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vented gas caps?

twas_brillig

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I believe vehicles pre-emissions vehicles had vented gas caps. We've hardly driven our BE at all this year and I just took it to fill up with non-ethanol premium and add fuel restorer. When I undid the gas cap, I got a real sound of pressure change. The cap is quite a new one, complete with ugly red safety warning label.
QUestios: (1) should the cap be vented (2) what do I look for to ensure that it is vented? Thanks, DOug
 

SD Bugeye

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So a bugeye tank has only two paths from the tank
fuel line to the pump and the cap.
i think you have to have a vented cap
I think that the pump would struggle to deliver fuel from a unvented tank
you need to move air into the displaced fuel tank
i bought a used snow blower at one time that had an issue with shutting off after 5 minutes of use .
let it sit and would start rite back up
took all winter to come to the conclusion that that gas cap vent was clogged with 10 year old dust.
 

gfholl

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I agree with SD Bugeye that the cap should be vented, my aftermarket cap is. The vent would be one way lets air into the tank but not out. Maybe you just had some expansion pressure it the tank. You opened the cap and air escaped?
 

JPSmit

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I put an Aston flip gas cap on Ms Triss - much venting as not a perfect seal - but looks awesome!
 

Joe Schlosser

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The statement is not should but must be vented. The fuel pump, mechanical or electric, cannot pump against a sealed tank. After a couple of pumps the flow would just stop as the vacuum in the tank would be greater than the pressure head of the pump
 
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twas_brillig

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Tom: had a look at the non-vented cap this evening - appears to have the outer 'decorative' cap, with an inner flat steel surface next, then the rubber seal, then the locking steel cup/flat surface and a centre plastic section with 'motorad' cast into the plastic. If this sounds like yours, please advise as to where to drill the hole. Thanks, Doug (PS: had only done a few km on a fillup, and lots of air flow into the tank when I pulled the cap.)
 
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twas_brillig

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been fooling with gas caps again, and it appears that the blast of air that was troubling me is actually air escaping from the tank. The plastic centre piece appears to contain some sort of flapper valve that opens when you suck on it - i.e. it lets air into the tank as the fuel is consumed. The flapper/whatever stays closed if you try to blow outwards, and nothing happens. So, no problems. Trivia: the gas caps I've got have those red circle warning things on their centres. A decently sharp jacknife does a good job of peeling them off. Doug
 
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twas_brillig

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THe BE was parked with its tush pointed towards the sun and a bit less than half a tank of gas in it. The low last night was about 9C and the high today in the low 20s, with lots of sunshine onto the car, and a good chance of air moving into the tank last night as everything cooled. Undid the cap this afternoon, and the 'whoosh' of air pretty much blew it into my hand. Sure seems the 'suck' portion of the vented cap is working as designed, as is the seal to keep fluids etc. from escaping. Doug
 
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