Evap vent valve question

[My Nameis]

So my understanding is under normal conditions (when the ecm is NOT checking for leaks) the vent valve is open to the atmosphere.

My question is, if that’s the case how do the stored fuel vapors in the canister not vent out? Does the charcoal hold the vapors regardless that the canister is open to outside air?

Or is my understanding of how it works wrong?

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[Todd Smith]

Yes, the charcoal acts like a filter and holds the hydrocarbons until the system can be purged.

[My Nameis]

[quote=”Tods213″ post=189471]Yes, the charcoal acts like a filter and holds the hydrocarbons until the system can be purged.[/quote]

So even though the canister is always open to outside air by the vent solenoid, the charcoal keeps the fuel vapors from escaping to the atmosphere?

[Todd Smith]

Yes. The vent valve is normally open and the charcoal canister holds the vapors so they don’t get out into the atmosphere.

[My Nameis]

[quote=”Tods213″ post=189480]Yes. The vent valve is normally open and the charcoal canister holds the vapors so they don’t get out into the atmosphere.[/quote]

Is there a reason the vent valve is normally open? Is it for when fueling the air has a place to go?

[Todd Smith]

Gas expands and contracts with temperature changes. For that reason, you would either want to admit or expel air to keep the gas tank equalized with atmospheric pressure. This equalization is pretty much what you want at all times.
Secondly, when fuel is drawn out of the tank, a vacuum will begin to form as the empty volume in the tank increases. To offset this, the vent allows atmospheric air into the tank to equalize the pressure. This idea is similar to putting your thumb on a straw and then pulling that straw out of a glass of water. Without a vent, your motor would be starved of fuel within minutes.

[My Nameis]

Thanks. The idea the vent valve is always open made it hard to believe the charcoal was the only thing keeping fuel vapors to the atmosphere if there’s an opening right there.

I’m curious, I have a repeated p1456 evap code in my 98 civic and I noticed on really warm days 70+ the code never sets and evap system passes the tests.

But if it’s colder than 70F the code always comes on. Any ideas on why? I’ve replaced the fuel cap and filler neck with no difference

[Todd Smith]

This is where there is one exception to the “always open” rule for the vent valve. The computer briefly commands the vent valve closed periodically to check for a pressure rise or drop that would indicate the system is sealed.
In the case of your trouble code, the computer attempted to close the vent valve and then found that there was enough of a leak to warrant the check engine light.
There are several reasons this could happen but some common causes may be that the vent valve isn’t working properly or that one of the lines of the system has rotted and is now leaking.
Under the hood of your civic, there is a green service port in the evaporative emissions lines. You can use a can of Smoke Check and fill the system using that port. It will take some time to fill the system with smoke. At least a minute. If there are any leaks in the system then you will see the smoke coming out.

[My Nameis]

I actually paid a shop to smoke test it and there weren’t any noticeable leaks. This is why it’s been so confusing to figure out.

If I apply 12v to the vent valve it does close.

Do you have any idea why when outside temps are 70F or higher the system passes the test?

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[Todd Smith]

Nope.

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