Topped off the tank and started rocking the wings a bit to check the cap. It started seeping fuel around the sides of the cap and no adjustment could fix it.
After receiving new gaskets (top & bottom) from ACS, I replaced everything using fuelube in the process and reattached the cap. Still no luck after replacing the gaskets. The left tank doesn't seep a bit so I swapped caps and still no change. Both lips of the tanks appear identical, no deformation, no cuts, dents, etc.
Any ideas? Do I need replace the whole cap or am I missing something here? Any different size (thickness) gasket I could use? Is there some kind of fuel safe gasket maker I could lightly line around the rim of the fuel tank opening? Can't figure out why either cap will seal the left tank but not the right"

Thanks,
Oly
 
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Did you replace gaskets or "O" rings?

The standard RV fuel cap has a large O ring and a small O ring, what type fuel cap do you have?
 
fuel cap

Thanks guys, when I said "gaskets" what I should have said was "O rings". I got the same brown ones that were on the fuel cap. I did not buy a new square stainless steel washer but did swap it out a couple times with spare ones. How about the white hard plastic "gaskets" that are on each side of the large O ring? Should I try replacing these? Didn't see them as available parts at ACS or the Vans website.

How would my filler neck get out of round? I haven't done anything unusual with the airplane. It doesn't appear out of round but I suppose I could measure and compare it to the other tank. If it was out of round how would I fix this? I have never heard of an RV fuel filler neck getting out of round so I am at a loss how to proceed. The Spring flying season is quickly approaching!
 
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Is it possible to wrongly install the newer brown O-rings on an older style fuel cap?

Could this be the source of the problem?

How can an old style cap be recognized by other than the O-ring color?
 
Fuel Cap

Dave- Thanks, when the snow gets manageable I will take you up on your offer.

Gill - You may be onto something. When this all started was after I got the engraved caps back from UP North Aviation. This was NOT their fault. I had a box of fuel cap parts so I put one together with their engraved lid and the innards with spare parts and new O Rings. Maybe I mismatched an old cap with new parts.

Still doesn't explain why both caps seal the left tank just fine but nothing I do works on the right tank.
 
Oly, I had a similar problem. On one flight, I saw fuel mist leaving the right fuel cap. As the fuel burned down from that full tank, the mist stopped. I checked the filler cap on landing, but could not determine anything wrong or different from the other fuel cap.

On the very next fill up, as I tried to remove the "problem" fuel cap, the fuel cap pivot pin broke. I replaced the broken roll pin with a stainless steel pin, replaced both large and small o-rings, fuel lubed the parts and all has been well since. No more fuel mist evident.
 
olyolson said:
How about the white hard plastic "gaskets" that are on each side of the large O ring? Should I try replacing these? Didn't see them as available parts at ACS or the Vans website.

How would my filler neck get out of round? I haven't done anything unusual with the airplane. It doesn't appear out of round but I suppose I could measure and compare it to the other tank. If it was out of round how would I fix this? I have never heard of an RV fuel filler neck getting out of round so I am at a loss how to proceed. The Spring flying season is quickly approaching!
Click to expand...

az_gila said:
Is it possible to wrongly install the newer brown O-rings on an older style fuel cap?

Could this be the source of the problem?

How can an old style cap be recognized by other than the O-ring color?
Click to expand...

I am not sure if the newer brown o-ring can be used on an older style cap but it is evident this is a new style cap. They have plastic (delrin?) inserts that makes for a slipperier ramp surface (as described in the OP's post above). The other way to tell is if you have to rotate the bottom plate of the cap to make clamping adjustements. The newer caps have a threaded plate and use teh nut as a jamb nut to lock the position. The older caps adjusted by just turning the nut.

The flange could get bent out of round if someone got rough with a fuel nozzle leaned over hard in the fill opening.

See if you can find someone with a set of bore gauges and measure whether the filler flange is out of round.

Are you sure the leakage is around the edge and not the center stem? There is a different problem that can allow leakage at that location.
 
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I have an RV-8 QB, first flight 11/13/11 and just started noticing the same symptom. If the right tank is full I will notice leakage from the center of the fuel cap if I am running off the left tank. Switching to the right tank will eliminate the seepage.

I have not yet been able to find the root cause.
 
Fuel cap update

Thanks for all the inputs by everyone,

As stated earlier, I was starting to get a bit of fuel seep out of the right fuel cap. I rebuilt the fuel cap with new O rings and adjusted it for a snug fit. Immediately after putting the cap on the tank I was still getting a bit of seep after rocking the wings back & forth in the hangar. Decided to "noodle on it" as my favorite mechanic frequently says (Tom Ward). I had other family stuff to do so I let it sit for 2 days before I was able to get back to the hangar.

I was able to go back and try some more ideas. So I rock the wings back & forth and guess what - NO SEEPING! ??? So I take the cap on & off a bunch of times rocking the wings each time and could not get any fuel to seep at all. Not that I wanted to see any seeping mind you, just wanted to make sure it was sealed.

So how did the cap "suddenly" start working. Do the O rings expand slightly over time when exposed to fuel/vapors? I'm not complaining just curious as heck.

Oly
 
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cap maintenance

On my -4, I find its good practice to lube the O ring with a few drops of oil, or LPS every couple tanks full. It will keep everything well seated, and even know you may have new O rings, they won't seat if all the moving parts are dry. Also, the plastic fulcrum can crack, and the lock loses its effect. I made my own from thin Teflon sheet to replace the original black plastic ones.
 
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