05 Outback Fuel Tank Venting Issue - Subaru Outback - Subaru Outback Forums
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post #1 of 46 (permalink) Old 05-14-2019, 04:54 PM Thread Starter
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05 Outback Fuel Tank Venting Issue

The Car: 2005 Subaru Outback 2.5i base A/T. 100% Arizona car, zero rust or corrosion on anything, 182,000 miles. This car is my unicorn, I’d drive it to the moon and back tomorrow.

Me: I have built many engines in my day, very competent with modern vehicles and have always done the work myself except for tire swaps and alignments. I am not a beginner and generally do not have issues I can’t figure out, however the issue below is STUMPING me so far.

The Issue: About 1.5 years ago, my fuel tank stopped venting at the gas station while pumping and I could only pump roughly .5 gallon at a time as pressure built up. The issue went away for 1 week and returned to its current state for the past 1 year. Over the past year, the issue has been the same, extremely consistent. Pump in .5 gallon, wait about 5 seconds to hear the pressure equalize through the filler neck, and repeat.

The Observations:
  1. No CEL’s
  2. No pressure/vacuum when unscrewing gas cap in any scenario
  3. No change at dozens of pumps at dozens of gas stations
The Teardown: I decided to finally rip this thing apart so I printed all the fuel tank diagrams and started from the end of the system, the vent to atmosphere.
  1. I took every hose from the vent to atmosphere back through the charcoal canister and to the vent nipple on the top rear of the driver side of the tank and blew through them, including the canister. Zero obstructions, completely free flowing.
  2. I disconnected the tank to canister hose and went to get fuel. Same issue, while pumping, NO air/fumes are coming out of the vent nipple on the tank. For giggles, I blew on the vent line INTO the tank and it was completely free flowing OUT the filler neck. All of the ~3/4” vent line from the tank to the atmosphere are completely free of obstruction, as is the canister.
  3. I then went to the filler neck side, pulled the filler neck and found it to be completely clear of obstruction. The mechanical valve on the top of the filler neck is working as it should via the flapper. This time though, I pressurized the filler neck with air to simulate filling the tank with liquid fuel. The only place that is venting ANY air/fumes from the tank is via the hose that goes to the TOP of the filler neck.

Does anyone know why there is no venting out of the ~3/4” VENT nipple/hoses into the charcoal canister?

Does anyone know why the only venting that is occurring is routing to the TOP of the filler neck?

I am clearly missing something and would happily call myself stupid and enjoy it being something dumb, but I am temporarily at a loss.

Thanks in advance everyone!! I am going on a 6000 mile road trip starting in July and would love to fix this by that time as that is a lot of fill-ups!

Appreciate the read and your help would be greatly appreciated!

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post #2 of 46 (permalink) Old 05-14-2019, 05:59 PM
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Possibly the Vent Valve (in the tank!).

Not sure exactly how that valve works (internally), but looking at the description and diagram, perhaps when blowing back into the 3/4 inch line to the canister, the valve will route to the hose outlet at the top of the fill tube. But when pressure is applied from inside the tank, i.e., from the fill tube, the valve opening to the 3/4 inch canister line remains closed. See attached.

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post #3 of 46 (permalink) Old 05-14-2019, 07:04 PM Thread Starter
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Originally Posted by plain OM View Post
Possibly the Vent Valve (in the tank!).

Not sure exactly how that valve works (internally), but looking at the description and diagram, perhaps when blowing back into the 3/4 inch line to the canister, the valve will route to the hose outlet at the top of the fill tube. But when pressure is applied from inside the tank, i.e., from the fill tube, the valve opening to the 3/4 inch canister line remains closed. See attached.
plain,

1) Is it fact that the vent valve is internal/built-in the tank?
2) I assume it is not serviceable?

THANK YOU for the reply! At least I would feel like I'm not going insane if there truly is an internal valve in the tank. I noticed that on the WRX and possibly the Forester, there is a valve EXTERNAL to the tank that is serviceable.
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post #4 of 46 (permalink) Old 05-14-2019, 07:34 PM
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is/was there history of the car sitting for periods without being driven?

I ask because, my MIL's '10 Forester had to have the tank dropped to replace some gummed-up valves on top. (or, the entire tank was replaced - I really should get the details from my BIL) seemingly caused by not being driven. It would not take fuel.

Any of the car's drivers in the habit of 'topping off'? you said you got flow thru the canister but, I wonder how sensitive the pumps are. I think you can find weights for the canister in the FSM? - like, Normal is 4 pounds, replace if 6 pounds.....w'ever.

anyone know about the FSM troubleshooting for this?

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post #5 of 46 (permalink) Old 05-14-2019, 07:46 PM
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is/was there history of the car sitting for periods without being driven?
Frank might resemble this comment. Look out for a PM ...

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post #6 of 46 (permalink) Old 05-14-2019, 07:52 PM Thread Starter
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Thanks for the reply! The car had 130k when I bought it 9/2016 and it was a local driver its whole life, so I'm going to assume it didn't sit much. Since I took possession it definitely has not sat, I am now at 182k. It was perfectly fine for I'd say 6 month then this issue randomly appeared, went away for a week, and has been present for about a year now. I do not top it off, but I cannot even get the thing to vent with the canister out of the system.

I found 'wally' with a similar issue here and am reading to see what's happening. I really wish I had a tank to one of these cars out so I could play with the various nipples on the tank to see what the heck does what.

subaruforester.org/vbulletin/f88/car-not-filling-gas-pump-4002/index5.html
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post #7 of 46 (permalink) Old 05-14-2019, 08:23 PM
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can you try filling after disconnecting the hose 'up stream' from the 'drain valve' ?

also, some cars seem to attract spiders that can block the final vent to atmosphere.

we need a better labeled diagram of the system.

also, be aware there are a coupla green connectors under the dash that, when connected, will cycle the solenoids on the evap valves - doesn't necessarily mean they are not clogged inside I guess......but you should be able feel/hear them.

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post #8 of 46 (permalink) Old 05-14-2019, 08:28 PM Thread Starter
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Originally Posted by 1 Lucky Texan View Post
can you try filling after disconnecting the hose 'up stream' from the 'drain valve' ?

also, some cars seem to attract spiders that can block the final vent to atmosphere.

we need a better labeled diagram of the system.

also, be aware there are a coupla green connectors under the dash that, when connected, will cycle the solenoids on the evap valves - doesn't necessarily mean they are not clogged inside I guess......but you should be able feel/hear them.
Yes, I have done the upstream disconnects and a check of the full vent line and tried getting gasoline after doing so to no avail. I'm going to draw up a picture right now of what I just confirmed, but check out my original picture and post. Thanks!!
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post #9 of 46 (permalink) Old 05-14-2019, 08:42 PM Thread Starter
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OK, I just drew up the attached image. On the rear of the tank (excluding the sending unit and fuel pump holes and their associated hoses), there are 5 connections.

When I am pressurizing the tank (by the way, keep the PSI down low for any future readers, I am doing about 8 PSI) there are 2 places I am getting venting:

1) Connection C

2) Connection D, HOWEVER, when the flapper at the top of the filler neck is open (when the fuel pump is in at the gas station), the valve closes, effectively NO LONGER allowing connection D to vent through its channels to the charcoal canister. This is how the system is supposed to work, and mine is working correctly in this regard.


NOW, I am getting NO venting from connection A with or without the canister hooked up, which is the problem. This is what is leading me to believe that what plain suggested with their being a valve INTERNAL to the fuel tank is true. Otherwise, we are kind of defying gravity here.....
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post #10 of 46 (permalink) Old 05-14-2019, 08:56 PM
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At this point I can only go by the Subaru documentation; this is what I found about the Vent Valve:

G: VENT VALVE
The vent valve is located inside of the fuel tank. During filling the fuel tank, fuel vapors are introduced
into the canister through the vent valve.
When the fuel vapor pressure becomes higher than the atmospheric pressure and overcomes the
spring force which is applied to the back side of the diaphragm, the port toward the canister is
opened. The vent valve also has a float which rises and block the port toward the canister when the
fuel is full.
[Source: 2005 "Mechanism & Function"]

There's a number of other valves related to the tank emission controls that are external and serviceable.

It does seem odd to make this one unserviceable by putting it inside the tank . . .

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