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Thread: Vacuum in fuel tank.

  1. #1
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    Vacuum in fuel tank.

    I have noticed for a while now a strong vaccum in the fuel tank when I take the fuel cap off.
    You can hear the pressure change as the fuel cap is being undone.
    Is this normal, or is there a problem with my fuel cap causing a fuel tank venting blockage?
    Any thoughts appreciated!
    Cheers, Pete.

  2. #2
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    Are you quite certain its vacuum, not pressure escaping? Warm weather means there will be a modest vapour pressure build up, that's what the cap is for. If in doubt, get a new cap. It is a universal style that should be available anywhere, to suit all major makes with screw in petrol caps.

  3. #3
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    Mine is the same, be like it forever, it's a 2001 Bosch

  4. #4
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    The venting of the fuel tank is via a solenoid valve into the inlet manifold under certain conditions eg overrun at over 1500RPM.
    If you are driving on a hot day on flat ground and stop for fuel you can get quite a pressure build up if you stop for fuel.
    Don't worry be happy as it is completely normal.

    Regards Philip A

  5. #5
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    Definitely a vacuum, there is no fuel smell when I open the cap. I would imagine if it was a pressure build up, there would be fuel smell as the pressure was released from the tank.
    I can not feel any escaping pressure with my hand either.
    Mine is a 2001 Thor as well, interesting yours does it as well Hammer H, might be a quirk of our model.
    I've never had any issues from it, but it did have me wondering if it was normal.
    Thanks for the replies!
    Cheers, Pete.

  6. #6
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    All the (working) caps I've met have a small valve in the centre of the cap facing the tank. Let the fumes dissipate from your cap and suck gently on the valve protrusion. It should take little effort to get the valve to open in the direction of into the tank. If not, try a new cap and compare. Pressure holding is higher and you shouldn't be able to easily blow through the cap valve. Some people block the valve as they can smell petrol vapour as they pass the car, this is not the best thing to do. Caps wear out and should be thrown away if no longer working properly.

  7. #7
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    I too have that faint petrol smell around the filler area as well, though my tank definitely holds pressure

    It seems the Tridon Non Locking Fuel Cap TFNL227 is a direct fit, so I may try one of those and see if it improves things

    Steve

  8. #8
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    An update, I got an aftermarket fuel cap, put it on, went for a drive, opened the cap and guess what?
    It's doing exactly the same thing.
    Oh well, it's not causing any issues, I'll just live with it!
    Cheers, Pete.

  9. #9
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    Mine does the vacuum thing too mate, even on long trips driving from full to empty without stopping. It’s never caused an issue or fuel surge for me, and likewise I’ve not worried about it.

    Cheers
    Keithy

    2002 P38 Range Rover HSE

    Sequential LPG - Redarc Charger - TPMS - Ashcroft Locker
    Wheel Carrier - Bullbar & Spotlights - 285/75/16 BFG KM3’s
    On Board Solar - Stainless Snorkel - 2” Suspension Lift

  10. #10
    Join Date
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    Well i fitted the Tridon cap and the fuel smell is much worse !

    Maybe I got a faulty one, I'll try swapping it next week

    Steve

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