All of the air being purged during refueling goes through the charcoal bed.
As a matter of fact, my car even had (I removed it) a charcoal filter in the induction system to capture any hydrocarbon molecules that try to escape that way. When the engine is shut off, there’s always one cylinder with the intake valve open that was drawing in fuel. God for bid a molecule should find its way back past the open valve and out to the world.
The pages I posted are directly from Toyota, much more detailed than third-party repair manuals. There’s an entire section on this system, but in the interest of brevity I tried to post the important pages. I’m glad you were able to read them. I no longer have access to the Toyota databanks, so I took pictures of the pages, downloaded them to my computer, and attached them that way.
Yes, everything in the pages you posted is perfectly legible.
Ok, as an example of unclear explanations, or at least unclear to me, here’s what my Corolla’s factory service manual says about the car’s gas cap and its role in the evap system.
The fuel cap check valve is provided to keep the pressure in the fuel tank constant. When the pressure in the fuel tank becomes positive and reaches its specified value, it opens the valve to allow vapor to flow into the canister. On the other hand, when the pressure in the fuel tank becomes negative, and reaches its specified value, it opens to allow airflow into the fuel tank.
I can understand the need to allow airflow into the tank when the tank pressure becomes negative, but I can’t understand why the fuel cap check valve would open if the pressure in the tank becomes positive. It seems like if it opened then, the air and fuel vapor mixture in the tank would just go out through the cap and into the atmosphere.
I think what they are trying to say is that when the pressure in the tank becomes positive, the canister’s check valve opens which allows vapors to flow into the canister.
Or is what they say actually correct?
It’s correct. When the pressure in the tank becomes positive, the air is directed to the canister then to atmosphere through that little port next to the fill pipe, the hydrocarbons having been removed by the carbon bed. When the pressure becomes negative, the tank becomes vented to atmosphere, which it draws in… or perhaps it would be clearer to say the atmosphere becomes vented to the gas tank… 
Both conditions, positive and negative pressure, are normal operations of the system. Volume of gasoline causing positive pressure change routinely occurs due to refueling, warming, and agitation of the fuel while driving down the road. Agitated fluids take up more space. Negative pressure change occurs routinely due to removal of the gasoline by the fuel pump, cooling of the fuel & air, and settling of the fuel and air after parking the car.
Said differently, the valves allow the gas tank to breath both in and out, but direct hydrocarbon-saturated air through the charcoal for removal of the hydrocarbon molecules. Said molecules are then allowed to be drawn into the car’s engine for burning every time you turn the car on.
Newer systems, such as the one in the documents I attacked, also include a self-test that pressurizes the system when you start the engine and check its ability to hold pressure… the famous “leak test” that so often confounds everyone when it detects a leak.
You can see why EVAP codes frustrate the heck out of everyone.
But why would the gas cap check valve open when the tank pressure becomes positive? I don’t see that helping the situation.
Pressurizing a gas tank has no benefit. But it would prevent the valves that operate the EVAP system from operating properly and prevent the fuel handle from shutting off, which is triggered when the valve with the float shuts off the vent line and fuel flowing into the tank creates positive pressure at the fill hole.
But the gas cap is completely removed during re-fueling. So the gas cap check valve, open or closed, isn’t an issue then. Once more, this is the sentence that doesn’t make sense to me, referring to the gas cap check valve.
When the pressure in the fuel tank becomes positive and reaches its specified value, it opens the valve to allow vapor to flow into the canister.
The gas cap being off is unrelated to the positive pressure response of the check valve.
In other words, the check valve directing the fumes to the charcoal canister closes and directs it that way whether the cap is on or off. It’s only AFTER the fumes go through the charcoal that the now-cleansed air goes to the vent hole next to the fill hole. When air needs to be drawn into the tank, it can get there through that little vent hole, because the check valve will be open under negative pressure… which is when the tank is drawing on air.
Perhaps it would help to think of the charcoal bed as a filter that filters out hydrocarbon molecules similar to the way the air filter filters out particulates. As air moves through the charcoal, the charcoal attracts the hydrocarbon molecules and retains them, letting the air continue through.
Charcoal is carbon chunks. In the case of a car’s charcoal bed, it’s “activated”, which just means treated to an acid bath to make it highly porous. That dramatically increases the surface area of the charcoal. Since carbon attracts carbon molecules, the hydrocarbons stick to the surfaces of the charcoal chucks as the air carries it past, just like iron shavings would stick to a magnet. The air being comprised mainly of nitrogen and oxygen, neither of which is attracted to the charcoal, it simply flows past the chunks.
Does this help? I really do care.
You don’t want a lot of positive pressure in the tank because when you remove the cap to refuel it would burp out a lot of gas fumes into the atmosphere.
Suggest to re-read that paragraph from the FSM again. I’m not saying your comments are incorrect, they are correct and I don’t disagree with them, but that’s not the issue. The issue here is whether that wording verbatim from the FSM is accurate or not. I think that wording isn’t accurate. Here it is again for reference.
The fuel cap check valve is provided to keep the pressure in the fuel tank constant. When the pressure in the fuel tank becomes positive and reaches its specified value, it opens the valve to allow vapor to flow into the canister. On the other hand, when the pressure in the fuel tank becomes negative, and reaches its specified value, it opens to allow airflow into the fuel tank.
The check valve that paragraph refers to – in both sentences – isn’t in the canister. It’s the check valve in the gas cap, the fuel cap check valve. The fuel cap – that is what I remove to refill the gas tank. It’s not in the canister.
It doesn’t make sense for the fuel cap check valve to be open both when the tank pressure is negative AND positive.
I think the fuel cap check valve is just a simple a one way air valve, which opens when the fuel tank pressure becomes negative to allow air to flow into the tank, replacing gas used by driving. And the fuel cap check valve closes when the tank pressure become positive, because that forces the fumes toward the canister, where it is captured in the charcoal and not allowed to re-enter the fuel tank by the canister check valve. But the canister check valve isn’t mentioned in the paragraph in question.
So I think the way the accurate wording as it should have appeared is
The fuel cap check valve is provided to keep the pressure in the fuel tank constant. When the pressure in the fuel tank becomes positive and reaches its specified value, it closes the valve to allow vapor to flow into the canister. On the other hand, when the pressure in the fuel tank becomes negative, and reaches its specified value, it opens to allow airflow into the fuel tank.
You may be right. The wording does sound a bit “off” now that you explain it that way.
Perhaps that’s what happened when they converted it from Japanese? Perhaps because it opens the path through the canister to the port next to the fuel cap the proper Japanese terms translate to “fuel cap check valve”?
The important thing is to understand how it works and what function it performs. That’s why I’m such and advocate for understanding why things do what they do. Words confuse my simple mind. 
And all this time I thought my obsession with understanding how things work was a compensating behavior for my horrible memory!
I completely agree with you TSM that the key to being able to repair something is understanding how it is supposed to work, when it is working correctly. That’s why it is so disconcerting to see the functional description of something so simple as the fuel cap check valve incorrect in the factory service manual.
Thanks for taking time to read through my posts. I think we’ve got to the bottom of this at last!
George, it has been a pleasure.
@“the same mountainbike” , thanks for the pictures of evap system. I wonder about the benefits of reduced gas fumes versus the complexity (and additional cost and additional repairs).
Environmentalists and regulatory agencies couldn’t care less about cost vs. benefit. Their perspective is that cost is irrelevant. We’ve had very lengthy debates before on that particular point, but the fact remains that environmental regulations aren’t made with cost/benefit considerations in mind.
My personal feeling is that the EPA is out of control, and often slaying a gecko when there are dragons all around them, but they haven’t asked me.
I recently heard a self-proclaimed (and obviously poorly educated in science) tree hugger saying to a reporter that unless we significantly reduce CO2 emissions from automobiles we’ll be contributing to a “greening of the earth” (his words). My first thoughts were ‘does he mean that deserts will become covered with forest, like they were millennia ago? Does he mean that the prior predictions that all the trees were being killed off is totally wrong?. What’s wrong with reforestation of the deserts?’ Scientists have found evidence all over the planet that many areas of desert were long ago forested. They’ve found fossils of aquatic life in the middle of deserts.
It isn’t my intent to open up an environmental debate again. I apologize if that happens. I’m only responding to the comment about cost vs. benefits of current emission control mandates for automobiles, and adding a few points as to why I feel the EPA is out of control. I realize that others disagree.
Unfortunately, the environmental arena is not the only area in which cost vs benefit is not adequately evaluated. You could make well-reasoned arguments in taxes, homeland security, energy policy, and many others; and there are many other issues that local governments get involved in.
I, too, don’t want to send this discussion in this kind of direction, but I agree that we have a lot of well-meaning laws with some positive benefits that simply get taken too far out of control.