i have a tj magna, and lately when ive been taking off the cap to fill up, there's this weird noise, kinda like a rush of air or something. Anyways, it hasnt happened b4, and i drive other cars around and have never heard it on those. Any1 know what it is?

Mine does it all the time. It's not a problem, it's just the air coming out. The only reason i can think of that it's never done it before is that the cap wasn't on tight.

Just a thought. If it turns out to be something serious.... i never posted this! :oops: 8-[

hey and welcome!

yep, its just hot air rushing out of your tank.

all that happens is that air inside your take gets hot, and builds up pressure. when you take the cap off, your bascally releasing the pressure, allowing you to fill up..

perfectly normal :D ... except if you have a diesel vehicle :oops:

Ohhhhhh YES!!!
I was right about something and i thought it up all by my little self!
\:D/ \:D/ \:D/ :D

O:)


i like your name oinkimapig.

I think it's even more amazing that Booya was right! :P hahahahah j/k I luv ya booya! ;)

I think it's even more amazing that Booya was right! :P hahahahah j/k I luv ya booya! ;)
This is a rare occasion that booya is right, we should treasure moments like these
welcome oink

Not to take glory away from him... but its a pretty simple answer... but I'll admit I didnt know at first either (theres a thread of me asking the same thing somewhere) and I bet thats where he stole the andswer from :lol: :lol: 8)

Not to take glory away from him... but its a pretty simple answer... but I'll admit I didnt know at first either (theres a thread of me asking the same thing somewhere) and I bet thats where he stole the andswer from :lol: :lol: 8)

ahaha.. actually, a question asked by a mate in a science class like 5 years ago! lmao! id be buggered how i rememberd it. .LOL

I luv ya booya! ;)
Don't even think about hitting on him! He's my biatch! :lol:

It is either what booyah said or i have another possible scenario.
You fill the tank with petrol therfore it replaces the space that was filled with air.
As the fuel goes down in the tank there is space for air. If the breath hole on the cap is blocked there will be a vacuum created in the tank.
Therefore when u open the cap the air actually rushes in, not out, to fill the displaced air.
Make sence? :badgrin:

It's ok Magna, between me and him, it's purely physical ;)

It's ok Magna, between me and him, it's purely physical ;)
When he told me I was the only one he cared for I suppose he was lying then aswell, or does he tell you the same thing? :shock:

no no no he cares for you, he doesnt care for me...it's just physical ;)

It is either what booyah said or i have another possible scenario.
You fill the tank with petrol therfore it replaces the space that was filled with air.
As the fuel goes down in the tank there is space for air. If the breath hole on the cap is blocked there will be a vacuum created in the tank.
Therefore when u open the cap the air actually rushes in, not out, to fill the displaced air.
Make sence? :badgrin:it makes sense but i dont think it could be possible. the vacuum it would create would cause way to much strain on the fuel pump and it would eventual seize at it would no longer be able to push any fuel out of the tank due to the pressure, and your car would stop. the only other option would be for your tank to crumple in with the pressure or for your fuel line to break.
id take a stab at the apperance of this problem being due to running the tank almost empty before filling. this means there is alot more air in the tank so the pressure is increased causing a larger woosh when you take of the cap. i thought this happened to everyone! oh well, i must just be the only pauper in here who cant afford petrol! :(

The problem with your scenario, MAGWGN, is why would the air pressure in the tank be higher than the atmospheric pressure? Unless something was heating the air in the tank it would be at the same pressure as the air outside so there'd be no gush of air.

When I bought my VT commodore it was missing the factory fuel cap and had a bendy yellow plastic thing instead. I seem to remember that when I went to fill it up the lid of the yellow cap would be concave. So I think Mitsiman is right.. there is a partial vacuum inside the tank.

I'm sure the vacuum would go away over time, but if you think about it it makes sense.. if you had great holes in the tank so that as the fuel level went down the air could come in to replace its volume you'd have somewhere for the fuel, or its fumes, to get out. So any way into the tank has to be watertight and fuel vapor-tight but not airtight, so air coming in to equalise the vaccum would only leak in slowly.

It makes sense to me anyway 8)

The problem with your scenario, MAGWGN, is why would the air pressure in the tank be higher than the atmospheric pressure? Unless something was heating the air in the tank it would be at the same pressure as the air outside so there'd be no gush of air.

When I bought my VT commodore it was missing the factory fuel cap and had a bendy yellow plastic thing instead. I seem to remember that when I went to fill it up the lid of the yellow cap would be concave. So I think Mitsiman is right.. there is a partial vacuum inside the tank.

I'm sure the vacuum would go away over time, but if you think about it it makes sense.. if you had great holes in the tank so that as the fuel level went down the air could come in to replace its volume you'd have somewhere for the fuel, or its fumes, to get out. So any way into the tank has to be watertight and fuel vapor-tight but not airtight, so air coming in to equalise the vaccum would only leak in slowly.

It makes sense to me anyway 8)

Sounds right bud,

mine doesn't always suck air when i open the cap - but rather only does it after i've been travelling for a fair while (200+ km's) and go to fill up. Where as if i just putter around town (stop/start) or go and fill it up in the morning when i haven't driven much it doesn't suck air.

So in my experiences that theory is right. - i never actually thought about it actually - i just assumed it was pressure blowing out!

Oinkimapig (hehe funny name) think about it - did the air sound come after travelling for a while? and previously when u didn't have the prob was it filling up without driving much??

Axle

The problem with your scenario, MAGWGN, is why would the air pressure in the tank be higher than the atmospheric pressure? Unless something was heating the air in the tank it would be at the same pressure as the air outside so there'd be no gush of air.

When I bought my VT commodore it was missing the factory fuel cap and had a bendy yellow plastic thing instead. I seem to remember that when I went to fill it up the lid of the yellow cap would be concave. So I think Mitsiman is right.. there is a partial vacuum inside the tank.

I'm sure the vacuum would go away over time, but if you think about it it makes sense.. if you had great holes in the tank so that as the fuel level went down the air could come in to replace its volume you'd have somewhere for the fuel, or its fumes, to get out. So any way into the tank has to be watertight and fuel vapor-tight but not airtight, so air coming in to equalise the vaccum would only leak in slowly.

It makes sense to me anyway 8)you really didnt read this thread closely at all did you! we all already discused the cause was from air in the tank heating up causing back pressure and gushing out. i was saying it happens more when your tank is low because there is more air in there! i.e. more air=more heated air=more pressure=bigger gush! oh, and have you never heard of a valve. they let things in but not out. and how exactly can something be petrol vapour tight but not air tight? if there was a vacum so great that when you opened the cap all this air sucked back in, a few things would happen. firstly you probably wouldnt be able to take of the cap in the first place. second your fuel pump would shit itself and seize up due to the huge amount of work it would have to do to continue pumping against the vacuum and finally if it got so great your fuel tank would implode.
again i may be wrong and id be happy to hear anyone elses opinions. just not from someone who is gonna bag out on me before even reading any other of the posts and knowing what they are talking about. i would be very interested to hear madmagna or mitsiman's opinions on this.

did the air sound come after travelling for a while?

Axle

Yeh it did, on a trip from Sydney to the South Coast. I've noticed it more when there's less petrol in the tank. It doesnt happen if i top it up when the tanks half full, which i do a lot. Only if it's, say, one line above empty.
Anyways, i didn't think it was anything bad, i was just curious about it. :)

you really didnt read this thread closely at all did you! we all already discused the cause was from air in the tank heating up causing back pressure and gushing out. i was saying it happens more when your tank is low because there is more air in there! i.e. more air=more heated air=more pressure=bigger gush! oh, and have you never heard of a valve. they let things in but not out. and how exactly can something be petrol vapour tight but not air tight? if there was a vacum so great that when you opened the cap all this air sucked back in, a few things would happen. firstly you probably wouldnt be able to take of the cap in the first place. second your fuel pump would shit itself and seize up due to the huge amount of work it would have to do to continue pumping against the vacuum and finally if it got so great your fuel tank would implode.
again i may be wrong and id be happy to hear anyone elses opinions. just not from someone who is gonna bag out on me before even reading any other of the posts and knowing what they are talking about. i would be very interested to hear madmagna or mitsiman's opinions on this.

I did read the other posts, which was why I was specifically refuting one opinion and trying to present my own theory and back it up with reasoned argument and examples, like the top of my dodgy old fuel cap being concave due to the inside of the tank having a lower pressure than the outside air - an example that couldn't happen in the "tank full of hot air" example.

It may be you are right, it may be you are not (and you made some good points). Just my opinion backed up by my own experience.

Anyway, at the end of the day, does it matter? We've established that ots of people have a car whose fuel tank goes *foop* when they open it, and so the dude's car isn't likely to explode because of it.

Ok, assuming that anyone besides me still cares about this :D I did a bit of reading up on fuel tanks, and it looks like both situations can occur, but an exhalation of gas from the tank is probably more common.

Here's an idea *logic attack*

Put your hand near the hole when you open the fuel tank.

So then you can know for sure if it's going out or in. ;)

Here's an idea *logic attack*

Put your hand near the hole when you open the fuel tank.

So then you can know for sure if it's going out or in. ;)

:idea: Geezart!!!.. where would we be without our inhouse Logic-tision!!!

that logic is surely to be documented somewhere in the AM Questions and ANswers of Fame! 8)

Or you could light it on fire..

If you get 3rd degree burns, the gas is going out..

If your car explodes, the gas is going in ;)

Or you could light it on fire..

If you get 3rd degree burns, the gas is going out..

If your car explodes, the gas is going in ;)

you definately need a piece of paper stating your new found degree

"Master Logics Degree" 8)

you go girl... or guy :oops: i think more guy, as youve just had *that* hair taken from you :badgrin:

Ok, assuming that anyone besides me still cares about this :D I did a bit of reading up on fuel tanks, and it looks like both situations can occur, but an exhalation of gas from the tank is probably more common.fair enough mate. sorry if i came back abit harsh. i was still pissed aboput the shit going on with the other topics. to be honest i originally thought it was a gas thing too. hence the saying "running on vapours" coz when your tank is empty (which i might add i was right about with the original post) you open the cap and it goes psssst! i honestly thought this happened to everyone. again though, i must be the only poor buuger who has to scrape 5c coins together from under his seat to get some more fuel. :(

Well ive always thought it sounded like sucking in.

It doesnt have to be a long trip, when i fill my car i turn the cap hard till it goes "CLICK" real loud.

I drove it until amost empty, then a few days later i drove 1k to the servo, 1k back (forgot my wallet lol) and 1 k back to the servo, so 3 ks in total and i still got that sound!

I thoguth it sounded like a sucking sound to me, i guess its hard to figure out, or theres some sort of vent that is clogged on ALL our cars due to dirt :P

I think it is usually air going out, and you could probably explain that it happens more on a hot day with less fuel in the tank.

I also think that Booya is in the perfect place to have a check for us. He could go out and do some driveway service (maybe even to the TL coppers) and record for us how many cars suck in, how many cars blow out, whether people have been driving for a while or if their tank is empty.

I look forward to the scientific analysis Booya!!! :lol: