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    Thread: Troubleshooting fuel tank ventilation

    1. Member
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      07-03-2012 08:55 AM #1
      Okay, let's refer to this diagram:




      For quite some time, I've had the infamous "gas flowing out from the filler gap and down my quarter panel" issue.

      When I remove the fuel tank expansion hose (#7 in the diagram) at the filler tube and blow into it, I can feel air moving freely out of its nipple on the filler tube. No blockage there.

      I disconnected my gravity valve and tested it, and it's working properly. I squirted some WD40 into both ends and gave it a shake anyway, for good measure.

      I can blow air into hose #26, which goes from the tank to the lower input of the gravity valve. Air moves.

      I can blow air into hose #25, which goes to the expansion tank in the passenger fender. It's not what I'd call "easy," but it's a long skinny hose and on the other end of the expansion tank is more long skinny hoses (to the charcoal canister and the rest of the emissions evap stuff) so I figure there's some inherent resistance because of that. Bottom line, I can blow air but there's some resistance.

      With everything hooked up, I can blow air into the other side of the expansion tank via the charcoal canister hose. About the same level of resistance as when I blow from the other end of the expansion tank.

      And just as a control test, I inverted the gravity valve and then tried to blow back to it through that charcoal canister hose and it was totally blocked...as expected.

      What am I missing? Does this all sound pretty normal?

      I'm thinking that to further troubleshoot this, maybe I should just disconnect the gravity valve altogether for now. From what I can tell, that should at least keep gasoline from backing out of my gas cap.

      Oh yeah, the gas cap is the chrome kind with the lock.

      What's a guy to do?

      Last edited by 84_rabbitGTI; 07-03-2012 at 08:58 AM.

    2. Junior Member RabidRabbot's Avatar
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      07-03-2012 01:44 PM #2
      is the hinged flap (1A) busted allowing gas to flow up the filler tube?

    3. Member
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      07-03-2012 01:59 PM #3
      That's a good question...I don't know. Is that possible? I would think that if the flap was easy going enough to open while filling up the tank, it would be not air-tight enough to resist high pressure build up.

      I'm intrigued...but wondering about the likelihood of this before pulling that apart.

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      07-03-2012 03:41 PM #4
      Were are you getting your parts i have the same problem. Thanks

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      07-03-2012 04:32 PM #5
      check and see if the hose (number7 on diagram) is still connected to the filler tube, mine was off and had the worst gas smell for awhile. it was much more prominent when i would fill up the tank all the way to full.

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      07-03-2012 04:36 PM #6
      Yes, #7 is connected. As I mentioned, I disconnected it so I could blow into it and test for blockage.

    7. 07-03-2012 09:14 PM #7
      What is the physical condition (missing, cracked, broken) of part #9?

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      07-03-2012 09:30 PM #8
      It's split, so it's not actually a ring, but it holds the valve in place...on a slight angle, not totally level.

      Which made me curious to test its threshold, so I steadily blew in it while tilting, tilting, tilting, until it blocked. It was tilted way further than how it sits in the wheel well before I felt any change. Not that I'm doubting you, I'm just putting that out there.


    9. 07-03-2012 09:43 PM #9
      Just checking that you were aware the roll-over valve could have been a possible problem if not secured and tilted too much.

      Also, I assume you are not adding fuel past the point that the gas station nozzle stops the filling when it is in its fully-inserted position.

      I would also suggest blowing some compressed air through the roll-over gravity valve, to make sure no debris is in there.
      Last edited by chickenfriend; 07-03-2012 at 09:51 PM.

    10. Member
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      07-03-2012 10:28 PM #10
      About fuel level: fuel will back out of the filler tube and gas cap with as little as a 1/4 tank of gas. It's a bad situation. I even left the cap OFF the other day with very little gas in the tank and observed gasoline pooling in the filler tube up to where the gas cap would be.

      Tomorrow I will try compressed air through all the hoses and the gravity valve (though today I blew in it, sprayed WD40 in it, and shook it real good and tested again...all seems well with it).

      I'm thinking about leaving the gravity valve off and letting the car sit, and see if it still happens. If there is no problem, maybe I'll add one hose at a time until the problem returns...

      Thanks for lending some thought to it man

    11. 07-03-2012 11:30 PM #11
      I would think there has to be a blockage somewhere.

      With a tank only 1/4 full, there is obviously plenty of room for the gas to expand in the tank.

      You might have to hook a pressure gauge up to the tank vent line to the gravity valve to figure out what is happening. The natural vapor pressure is going to be around 10-15 psi, I think, so you would be looking for numbers higher than that.

      I agree it is quite puzzling.
      Last edited by chickenfriend; 07-03-2012 at 11:38 PM.

    12. Geriatric Member JUS_GT_EYEZ's Avatar
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      07-04-2012 10:29 AM #12
      Quote Originally Posted by chickenfriend View Post
      I would think there has to be a blockage somewhere. .
      It seems like you eliminated 7 and 26 which would only IMO leave you 3,1a,11 and the tank itself as the issue. with as little as 1/4 tank there is no way it would leak there without a blockage close by. have you peaked inside the tank? the access panel on top of the tank should let you see if there is anythign obviously in the tank itself. I can not imagine what it might be tho.

      I belive there is a o-ring seal on there, make sure you have a replacement before you try opening it tho.

      maybe i missed it but have you checked 11?
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    13. 07-04-2012 02:19 PM #13
      I have two mk1 Rabbits and both have this issue. One time I opened the filler cap and fuel actually sprayed out. It's like pressure is building up in the tank and pushing fuel up the filler neck. I just make an effort to not fill the tank right to the top. I know it's not a fix but you won't have the fuel running down the side of your car.

    14. Junior Member RabidRabbot's Avatar
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      07-04-2012 09:06 PM #14
      Wait a minute! Its got to be that chrome locking cap. it probably does not vent the way the stock gas cap vents. if it does have a venting mechanism maybe it is clogged? check the cap bro.

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      07-05-2012 08:30 AM #15
      Quote Originally Posted by JUS_GT_EYEZ View Post
      maybe i missed it but have you checked 11?
      I haven't checked 11 directly (like by taking it off and looking at it), but I have passed air through it...when I disconnected #7 from the filler tube and blew in it, I felt plenty of air escaping out of its nipple on the filler tube. So I don't think #11 is blocked.

      Quote Originally Posted by RabidRabbot
      Its got to be that chrome locking cap. it probably does not vent the way the stock gas cap vents. if it does have a venting mechanism maybe it is clogged? check the cap bro
      What does the stock cap look like? This is the same one I have.


      Yesterday I gassed up half a tank and then disconnected #7, both hoses to the gravity valve, and the line under the hood that runs from the expansion tank to the charcoal canister. I left the gas cap on. I'm going to wait and see if the problem returns. If not, I'll hook #7 back up. If all is well, I'll hook up #26, then the gravity valve, then #25, and finally the charcoal canister. At some point in that sequence, the problem will return...it may take a week, but I think I can pinpoint it that way.
      Last edited by 84_rabbitGTI; 07-05-2012 at 08:41 AM.

    16. 07-05-2012 10:04 AM #16
      Good luck finding the problem.

      Quote Originally Posted by RabidRabbot View Post
      Wait a minute! Its got to be that chrome locking cap. it probably does not vent the way the stock gas cap vents. if it does have a venting mechanism maybe it is clogged? check the cap bro.
      The stock cap does not vent to my knowledge.
      Last edited by chickenfriend; 07-05-2012 at 10:23 AM.

    17. Junior Member RabidRabbot's Avatar
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      07-05-2012 10:17 AM #17
      Yup you are correct, chickenfriend. I took a better look at my stock cap, no vent.

    18. Member
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      07-07-2012 03:58 PM #18
      I just tried some compressed air in hose #25 to the expansion tank. When I pump it up with air, it shoots air back out at me when I take the nozzle away, like it's storing some of it up rather than passing it on to the charcoal canister (canister is currently detached so I can rule that out). I was putting about 60 PSI into the hose and when I stop, the hose "exhales." I should add that it behaves the same way at 10 or 20 PSI as well. Blockage?

      Otherwise, my other experiment is coming along. The ventilation tube (#7) is hooked up, and so is the hose from the tank to the gravity valve, and the gravity valve is hooked up. Sitting in 100 degree sun, 1/2 tank of gas, and no overflow. I'm thinking it's the hose to the expansion tank.

      So, back to my first paragraph here, how much resistance should I feel in that hose to the expansion tank?
      Last edited by 84_rabbitGTI; 07-07-2012 at 04:20 PM.

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      07-07-2012 10:54 PM #19
      Not to hijack the thread or anything; just a quick question that's tank ventilation related..

      When I fill up my tank gas squirts out a bit from under the car right around the fuel pump area. Is there an opening in the tank that causes this? A friend of mine says they all do that, but that doesn't seem right to me. >.>

    20. Geriatric Member JUS_GT_EYEZ's Avatar
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      07-20-2012 09:23 AM #20
      Quote Originally Posted by Schpamb View Post
      Not to hijack the thread or anything; just a quick question that's tank ventilation related..

      When I fill up my tank gas squirts out a bit from under the car right around the fuel pump area. Is there an opening in the tank that causes this? A friend of mine says they all do that, but that doesn't seem right to me. >.>
      It should not do that. Probably a loose vent hose at the filler neck if you only see it at fill up.

      OP no idea on resistance.. but free bump.
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    21. Member RudyThree's Avatar
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      12-10-2012 08:49 AM #21
      Depending on how much air you are trying to blow through the vent tube will vary the back pressure. So, a light level of flow should provide little or no resistance. If you have resistance there, sounds like blockage or maybe the line got crushed somewhere.

      Excellent troubleshooting approach in this thread. Systematic. One question I have on the system is why the expansion tank is needed? Best I can tell is that the charcoal filter is always open to the atmosphere (no valve). So what is the purpose of the expansion tank? I'm thinking of deleting mine.

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