Fuel Tank Vent?

Engine, Transmission and related drivetrain.
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TheHeretic
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Fuel Tank Vent?

Postby TheHeretic » 21 Mar 2016 22:17

What is everyone doing with the stock vent that was supposed to be tied into the stock intake system? This is mine as it sits...
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Byron510
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Re: Fuel Tank Vent?

Postby Byron510 » 21 Mar 2016 22:45

If you remove it, you must run either a vent onto the ground under the back of the car or run a vented gas cap.

If running a vent to the ground is your option, a one way valve is always a good idea in conjunction with a line that runs up to the bottom of the package tray before coming back down through the trunk floor to somewhere under the car.

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Re: Fuel Tank Vent?

Postby Three B's Racing » 23 Mar 2016 07:43

If you still have the original vent tube to the air cleaner housing why not simply plumb it into you cold air intake air cleaner or whatever setup you have? You should still run a one way check valve.
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TheHeretic
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Re: Fuel Tank Vent?

Postby TheHeretic » 09 Apr 2016 13:28

Dual mikunis with ramair foam filters...doesn't look like I could tie in the vent very well.
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Re: Fuel Tank Vent?

Postby datzenmike » 09 Apr 2016 21:56

The tank vent should go to the flow guide. The flow guide opens under a predetermined pressure of the fumes generated by the sealed gas tank by evaporation. The fumes are directed into the block vent tube and stored in the crankcase. When the engine is started the PCV valve sucks all fumes from the crankcase and burns them. When enough fuel has been used to cause the tank to have a slight negative pressure the flow guide opens the line to the air filter so that air can get into the tank to replace the fuel used. You need a working PCV valve for this.

If you don't have a flow guide just grab a carbon canister off a newer car. All you need is a vacuum line to the intake and a another connected to the vacuum advance line to act as the purge signal... and the tank vent line. Gas fumes from the sealed tank are stored in the activated charcoal. The vacuum advance is only active above idle so a signal from it opens the purge valve and the intake vacuum draws the fumes into the intake to be burned. The charcoal canister is designed with the bottom open to the air so as the gas tank empties, air is free to flow back into the tank to replace it. You don't need a PCV valve for this.
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SteveEdmonton
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Re: Fuel Tank Vent?

Postby SteveEdmonton » 10 Apr 2016 19:59

Related question, as I too am working on getting this system working on my 510. Since installing my new engine (L20B with twin SUs) I have had a very noticeable problem with fuel odour in the trunk & cabin. I'm guessing that the vapours that build up in the gas tank aren't being properly disposed of. And so my first instinct is to "go through" this system and get it working properly again. (Like TheHeretic, the vent line from my gas tank is currently open to the air, in the engine bay.)

So here's a schematic from the FSM, illustrating the verbal description Mike gave above.
emissions- fuel evap system CLOSEUP Apr2016.jpg
emissions- fuel evap system CLOSEUP Apr2016.jpg (154.32 KiB) Viewed 1022 times


Two issues / questions.

One: My PCV setup is pretty simple. I adapted the standard L20B vent-pipe from the crankcase to fit between the branches of the exhaust manifold and tuck below the intake manifold, then connected it to the PCV valve on my intake using the "SSS" rubber/fabric hose. The issue is that there is no secondary "inlet" in this kind of setup for a tube from the flow-guide valve, carrying fuel-tank vapours into the crankcase. (Such an inlet is circled in red on the schematic above.)

So my question is: I suppose I need to add such a pipe, in order for the fuel-venting system to work properly?

Two: The other thing I'm missing so far is a small-diameter line from the air cleaner to the flow-guide valve, supplying air to the fuel tank as needed. On the schematic, this line is circled in blue. But here's the thing: on my stock L16 air cleaner (1971 510), this line draws its air through a nifty little valve that's controlled by a bi-metallic strip. (Picture below.)
IMG_2115.JPG
IMG_2115.JPG (159.08 KiB) Viewed 1022 times

I tested this valve and discovered that it opens at about 140 - 145 degrees Fahrenheit. Presumably, until the temperature inside the air cleaner reached that temperature, this valve would not open-- and air would not feed from the air cleaner into the flow guide valve, and thus back to the fuel tank.

But does that really make sense? The description in the FSM hasn't helped me figure it out. The only place I see it talking about this feed from the air cleaner, it says: "When the pressure of the closed type fuel tank, vapor liquid separator and vapor vent line becomes negative by decreasing the fuel [that is: when the fuel level in the tank goes down, creating a vacuum], the flow guide valve opens to send fresh air from the carburetor air cleaner to the fuel tank" (page EC[A]-33) There's no mention in that description of a heat-sensitive switch opening this feed line from the air cleaner. Instead, it sounds like the feed from the air cleaner is always open.

So my second question is: What possible advantage would there be, to having the feed-line to the flow guide valve open only beyond a temperature of 140-145 degrees within the air cleaner? And flowing from that question, when I mount this valve in my own new setup, should I keep this temperature-controlled valve in place, or just lop off the little bimetal strip so that the flow-guide valve itself is the only "control mechanism" that determines when air feeds back into the gas tank?
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Re: Fuel Tank Vent?

Postby loungin112 » 25 Oct 2016 11:10

This seems simple in theory....until all the options start running through your head, only to result in confusion! I have a similar problem i am working on. Details: L20b, dual SUs (38mm sss) with overflow port (on carb nearest firewall), mechanical fuel pump with spacer, breather line into engine compartment, k&n air filters. When running the engine, there is fuel that blows by the SU bowls. PO had the overflow routed through the breather line that dumped back into the expansion chamber. Stupid mechanic recently blocked it off (poorly) resulting in a fuel leak that spilled into the engine bay.

So my problem is two fold, but aparently with many solutions.
Problem one: what to do with the overflow? Return to expansion tank, or add fuel regulator and gauge and block off return line?
Problem two: what to do with fuel venting? Add venting fuel cap or hook up flow guide valve and leave venting side open to engine bay? This would be done in conjunction with blocking off SU return line. (I think?)

This is stupidly frustrating and yes, i now hate the smell of gasoline. But, i will add a few pics tonight, given i can get photobucket to cooperate.

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Re: Fuel Tank Vent?

Postby loungin112 » 25 Oct 2016 11:12

Heretic and Steve...what did you end up doing?

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SteveEdmonton
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Re: Fuel Tank Vent?

Postby SteveEdmonton » 27 Oct 2016 20:10

Nuts-- I just finished writing this all up, then the internet connection was too slow in preview mode and I lost it all. So let's try again:

I solved the overflow problem a little differently by running a metal line (a cunifer line, easy to bend by hand) across the firewall then down, near the clutch slave line, where it dumps onto the ground. This is basically the factory solution too. You can't see the top end of the metal pipe in this picture, but you get the idea.
IMG_8364 comp fuel vent-line.jpg
IMG_8364 comp fuel vent-line.jpg (197.4 KiB) Viewed 711 times

On my carb setup there are 2 overflow ports, one from the top of each float bowl. They merge into one via a factory-provided pipe. You can see this in the pic below, even though I was still running a rubber hose from the junction to the drain location at the point I took this picture (before I even got the motor running). Nowadays the metal line in the previous picture connects up to this junction instead.
IMG_6879 comp float-vent lines.jpg
IMG_6879 comp float-vent lines.jpg (224.86 KiB) Viewed 711 times


Re. your other question: I too thought that the venting issue was causing all my gas odours. But it wasn't. Other guys here helped me realize it was probably related to the gas-tank filler pipe. And they were right. Here's how I fixed that problem:
viewtopic.php?f=30&t=29236&p=255966&hilit=subaru+neck#p255966

Once that was fixed, the odour problem largely disappeared. I do still have the piping hooked up from the flow valve to all the stock locations, because I think that's the easiest / best way to dispose of those vapours properly. But this system did not turn out to be as significant of an issue in the smell department as I had thought.

Hope this helps. Best of luck!
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Re: Fuel Tank Vent?

Postby loungin112 » 28 Oct 2016 09:13

Thanks Steve.
I hadnt thought of adding another line to the equation but that would give another option for handling the excess. I'll be working on my setup soon, well, as time permits. I still need to add photos though.

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Re: Fuel Tank Vent?

Postby loungin112 » 05 Nov 2016 17:02

So I am a few steps closer. I received my expansion tank back from being sandblasted (internally), coated, and painted. Picked up some new fuel lines for to run to the expansion tank. And, to my surprise, I even found a vented gas cap!! So, I think my issues are sorted. Pics upon install...seriously.

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Re: Fuel Tank Vent?

Postby loungin112 » 09 Nov 2016 18:07

Engine is much cleaner now, but the fuel return line is the same. You can see the return coming off the SU closest to the firewall, connected to the line going across the BMC and Clutch Cylinder.

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Closer

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Gas Tank Setup

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Vented Gas Cap!! (I didn't think they exist)

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With the vented gas cap, I'm thinking I will leave the setup as is and take advantage of circulating gas through the system. Vented gas cap and use of the expansion tank should take care of both vapors and pressure yes?

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bertvorgon
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Re: Fuel Tank Vent?

Postby bertvorgon » 10 Nov 2016 06:54

One thing I did, as I kept the vapour tank in the trunk, was to put a line from that vent and snacked it down and out through a small hole over near the wheel well.

That accomplishes two things.....the vapours go out UNDER the car and get swept away by the air stream, and...if you ever end up on the roof, that vent line will be the highest point, so no fuel would escape.

Not sure what to say about the vented fuel cap, never sued one, but, if cornering forces let fuel come out, then you will need to revisit that...which then takes you back to the vent line going under the car, as that then lets the system "breath", so you will never get a negative pressure in the fuel tank.
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Re: Fuel Tank Vent?

Postby loungin112 » 12 Nov 2016 15:27

Thanks Bert. As with most things, my idea off this 510 had changed over the last few years. Originally wanted it to be a commuter/track day car, now it's looking like a commuter/cruiser. Good note about vapors/pressure and sloshing...something to remember when the purpose goes back to something more spirited.

I'll report back if there are any useful observations from this setup.


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