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tow bar
by G Walker - 24/08/26 06:52 PM
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Jun 14th, 2026
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Joined: Nov 2015
Posts: 8,663 Likes: 489
Just barreling along Needs to Get Out More!
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Just barreling along Needs to Get Out More!
Joined: Nov 2015
Posts: 8,663 Likes: 489 |
Well, It's a Stant cap marked "Caution: Rotate cap one-half turn to vent." and "After Hissing Stops Continue Turning to Remove."
Seems like it's supposed to "hiss" or Swoosh. Fuel tanks need to be vented to allow air to replace fuel drained from them and thus prevent tank collapsing not to mention compromising fuel pump suction. Irrespective of what is printed on your present cap which is likely not OE, it's a simple test to go for a short drive with cap removed and some form of plug from a freely breathing rag in its place. BTW I have heard of ethanol bloating seals in caps that otherwise allow some air flow. I've often wondered about this as my fuel cap is none vented, but I suspect the the vaccum that would be created by using fuel must be relieved via the evap cannister. I think that there is a one way valve on the cannister, because if there wasn't, it would defeat the purpose of it think. I get a whoosh when I open my fuel cap, but I'm sure this is the build up of fumes coming out as opposed air being sucked in. I wonder if those with a vented fuel cap suffer from fuel evaporation. Leroy, my previous S1 was just the same, it's an inward whoosh of suction, often worse after a long sustained run, however never caused any running issues I would not replace the cap with a breather one as modern petrol goes off much quicker and loss of the more volatile VOC's via the vent is not good
Jon M
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Joined: Oct 2021
Posts: 58
Just Getting Started
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OP
Just Getting Started
Joined: Oct 2021
Posts: 58 |
"Yes - I'd give it a run with the cap off and a bit of rag stuffed in the aperture if it were me.."
Creates visions of a green molotov cocktail driving down the road.
Seriously tho: I think I'll wait till the new fuel pump and filter get here.
Thanks for all the input.
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Joined: May 2010
Posts: 9,545 Likes: 132
Needs to Get Out More!
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Needs to Get Out More!
Joined: May 2010
Posts: 9,545 Likes: 132 |
You need to check the vents are ok. I’d do that before changing the pump. If the vents are the problem then a new pump is a waste. The inward whoosh is the big clue. The tank is effectively sealed with no mechanism to allow air in to fill the void as petrol is removed.
As an analogy, get a plastic pop bottle. It feels solid when full. Release the top. Whoosh! The CO2 is released. It has stayed in the pop due to the pressure in the clear space below the cap reaching equilibrium and no more is released from the liquid. Now, when the pop has all gone place your lips round the open top and suck. The bottle will collapse. Punch a hole in the cap and the bottle never feels solid as the CO2 just vents out.
This is roughly what happens in a fuel tank UNLESS there is a vent/valve to manage the pressure changes.
Last edited by sospan; 29/08/22 12:50 PM.
Plus Four MY23 Furka Rouge
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Joined: Dec 2017
Posts: 2,221 Likes: 181
Talk Morgan Expert
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Talk Morgan Expert
Joined: Dec 2017
Posts: 2,221 Likes: 181 |
"Yes - I'd give it a run with the cap off and a bit of rag stuffed in the aperture if it were me.."
Creates visions of a green molotov cocktail driving down the road.
Seriously tho: I think I'll wait till the new fuel pump and filter get here.
Thanks for all the input.
If you don't want the molotov cocktail vision another way is to screw the gas cap in till it clicks and the cap is secure then back off the cap till it is about to start to unscrew, this allows the cap to breath.
Bruce
1964 4/4 Series V Comp (Megan)
1994 +8 (Maurice) tis no more
2013 M3W (Olga)
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