OK, so I've checked the parts diagram, it shows one vent valve with two hoses connected, right in the center of the top of the tank, in between the two access panels. all of the pics of tanks I have looked at show 2 vents with one hose connected to each, just to the inside of the access panels. Not sure which one is accurate.










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I also think I was looking in the wrong place. I found a check valve type thing accessible from underneath the car at the passenger front side of the tank. I disconnected the 2 hoses to it and a small amount of fuel came out. The valve seems to be operating properly.


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I have to imagine I'm still in the wrong place.
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How does the venting system work when fueling? I have to imagine the vent hose is large, as there is a great deal of vapor expelled rapidly during fueling. The only large hose I can identify is immediately next to the filler neck, on the tank. When I applied compressed air to this line, I heard gurgling out of the filler neck, when the pressure was removed, a flatulent sound came from the hose, sounded like it came from where the hose meets the tank.
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Is there a check valve on the tank side of this large hose? If so, is there anything that controls it?

 

Fuel tank vent valve operation question

After looking into my fueling issues, I'm pretty sure I have an issue with the fuel tank vent during refuling. When I disconnect the hose running from the tank to the charcoal canister (at the canister) and apply vacuum, the vacuum is held for far too long.


I can blow air into it though.


Is there something that controls this valve either electronically or mechanically? I want to eliminate any other possible issue before I pull the tank to replace this valve.


Thanks,
Bob

 

I'm no expert on this, but I think the tank vent valve is controlled by remote solenoid, and if something were wrong with that solenoid you'd get a code and CEL. You may be on the right track.

If you pull the tank, inspect the steel brake lines that route over it. Now would be the time to replace them. I've heard of some rusting through.

 

the evap sysem is kinda complex, plus, issues can arise from drivers being in the habit of 'topping off' when refueling - leading to flooded canisters. Or, insects/spiders might nest in some of the systems tubing.

I'm no expert either but, there is 'drain' hose with a 2-way solenoid I think - it is supposed to allow fumes to enter the canister while allowing de-fumed air to escape - but, when driving , it can allow atmospheric air to enter the canister as the purge valve sends gas fumes into the engine to be burned.

I believe on some models, people have had to replace or modify a valve.

something like that

 

Update:


I pressurized the tank via the rubber hose that attaches at the very top of the filler neck. Nothing came out of the large vent line to the charcoal canister.


I then relieved pressure on the line that attaches at the top of the filler neck (took the air hose away), bam vapor shot out of the large vent line to the canister.


Similarly when fueling, if I have the hose to the top of the filler neck connected, it won't take fuel. If I disconnect it, it will take fuel, although the auto shut off function of the dispenser seems to be delayed a hair.


What does the hose connected to the very top of the filler neck connect to and what does that hoobajoob do?

 

There should be three hoses attached near the top of the filler tube. Two connect to a device near the top of the tube -- the "shut off valve". The shut-off valve is normally open, but when the gas station pump nozzle is inserted into the fuel filler pipe, it moves a lever in the pipe that closes the shut-off valve.

The third hose, which attaches to a fitting on the filler tube directly, goes back to the "vent valve" on the top of the fuel tank. The hose provides atmospheric reference pressure to the vent valve during fueling. Also, the vent valve has a spring that keeps the valve closed (even if the float is down). When pressure in the tank increases above atmospheric, the spring allows the valve to open and fumes can flow to the canister.

Here's a diagram of the evap system, supposedly for the 2000 Legacy, but it should be similar to, if not the same as, the 1999 especially to explain the refueling function.

My approximate explanation of how it works follows . . .

During refueling, as noted, the shut valve is closed. The vent valve has a float that comes down into the tank. When the fuel level is below the intended "full" level, the float is down and the vent valve is allowed to open. Also the vent control solenoid valve (aka drain valve) at the canister should be open, and the purge valve should be closed. With these conditions there's a single path for fumes in the tank to follow while liquid fuel is coming in. The path is from the tank, through the vent valve and the (diagrammed) vertical, then horizontal pipe, to the canister. The fumes are trapped by the charcoal in the canister, and fume-free air is discharged through the drain valve to the atmosphere.

When the fuel level rises, the vent valve float closes the valve, and there is no venting path. As fuel continues to come in, there is a pressure build up, and a sensor in the fuel delivery nozzle detects the back pressure in the filler tube, shutting off the nozzle.

When the fueling nozzle is removed, the shut valve opens. This provides a path from the top of the fuel tank, via the fuel cut valve and pressure control solenoid valve, through the open shut valve, to the canister when the vent valve itself is closed.

If the filling nozzle cuts off prematurely, it means that pressure is building up in the tank. This can happen if the vent valve is stuck closed, or if the drain valve at the canister, is stuck closed or the canister is clogged.

Hope I have it close to right, and that this will help narrowing down the cause of the premature shut offs.

 

Found it! Rammed a wire through the large nipple at the top of the filler neck, reconnected the hose, disconnected the battery. BAM! car takes fuel and no CEL!

 

Thanks Plain OM!


Found it! Rammed a wire through the large nipple at the top of the filler neck, reconnected the hose, disconnected the battery. BAM! car takes fuel and no CEL!

 

so, what came out?

 

merged threads.

 

yeah did you have to remove something, like a protective thing or something from the molding.

or did you have to pierce a hole in something.

was it defective,..

.was their directions that said remove or pierce? (edit: whoops I see you got a used one)

 

I just stuck some 10 gauge insulated braided wire through the hose barb on the filler neck, it is just astraight through port the vent valve uses for an atmospheric pressure reference. Nothing came out stuck to the end of the wire, it did give some initial resistance though. Maybe a mud dauber/wasp home? Its in the bottom of the tank now.

 

How far down did the wire go?

I haven't examined a vent valve close up, but would imagine that the hose from the top of the fill tube going to the valve ends at some sort of diaphragm in the valve. That being the case, whatever was in the tube is probably down there, not in the tank. Probably of no importance now, but perhaps something to keep in mind if there's issues in the future.

Then again, as the fuel fill tube is normally capped, I have to wonder how anything could get in there (from the top) to develop a blockage. I wonder if there might have been rusting inside the metal part (barb) at the top and that was blocking the hose; the wire might just have pushed it aside -- just as well.

But, help me out here. My 07 is also supposed to have a vent line to the upper part of the fill tube, but when the fuel cap is removed, there's only the restriction plate that's visible -- I can't see where the vent tube comes into the fill tube, which then begs another question: Where did you insert the wire?

 

I just probed the hose barb/nipple in the neck itself. From the end of the fitting with the hose off, into the filler neck. The hose barb is just behind the filler flap. Literally just two inches of barb. I guess that's a problem that can be expected when a used filler neck is purchased. Who knows how long it had been of a shelf in a warehouse. I'm going to bet a LOOOOOONG time seeing that it came from Maine or Vermont and had no rust.


What an unbelievably simple solution to an immensely complex problem. And to think I scoffed at the idea of spider webs causing a blockage...

 

Got it. Probed from where the hose attaches to the bottom of the barb (fitting near the top of the fill tube). The blockage was in the replacement fill tube fitting, not the hose going down from the barb to the vent valve. And, yes, if probed upward, the blockage would have gone down the large fill tube, into the tank. Thanks.