What's Inside: Fuel Tank Edition (Leaking Fuel Tank Autopsy) - Subaru Outback - Subaru Outback Forums
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post #1 of 23 (permalink) Old 05-15-2019, 09:50 PM Thread Starter
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What's Inside: Fuel Tank Edition (Leaking Fuel Tank Autopsy)

Recently replaced my fuel tank due to a leak and some members were interested to see what's inside the old one to help diagnose a fuel filling problem. Sliced it open and took a bunch of photos. This is the original, 196K rust belt miles, tank as far as I know. Note in the photos the outer nipple which goes to the tank pressure sensor has rusted off (it's the one with no internal connections).

Purging the tank with a fan for a few hours before cutting around the seam with an angle grinder. Fuel was previously drained obviously...


Inner Top of Tank


Inner Bottom of Tank


Closeups of Evap. & Filler Plumbing


Filler Check Valve Details:


Rear Float Valve: Conical seal/seat at the top of the float.


Front Float Valve. Conical seal/seat at the top of the float.


Evap Canister Valve (Vapor Vent Valve). There was a diaphragm in the upper part of the housing which was ultrasonically welded together; I didn't try too hard to get it apart. See US Pat. 4714172 for principal of operation.


Connectors and Hoses:


Tank Fittings:


Continued below...


Last edited by aero901; 05-15-2019 at 11:04 PM. Reason: Tidy up formatting
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post #2 of 23 (permalink) Old 05-15-2019, 09:51 PM Thread Starter
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Tank Seam Section (appears to be spot welded):


Tank Seam Section where I had the leak. Note the rust penetrating into the weld from the outside (left) edge.


Tank side seam section where the leak was (pass. front corner of tank).


Vapor Vent Valve Internals:








Hope these can be helpful to someone!


Last edited by aero901; 05-17-2019 at 10:48 PM. Reason: Added photos of vent valve internals
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post #3 of 23 (permalink) Old 05-15-2019, 11:23 PM
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I have no words to describe my appreciation for you providing this Aero.... the photo skills and detail is something I’ve never seen before on a forum of any kind. Seriously, hats off to you...

I’m going to digest this a little more tomorrow but my problem is the black valve assembly. Do you think you could hang on to the black valve and perhaps ship it to me if needed after I mess with the car a little more? I’d obviously send cash your way should it come to that. Thank you!!!

Seriously, wow, thank you so much...
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post #4 of 23 (permalink) Old 05-16-2019, 08:18 AM Thread Starter
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Quote:
Originally Posted by STepeci View Post
I’m going to digest this a little more tomorrow but my problem is the black valve assembly. Do you think you could hang on to the black valve and perhaps ship it to me if needed after I mess with the car a little more? I’d obviously send cash your way should it come to that. Thank you!!!
Sure, let's PM details if you want it. The plastic tabs that hold the spring retainer/float stopper in place broke when I disassembled it but seems to be functioning otherwise.
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post #5 of 23 (permalink) Old 05-16-2019, 08:25 AM
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OUTSTANDING report!


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post #6 of 23 (permalink) Old 05-16-2019, 08:42 AM
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Ditto on the work, the great photos, and even finding that patent paper for more background.

Seeing four "devices" inside the tank. The orange fuel filler tube (with the spring-loaded flap at the end!), and the black,Tubular, Vent valve with a smaller and a larger hose going out of the tank. Noted that the filler tube is pointed toward the right side, i.e., where the fuel pump is located and appears to be supported by a metal brace. In this regard, is that orange fill tube just press-fit onto the metal tube going out of the tank, or is the black ring some sort of retainer?

Then there's two other devices, one orange at the filler end of the tank, and a lighter, yellow device at the front. There's a metal tube from the yellow device to a "T" in a hose going from the orange device to an external nipple. Each seems to have a spring-loaded float to close an opening at the top. The nipple that these are connected to, I believe, goes to the Shut Valve, the device at the top of the fuel fill tube that switches when the fuel filler nozzle is inserted. Both these devices appear to provide venting from the tank via the Shut Valve, but would not be related to a filling problem because the Shut Valve is supposed to be closed when filling. (Unless there's some additional plumbing we can't see in the photos.)

So the problem of filling the tank, where the tubing to the canister has been disconnected, could be with the Vent Valve, or, perhaps, now that it's apparent, that spring-loaded flap at the end of the fill tube (e.g., not opening enough to allow full flow of fuel into the tank).

In regard to the Vent Valve, if air is blown into the large hose that goes to the canister (as was done in the related thread -- https://www.subaruoutback.org/forums...ing-issue.html), that air should go into the tank and come out the fill tube (gas cap off). The hose and diaphragm valve both appear to have similar diameters, so there should be fairly free flow in this case (inhibited only by the diaphragm spring that tends to close the diaphragm valve).

As discussed in the related thread, if the Vent Valve float sticks in the up/closed position, that would block the path between the tank and the diaphragm, preventing air from going out. But if air is blown back into the canister tube, and it comes out the fuel fill tube, that should mean there's an open the path from the tank, at least until the next time the fuel level in the tank again raises the float to the closed position. (I believe the spring associated with the float in the Vent Valve photos is positioned to hold the float in the open/down position, thereby ensuring a normally open path except when the fuel level is very high. @aero901, is that correct?)

For the Vent Valve to work properly when filling, pressure builds up inside the tank, and that causes the diaphragm to lift away from the Valve opening (seen in photo #37), providing a path for vapor to the canister tube. The diaphragm has to be intact for the tank pressure to cause it to lift. @aero901; if you blow lightly into the atmospheric fitting on the valve (the smaller one), can you tell if diaphragm is intact? I'm wondering if in @STepeci 's case, the diaphragm is broken, so that pressure in the tank doesn't cause it to lift.

(Related: aero901's original thread with photos and diagram: https://www.subaruoutback.org/forums...ml#post5827689)

But again, that was some fantastic work and a very significant contribution to the overall knowledge base of this forum. Very much appreciated!
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Last edited by plain OM; 05-16-2019 at 10:00 AM. Reason: correct links
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post #7 of 23 (permalink) Old 05-16-2019, 11:26 AM
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The spring that is part of the black valve assembly has me scratching my head. It looks awfully long for the amount of space in the body when taking into consideration the length of the float.

Also, after pondering the patent image attached, I'm really questioning if tank pressure should EVER be able to escape the atmospheric reference line/signal line (small nipple on vent valve) on a correctly functioning system, I'm leaning to NO.

If hose A in my attached image is disconnected or broke, tank pressure would escape to the atmospheric reference line/signal line.

If the diaphragm body was broke or cracked, tank pressure would escape to the atmospheric reference line/signal line.

If the diaphragm itself was broke or cracked, tank pressure would escape to the atmospheric reference line/signal line.
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post #8 of 23 (permalink) Old 05-16-2019, 11:49 AM
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Quote:
Originally Posted by STepeci View Post
The spring that is part of the black valve assembly has me scratching my head. It looks awfully long for the amount of space in the body when taking into consideration the length of the float.

Also, after pondering the patent image attached, I'm really questioning if tank pressure should EVER be able to escape the atmospheric reference line/signal line (small nipple on vent valve) on a correctly functioning system, I'm leaning to NO.

If hose A in my attached image is disconnected or broke, tank pressure would escape to the atmospheric reference line/signal line.

If the diaphragm body was broke or cracked, tank pressure would escape to the atmospheric reference line/signal line.

If the diaphragm itself was broke or cracked, tank pressure would escape to the atmospheric reference line/signal line.
Perhaps @aero901 can clarify where the spring goes when assembled, and how compressed it is. It could be a long spring, but not very resistant to compression. But, yes, it does look odd. The other two vents also appear to have long springs, presumably also to hold the floats down, away from their outlets, except when the tank is (over?) full.
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post #9 of 23 (permalink) Old 05-16-2019, 12:21 PM
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yeah, that float is what helps prevent flooding the canister with liquid fuel.

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post #10 of 23 (permalink) Old 05-16-2019, 12:46 PM
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So, these guys seem to be called fuel cut valves in the diagram...are they involved in signaling the pump to shut off? Could a gentle application of air, or some solvent into their line be helpful?


Quote:
Then there's two other devices, one orange at the filler end of the tank, and a lighter, yellow device at the front. There's a metal tube from the yellow device to a "T" in a hose going from the orange device to an external nipple. Each seems to have a spring-loaded float to close an opening at the top. The nipple that these are connected to, I believe, goes to the Shut Valve, the device at the top of the fuel fill tube that switches when the fuel filler nozzle is inserted. Both these devices appear to provide venting from the tank via the Shut Valve, but would not be related to a filling problem because the Shut Valve is supposed to be closed when filling. (Unless there's some additional plumbing we can't see in the photos.)



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