As the title suggests, when I shut my car off, you can hear a hissing noise coming from the gas fill area. Unscrewing the gas cap lets a big whoosh of compressed gas out, and stops the hissing for a bit, but sure enough, after about 30 secs, the hissing is back. It has been suggested to me that the breather hose is blocked, which makes perfect sense to me.

Any other ideas?
And any solutions?

I did a *quick* search and found nothing.

cheers guys..

 

same thing with me. I dont have a constant hiss, but every once in awhile when I fill up I get a big hiss of gas as I unscrew the cap. I havnt thought much of it, actually seems sorta cool to me :P but itd be nice to know whats going on and if I should be worried

 

There's a vent line going from the gas tank up to the charcoal canister. There's also a valve on the top of the gas tank that's supposed to let pressure out. One of them is clogged, crushed, or not working. I believe the valve is called the "check and cut" valve, or something like that.

-=Russ=-

 

well im slightly worried, cause petrol/gas is much more volitile when it is in gas form, so if it builds up it could reach "critical mass" (i think it is called) it is the point where it ignites because of the pressure, I guess I am probably safe cause it is escaping and will never reach the explosion point..

I think it is usual to have some built up pressure in the tank, I get the whoosh every time I unscrew the cap on my other car, a Mazda 626.

 

thanks Syonyk, I will have to check both of those out. I am not sure where the charcoal canister lives and also, the valve you menioned, it is under one of the metal plates underneath the boot carpet?

thanks for your help

 

The charcoal canister is a large black cylinder on the (uh, passenger's side? Do you guys have right hand drive or left hand drive down there?) right side of the engine bay near the back. There will be a few lines coming off it. One of these lines will be going to a hard metal line on the firewall. Pull the hose off the metal hardline. If you get a rush of gas vapor coming out, then the valve on the gas tank & the line are OK, and the problem is a clogged charcoal canister (haven't heard of it happening, but that doesn't mean much). If you don't get a rush of gas vapor coming from the line, you'll probably want to check the valve. I'm not sure how to check it, but the FSM should have some info. You also may want to trace the line back and see if it somehow got crushed. I can't imagine something crushing the line enough to prevent it from flowing gas, but I suppose it's possible.

-=Russ=-

 

here is the diagram of the tank area fuel system, the valve seems fairly accesible... ill jack up my car in the morning and see if I can find a clog in mine!



and yes they are correct, the FSM states that the cut/check valve is for regulating fuel pressure

 

couldnt find any other info on the problem in the FSM, so I guess it cant be that serious

 

yea true... and is that from a s4? cause my picture doesnt look the same as that...

but I can see what is what.

should be sweet

thanks for all the help Jono20 and Syonyk

 

Had the same problem on my Lexus, it was the cannister.

 

I'm not entirely sure about this, but I think the cut/ check valve serves a dual purpose. It relieves positive AND negative pressures at about a 1 psi differential for both.

In other words, if the fuel heats up and expands enough, the valve vents the vapors to the charcoal canister, and if the tank develops a vac pressure above 1 psi under ambient, the valve will relieve that also. Mainly to keep the tank structure from imploding in on itself, I would think.

I think I read this in the FSM years ago (or what seemed like years ago), or in another car's service manual, who knows? Gettin' old, ya know

Our gas caps also have a valve in them...

 

My car hisses too, but only when unscrewing gas cap.... I just figure its built up pressure...

 

That's normal- that's the vac created by using the gas in the tank, and the negative pressure hasn't reached the point where the check and cut valve has dissipated it.

 

oh, ok... kewll beans... thanx...

 

get a new gas cap or have your old one tested.

 

I had my car jacked up today, I thiknk I saw what was the cut valve but im not positive. I had to go to work so I didnt bother disassembling it.

I also tried to yankt he tube off the charcoal canister but it was on too tight and I couldnt pull it off.