Evap vent valve question

The evap vent vale is a normally open valve. So applying voltage causes it to close. It allows fresh air to enter the canister. I'm all good with that. My question is what prevents fuel vapors from exiting the canister through the vent valve and into the atmosphere? It's supposed to have fresh air go in but if it's normally open and the vehicle is off then hydrocarbons should be able to go out causing an evap leak..? Or does it have some kind of one way check valve in it? Can this be a source of an evap leak when the solenoid portion works perfectly fine?

Solenoid working doesn't mean it seals. I believe charcoal doesn't let HC and fumes escape.

borntoroll wrote: Solenoid working doesn't mean it seals.

Oh ya, I know about that. Just wondering if it has anything built into it to prevent HCs from exiting it. If it did that would have to be separate from the solenoid portion of the valve as it's normally open.

I wondered if the canister relied solely on the carbon capturing and holding all the HCs as its means of preventing them from going out the vent. Just seems like it would be putting a lot of faith in that for how much these systems are regulated by law. Plus whenever someone overfilled their tank up to the filler neck that would saturate the canister which should cause venting of HCs...

I can't really seem to find anything definitive on the web. :huh: Maybe next time we pull one out I'll rip it apart and take a look see.

ecwurban wrote: I wondered if the canister relied solely on the carbon capturing and holding all the HCs as its means of preventing them from going out the vent.

This has always been my understanding, but it never really satisfied me. Seemed silly to go to all this trouble for EVAP systems and leave the vent valve open 90% of the time. :silly:

Maybe I was misinformed? If anyone has a better answer, I'm ready to hear it. :lol:

vent is open at least 90% of the time the fuel tank has to be able to vent. the charcoal canister is solely responsible for filtering the fuel vapor , I wouldn't say this means it does it 100% but that is the way it's designed to work . On a side note I wonder what a 5 gas would say at the vent valve while adding air to test port (obviously very low pressure)

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Ben wrote: On a side note I wonder what a 5 gas would say at the vent valve while adding air to test port (obviously very low pressure)

Oooooh, that's a good idea. :ohmy: Like, 1 PSI, right? Maybe just hook up the smoke machine. :huh:

I bet it would read a LOT of Hc's.
I read something a while ago that said the charcoal in an automotive charcoal cannister is adsorbant (yes with a D).
Basically, if memory serves, this property is desirable because it allows the charcoal to not only capture Hc's (hydrocarbons) but to also release them when the purge valve is opened. I would think that pressurizing the system would have the same effect on the charcoal in the cannister, and the Hc's would be released through the vent.

Side story;
I've been filling my Navigator at the junkyard for YEARS with a syphon hose and 5 gallon buckets. There's no automatic shut off, so when it's gurgling out the filler neck, you're full.
Well, I had to stop at a gas station and actually BUY GASOLINE! WITH MONEY! :sick: NOT so much to my surprise, the pump kept shutting down on me during refueling.... :whistle:

Noah wrote: I bet it would read a LOT of Hc's.
I read something a while ago that said the charcoal in an automotive charcoal cannister is adsorbant (yes with a D).
Basically, if memory serves, this property is desirable because it allows the charcoal to not only capture Hc's (hydrocarbons) but to also release them when the purge valve is opened. I would think that pressurizing the system would have the same effect on the charcoal in the cannister, and the Hc's would be released through the vent.

:whistle:

Side story;
I've been filling my Navigator at the junkyard for YEARS with a syphon hose and 5 gallon buckets. There's no automatic shut off, so when it's gurgling out the filler neck, you're full.
Well, I had to stop at a gas station and actually BUY GASOLINE! WITH MONEY! :sick: NOT so much to my surprise, the pump kept shutting down on my during refueling.... :whistle:

:lol: :lol: :lol: I actually have the same problem with the Scion. I think the previous owner brimmed it one too many times? Anyway, I'm too cheap to replace the canister, so I've gotten REALLY good at shooting the gas into the filler neck on the car. The pump can't kick off if it doesn't know there's a problem.

Anyway, I'm too cheap to replace the canister, so I've gotten REALLY good at shooting the gas into the filler neck on the car. The pump can't kick off if it doesn't know there's a problem.

We actually just took apart a canister on a vehicle that couldn't fill up at a gas station without the pump shutting off constantly. All the pellets had completely solidified together into this brick. If it were my vehicle and I couldn't get one at the local salvage yard I'd be tempted to partially gut it out

Noah wrote: I've been filling my Navigator at the junkyard for YEARS with a syphon hose and 5 gallon buckets.

Hard to turn down free gas but that can be risky! We have a buddy that works at a metal recycling plant here that used to do that until he put in some nasty gas that killed his truck. Towed it to our shop and it had zero compression. It had washed the cylinders so bad that it took us a day and a half to resurrect that thing!

ecwurban wrote: We actually just took apart a canister on a vehicle that couldn't fill up at a gas station without the pump shutting off constantly. All the pellets had completely solidified together into this brick. If it were my vehicle and I couldn't get one at the local salvage yard I'd be tempted to partially gut it out

Noah wrote: I've been filling my Navigator at the junkyard for YEARS with a syphon hose and 5 gallon buckets.

Hard to turn down free gas but that can be risky! We have a buddy that works at a metal recycling plant here that used to do that until he put in some nasty gas that killed his truck. Towed it to our shop and it had zero compression. It had washed the cylinders so bad that it took us a day and a half to resurrect that thing!

You have to know what you're looking at. I could make ​a whole thread about how to grade recycled gasoline by eye.
I only run the primo stuff. Once it's got that turpentine stink, it's no good.
You can tell a lot by the color and clarity, you can get a good idea of water content just by the weight of the tank...

Noah wrote: You have to know what you're looking at. I could make ​a whole thread about how to grade recycled gasoline by eye.
I only run the primo stuff. Once it's got that turpentine stink, it's no good.
You can tell a lot by the color and clarity, you can get a good idea of water content just by the weight of the tank...

I would read the crap out of that thread. :ohmy: