Gas Tank Pressurizing - Check & Cut Valve?
 

I've been noticing that when I fill up the tank, if its pretty low there will sometimes be a big rush of air when opening the cap. I think its a rush outwards, since I can smell fuel after it happens but before fully removing the cap. Anyway, after some searching I figured that my check-and-cut valve was probably dirty or broken. However, after taking it apart, cleaning it, and checking out how its supposed to work, I'm not sure if its the correct part for the car.

https://lh5.googleusercontent.com/-0...2+15.36.36.jpg
https://lh4.googleusercontent.com/-S...2+15.36.43.jpg

I could blow & suck freely through either of the bottom nozzles, regardless of valve orientation (they were connected together in a chamber, disconnected from the top nozzle), and the top nozzle (in the photos, actually points down when installed on the car) connected to both of them as long as you blew through it (there was a check valve, no sucking). What seemed odd was how everything was hooked up. The bottom nozzles were connected to the fuel tank and to the fuel line that runs up to the charcoal canister, while the top nozzle is connected to a fitting that looks like it leads into the cabin of the car, but doesn't have a corresponding fitting, connection, or any sign of it on the inside of the passenger-side bin area (not really a bin since it's underneath the spare tire cover in the convertible, but similar stamping to the bin area on a hatch).

https://lh5.googleusercontent.com/-O...2+15.40.00.jpg
https://lh4.googleusercontent.com/-t...2+15.45.27.jpg

Having no idea where the line that connects to the top nipple goes, (it definitely doesn't seem to run to a vacuum or pressure source near the engine), I can't see how the valve is supposed to work, cut off a potential fuel flow if the car flips, or anything else. The valve itself looks different than the picture in the FSM too, which leaves me more confused. Also, there was mention in the FSM of a ball valve that would close if the car were to flip, but I didn't see anything like that when taking it apart, nor did the operation of any of the nozzles on the valve change when I flipped the valve upside down or held it in any other orientation.

Does this part look familiar to anyone? Is it actually the check-and-cut valve?
Does anyone know where that unknown connection to the chassis seems to go/what it's purpose is?

Background - S4 convertible, turbo-swapped but I haven't touched the tank (besides new fuel pump), this valve, or the line to the charcoal canister.

look on top of the tank.

Oh, so this isn't the check & cut valve? Then what is it? I haven't seen anything else mentioned as being between the fuel tank evap port & the charcoal canister, either on the forum or in the FSM

look up 4200 in the Factory parts manual.
it will show your valve as a check valve but also a valve on top of the tank as a fuel vapor valve.
number of your valve is 42-710..
I am talking about 42-720. here is a pic..hopefully it shows.
I am not saying this IS your problem,I am just letting you know there is a fuel vapor valve on the tank.

Mine does the same thing but i disregared it because it says right on my gas cap that the tank may build pressure

Hmm, I'll check into that valve 42-720 then. Didn't know that was there, thanks!

+1, The fuel system allows pressure to build in the tank, there's nothing wrong.Actually pressure in the tank confirms your evaporative emissions system is functioning well. I do applaud your willingness and dedication to disassemble your car for this though. :nod:

How did you fix this issue?

Yes that is what is on the gas cap but should only release for a few seconds and not for a prolong time for it to release. In addition to hearing a hissing sound under the car specially after a spirited drive.

funny how every car does this but only on the RX7 club will people disregard this most important post.

why wouldn't it? you specifically mentioned it does this more when the fuel is lower. know what the relationship between the vapor volume in the tank is when the fuel level is high versus low? i actually am not going to answer that and let you take a stab at it.

as cone crushr said, pressure points to there being no leaks in the system, which is actually good.

My wory is not that it releases presure when I remove the gas cap. But more so of the hissing sound, I am affraid that if there is excessive fuel pressure it may rupture the tanks or some fuel related lines and I also smell fuel and I dont see any leaks.

you watch too many movies, where cars violently explode. which also does not happen in the real world.

is the fuel smell strongest around the firewall in the engine bay on the passenger side? or is it strongest directly over the engine? the fuel smell is unrelated to your pressurized gas tank, so you're starting at the wrong end of your problem.

some fuel vapors are normal, the charcoal canister cant eat all the hydrocarbons on a really hot day. the charcoal canister is also 30 years old, i don't know how long they last but i wouldn't assume it is indefinite.

The smell underneath the drivers side. I understand that in real life cars just don't go up inflames. (Lol). Any help and knowledge I can get is greatly appreciated.

my entire system including the valve on top of the tank is removed.
I did put a vented cap in which even then allows a certain amount of pressure to build before it vents