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Thread: Fuel Cap Vent

  1. #1
    sierrasplitter's Avatar
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    Default Fuel Cap Vent

    I am finally started working on my right tank fuel issue, mine is fuel starved with 9 gallons in the Tank climbing at 500 fpm my engine quits. So, I will be going through the tank, hoses and filters. Changing the gascolator on the right side and blowing out fuel lines.
    What about a bad gas cap ? How can I check the vent if its working ?

    I have these caps

    https://www.univair.com/piper/parts/...iper-fuel-cap/

    And I plan to clear out and inspect everything up to the fuel selector. I may end up going with the maul selector without re-routing the fuel line. I would like to reroute the fuel line but I know its a big project on a finished plane

    But, what happens if your cap vent is plugged ?

  2. #2
    mmoyle's Avatar
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    Default Re: Fuel Cap Vent

    If your aircraft is a PA20? Should have a placard that says, “Level flight only on right tank”. Your preflight take off check list needs to specify “fuel valve on left tank”.


    Sent from my iPad using Tapatalk Pro

  3. #3
    sierrasplitter's Avatar
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    Default Re: Fuel Cap Vent

    Mine is a PA22 conversion. It does have a placard but it says
    “ No Take Off / Landing on right tank below 1/3 full which would be 6 gallons
    My engine quits when I have 8-10 gallons
    I have two friends with the same plane and theirs do not act that way

  4. #4
    Glen Geller's Avatar
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    Default Re: Fuel Cap Vent

    My guess would be the rear pickup of right tank is restricted in some way, either blocked by debris or the hose in wing root is kinked
    Have you tried simple fuel flow tests?
    For obvious reasons, start with full tanks if possible.
    Tail up on stand in approx cruise attitude.
    Fuel selector off.
    Remove fuel hose from carb inlet and place end into a clean large transparent graduated (gallons) vessel.
    Select Left tank and time how long to fill one gallon, then selector to off.
    Select Right fuel tank and time how long to fill one gallon.
    Are flow rates the same?
    Now put tail down maybe block the mains on jackstands etc to simulate climb attitude and repeat left and right flow tests.

    Actually, doing the "climb attitude" flow test first might prove/disprove a right rear pickup clog.
    If different flow rates from right tank in cruise vs climb, there's probably an issue with the tank (rear pickup clog/kink/etc.)
    If different flow right vs left tank, then more likely a selector valve issue.

    The original selector valve is not difficult to disassemble and rebuild, there are several threads on this site discussing it.
    I rebuilt a very stiff selector on my Tri-Pacer a few years ago, a bit of a learning curve but way easier and cheaper than replacing it.
    It's smooth and reliable now.

    GG
    Glen Geller
    1955 PA22-150 "One For Papa!"

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    sierrasplitter's Avatar
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    Default Re: Fuel Cap Vent

    Im thinking along the same lines Glen. I have the tank empty and will be using a borescope. I plan on removing the Bungs to check the screens at the fuel tank, check hoses, blow out lines with air and install a new gascolator ( Steve's) under the right seat. I dont believe its the fuel selector because all works great until I get to around half tank on the right . If needed I may replace the fuel selector

    Thanks Guys

  6. #6

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    Default Re: Fuel Cap Vent

    Quote Originally Posted by sierrasplitter View Post
    Im thinking along the same lines Glen. I have the tank empty and will be using a borescope. I plan on removing the Bungs to check the screens at the fuel tank, check hoses, blow out lines with air and install a new gascolator ( Steve's) under the right seat. I dont believe its the fuel selector because all works great until I get to around half tank on the right . If needed I may replace the fuel selector

    Thanks Guys
    The right underfloor gascolator from the factory was installed without a fuel screen. When installing mine in that location I recommend removing the screen and clip and retain them for your firewall gascolator if you ever need them.

    Brian.

  7. #7
    Gilbert Pierce's Avatar
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    Default Re: Fuel Cap Vent

    I would start with the rubber lines from your tank outlet’s to the aluminum tubing in your wing roots. If you have a blockage that is the most likely location.
    Those screens in the tank outlets only stop rocks and dead vegetation. They are usually really hard to get out. Hopefully you can see them with your bore scope as the screen protrudes into the tank.

  8. #8

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    Default Re: Fuel Cap Vent

    Quote Originally Posted by sierrasplitter View Post
    fuel starved with 9 gallons in the Tank

    So it does not quit with more than 9 gallons in the tank?


    I dont think thats an obstruction issue. I guess it could be... but I might look at flow issues first.. have you swapped gas caps to see if the problem followed the swap? Never had an issue with a plugged up cap vent, but maybe it happens.

    The original gas caps were Harley Davidson Knucklehead gas caps, you cant buy them new anymore, ebay gets high dollar for them used, you can buy 5 dollar repops https://www.bmikarts.com/Vented-Chro...son_p_910.html
    Wagaero sells repops for $7. https://www.wagaero.com/flange-vented-fuel-cap.html

    Have you tried the left cap on your right tank yet?

    My plane only has a gas line on the front of the right wing gas tank, not one on the front and the back like the left wing tank has.. my engine will also quit in a nose high attitude when on the right tank because the gas is not in the front of the tank unless its more than half full.
    Attached Images Attached Images
    Last edited by PA-16; 11-09-2022 at 08:21 AM.

  9. #9
    Administrator Steve Pierce's Avatar
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    Default Re: Fuel Cap Vent

    I have seen a vented gas cap that was not working suck the tank in. I would start at the outlets of the tanks, the finger screens screwed into the tank and the rubber hoses coming out of those fittings leading into the aluminum line.

  10. #10
    sierrasplitter's Avatar
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    Default Re: Fuel Cap Vent

    Thanks for the advice and excellent product Brian .I will be reading the instructions today and installing Friday

    My plane only has a gas line on the front of the right wing gas tank, not one on the front and the back like the left wing tank has
    Oh man, I hope that's not the case but could be.

    I may have to use a mirror with a light or maybe even a gopro if it will fit down the hole. Tethered of course.

    I will keep everyone posted about what i find

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