gas tank vacuum

Discussion in 'Parallel Universe' started by loph917, Nov 7, 2010.

  1. loph917

    loph917 Beard Bros Racing

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    i'm wondering if i have a blocked vent tube somewhere...

    when i fill up with gas while i'm opening the gas tank i can hear what seems like a very good vacuum being released...i don't seem to remember this behavior before.

    my question is; am i suffering from a blocked vent tube or other form of blocked venting somewhere? can other f8 owners (us) chime in and let me know if i'm going crazy (ok, we already know that answer)?

    thanks.
    #1
  2. Mr B

    Mr B Been here awhile

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    If you are hearing a vacuum sucking sound when you open be warned you are not venting the fuel tank. Here in California we have these nifty little charcole filters, I dont understand them completely but believe they are suppose to eliminate teh fuel vapors when you park the bike in the garage.
    Anyway last summer I was riding and it was extemely hot, we were climbing up out of the Kings River Canyon, about 3000 feet. As I was climbing up the bike stutterd and stopped running. I stopped and restarted no problem and then about 1/4 mile later, same thing.
    I opened the fuel tank to ensure that there was fuel still on board, I was not out of gas but did hear that sucking sound. This repeated itself once more at which I opened the fuel filler cap, wedged some paper under and taped the cap down. this let the tank vent I we climbed and no more stalling.
    The next morning, we camped that night up fairly high, +/- 7000 feet, I closed the fuel tank and went home, I did not want to break down on a trail miles from home, so I slabbed it home, no problem. When i got home I inspected the fuel vent line that travels aft of the fuel tank and ties into the charcol canister. I did not see anything but I did relocat the vent line in an attempt to ensure that it would not be blocked when I carry a load on the back of the bike.
    I guess what I am saying is be careful if you pinch that vent line you will can create a vacuum in the fuel tank, and eventually the fuel pump will not be able to overcome this vacuum.
    Hope this helps, sorry so wordy...
    :D
    #2
  3. johnparjr

    johnparjr Been here awhile

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    Mine has a little suction when I open it and I have never had a problem. There is an older post on this somewhere and most people had some suction If I remember correctly.
    #3
  4. JRWooden

    JRWooden Long timer

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    Not sure which bike you have, but the charcoal canister on the F880GS/F650GS twins can cause this issue.
    There is a recall active on it.
    If you have one of the twins you may have a plugged canister.

    If you have a different bike it still could be same/similar problem

    Search for "charcoal canister" there was a lot of chatter on this about 6 months ago but it has died down now since the recall and etc...
    #4
  5. ebrabaek

    ebrabaek Long timer

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    As stated above..... there is a recall on the Carbon canister. That said..... If you want to investigate it your self...... Take off the yellow plastic on the top of the tank..... There will be a little 10mm or so od hose running from the top of the tanks backwards then under the seat, and down to the CC. Right before said hose runs under the seat, you will find a little tubular shaped valve. Make sure the arrow on it points towards the tank....NOT away from it. I have heard of some claiming theirs was installed incorrect. When you remove the hose from the tank.... try to apply suction to the tank end.... It should flow freely... If not... Said hose is the key to you problem. If the CC is plugged.....or the valve is not working the vacuum is not alleviated in the tank.
    :thumb:thumb

    Erling
    #5
  6. Dansrc51

    Dansrc51 I need a cape....

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    or just remove the charcol filter and eliminate that and a few other problems all together... :wink:
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  7. EnderTheX

    EnderTheX Dirt Rider

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    :clap That's what I did. Screw the environment, at least I'm not driving a giant truck like everyone else in Texas...
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  8. Dansrc51

    Dansrc51 I need a cape....

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    I'd be willing to bet the difference in emmissions is minimal, by comparison. I mean between with canister and w/o.
    #8
  9. ebrabaek

    ebrabaek Long timer

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    Yes .....as I did as well...... But you still need to check the valve...thus removing the plastic..... some have been reported to be installed the wrong way......:thumb:thumb

    Erling
    #9
  10. JRWooden

    JRWooden Long timer

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    Erling:
    I should allow air to move freely INto the tank and also allow air to come out of the tank after it reaches some modest pressure of ... several PSI?

    Is that correct?

    Thanks!
    #10
  11. johnparjr

    johnparjr Been here awhile

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    So what is the vacuum effect doing to my gas tank?
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  12. EnderTheX

    EnderTheX Dirt Rider

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    It's robbing you of your mojo man! Dude... :stoned







    :zilla
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  13. ebrabaek

    ebrabaek Long timer

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    Yeppers JR.... your correct. I cant speak of the correct pressure..... But when I applied 5psi against the arrow (away from the tank flow) my valve began to slowly vent. If you applied any suction on the discharge side of the valve...( away from the tank)...it instantaneously opened up and flow free. That had me concerned..... because when the .....purge valve i think it`s called.....opened up during engine operation....a vacuum vas applied from the manifolds....through the CC....up through the said valve....and it opened without any resistance. I wondered if that was the way it was meant to be......then decided that it was not detrimental, since the motor was not running while parked....and pressure was still being vented....so off went the CC.....no issues since. I am not an advocate of just yanking stuff off the bike.....specially emission stuff..... But when BMW cant figure this one out....even with the y hose installed..... Then it gotta go. I think that next time I`m in there..... I`l just remove the valve itself.....and everything will flow free.....in either direction..... I think there is a tip over valve in the tank before the nipple where the hose is attached.....that prevent any fluid to escape if the bike takes a nap......so the little check valve is not needed..... IMHO... but as usual ...perhaps I`m missing something.......
    Joel should bee here shortly to correct me......:D
    :thumb:thumb

    Erling
    #13
  14. ebrabaek

    ebrabaek Long timer

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    I a perfect world....there should be no suction when opening the tank......Thus no vacuum present...... A little is propperly ok..... But if you have a swoooosh..... there is something going on.......and you are making the Fuel pump work overtime.....and it has enough issues with the ethanol plugging the artery`s...:lol3...... You should have a look.....:thumb:thumb

    Erling
    #14
  15. loph917

    loph917 Beard Bros Racing

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    thanks for all the replies everyone; i haven't had a chance to go out and check things over yet (since posting the original query) but its good to know that this is just another hiccup in the life of an f800gs and seems to be relatively easily solved.

    i am beginning to think that all the quirks the f800gs has exhibited has worked to help us better understand how all the various systems on the machine actually work. if it ran perfect all the time would we be as educated about it?
    #15
  16. JRWooden

    JRWooden Long timer

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    Thanks ...
    When you look at the parts diagram there seems to be more valves there than might be "required' ... a tip-over valve (yah good) and then at least two more valves....

    sigh................

    Work gets in the way again of me learning important stuff.... :cry
    #16
  17. itsatdm

    itsatdm Long timer

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  18. JoelWisman

    JoelWisman Long timer

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    Am I correct that your not experiencing any starting or running issues?

    When I run into this question at my dealership, the first thing I do is to T in a pressure / vacuum gauge, just past the tip over valve on the fuel tank and ride the bike. Nearly always nothing is wrong amd pressure and vacuum remain bellow 1 psi.

    Next both right after a ride, and after the bike has set, both cooling and warming, I hold a thin piece of tensile next to the filler cap. Almost always what the customer perceives as a strong vacuum is infact pressure. I've begun taking bets on this and have earned 40 us dollars so far.

    Up to 4.5 psi of pressure, and 1.5 psi of vacuum below ambient is normal.

    There is a sure sign of the fuel tank holding excessive vacuum, the fuel cap will not spring open and must be pulled on to break the vacuum.

    In the rare instance that vacuum is excessive, the problem has always been a blocked canister vent tube or a stuck roll over valve.

    The same additives that protect fuel injectors and fuel pumps will keep the roll over valve from sticking. Chevron techron concentrate fuel system cleaner, or, BMW fuel system cleaner + with techron.

    The canister drain valve can be prevented from blocking by removing it occasionally and blowing it out.

    How often it needs to be blown out varies, but it will never block unless the bike is run in deep mud or a spider builds a web in it.

    It also helps to route the canister drain line so that it terminates high and near the side stand, protected by the side stand frame. Cutting the end of the vent line at a 45 degree angle also helps.

    I don't know what is happening with your specific bike, but statistically in my personal experience, nothing is usually wrong, the owner just noticed the sound when enviornmental and other variables were right for the first time.

    The gold standard of tests is to have a runnability problem that disappears when the fuel cap is left open.

    This test carries risk that fire proof BMW techs are untrained to deal with :)
    #18
  19. johnparjr

    johnparjr Been here awhile

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    Mine is in the shop as I type this I decided better safe than sorry hopefully I won't be waiting long.
    #19
  20. johnparjr

    johnparjr Been here awhile

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    1 1/2 later I had a pinched hose and a bad canister will have to go back next week as they didn't have the canister in stock I asked if this could have caused any damage to the tank and I got no response.
    #20