There’s been some interesting discussions here of late about how the evap system handles re-fueling events in modern cars. The problem of course is the laws of physics; if you pump 10 gallons into the tank, 10 gallons of air has to come out some way or another. And it has to come out very fast to prevent the pump from shutting off prematurely.
The impression I got from the recent discussions is that with new car evap designs the air comes out 100% via the charcoal canister during re-fueling. But I noticed the other day on my 20+ year old Corolla there’s a small hole adjacent to the filler hole where I insert the nozzle. That small hole is covered air-tight by the gas cap when the gas cap is on, but is open to the atmosphere during re-filling. I’m thinking that’s where the re-fueling air comes out, rather than the charcoal canister. Or does it exit both ways? It seems like allowing that air to exit during re-fueling might contribute to air pollution. Is that vent hole under the gas cap removed in later models and the re-fueling air exits entirely through the charcoal canister?
edit: hmmm … or on my Corolla is that hole adjacent to the filler hole connected to the air exit point on the charcoal canister? So the re-fueling air in fact does exit 100% via the charcoal canister, but it just vent via that hole?
Your vehicle is OBDI. So stop thinking it’s a modern vehicle.
On OBDI vehicles, the gas pump nozzle was suppose to collect any fuel vapors while filling the tank. Remember the rubber bellows on gas pump nozzles years ago? Well, that worked if the gas stations maintained the pump nozzles. But most didn’t so gas vapors escaped into the atmosphere.
So when OBDII came out, the EPA required all vehicles to have the Onboard Refueling Vapor Recovery system. That way, you’re not relying on an outside entity to make sure no vapors escape into the atmosphere. The vehicle makes sure it doesn’t happen
I didn’t mean to imply that a 20+ year old Corolla is a modern vehicle … lol … I was just curious what that hole was for, and how the evap function which handles the re-fueling air differs for modern cars vs 20 + year old cars. Ok, so on 20+ year old OBD I cars that hole really just vents the re-fueling air directly from the gas tank? And that air isn’t scrubbed by the canister at all? And the pump is what is supposed to prevent that gas-laced air from escaping into the atmosphere? I can see that. Here is Calif we still have a complicated bellows gadget attached to the nozzle, so it should work somewhat at least.
On OBD II more modern cars, does that hole then connect to the charcoal canister instead? So the re-fueling air gets scrubbed first?
There is no hole at the filler neck for the gas tank.
The ORVR system works like this.
When you stick the gas nozzle into the filler neck and turn the nozzle on, the flow of gasoline down the filler neck creates what’s called a venturi effect. This venturi effect forces air into the gas tank pressurizing it. The pressure then forces the gas vapors into the carbon canister where they’re collected and then the air exits out the vent valve.
Then under certain conditions, the purge valve opens so the collected vapors in carbon canister can be drawn into the engine and burned.
While all this is going on, the vent valve is always open. The only time the vent valve closes is when the EVAP system does it’s self test for leaks.
So the canister vent valve on modern OBD II releases the scrubbed re-fueling air into the atmosphere directly from the port on the canister rather than piping it somewhere first?
I think you are referring to the small tube connected to the side of the fuel fill tube.
On a 2000 and newer vehicle when the tank reaches the intended full level the fuel fill control valve closes the vent to the canister then fuel will be driven up the small tube, splash across the pump nozzle turning it off.
hmmm … ok, I think I understand what you mean. What Tester above says that the canister vent valve on newer models just exits to the atmosphere right at the canister is correct, but that on the newest models – in order to prevent owner over-fueling that can damage the canister — that vent gets blocked off when the gas tank is completely full, in order to shut off the pump. Which in turn causes the gas to back up the smaller tube and turn off the pump, before the canister can get overfilled.
The canister vent and the second tube on the fuel fill tube are two different things. The second tube on the fill tube is to shut off the fuel pump nozzle on OBDI cars as well.
Some vehicles have the canister vent hose routed to a location behind the vehicles fuel door. I don’t have a picture of one but here is a drawing of one example;
Ok, confused again. Back to the Corolla, a 20 year old OBD I car. So are you saying Nevada that that small hole I mentioned adjacent to the re-fueling nozzle hole, that small hole has a second function of turning the pump off? In addition to venting the unscrubbed re-fueling air?
Edit: hmmm … I’m now thinking on a 20 year old Corolla there’s actually two tubes close to the main filler tube. One as pictured in the diagram above, which allows gasoline to flow to where the end of nozzle is, inside the main filler tube. And another which only vents re-fueling air, not to the inside of the filler tube, but to the small hole which exits to the side of the filler tube.
The small hole by the fill hole is to vent the fumes as the tank fills. As Tester described earlier, the old fill pump systems has a vapor recovery system built into the fueling system on the pump that you filled the tank with, and fumes were captured via that system. There was no system to recover hydrocarbon vapors on the car during refueling. They were simply vented with the air through that little hole under the premise that they’d get drawn into the station’s vapor recovery system. That was the system with the bellows. Nevada’s attached drawing shows that vent line clearly.
Regulations were changed to require vapor recovery systems to be on the cars themselves, and that vent hole is now simply to allow venting of the air after it’s scrubbed of hydrocarbon molecules by the charcoal canister. That vent line in Nevada’s drawing now goes through some control valves and the charcoal canister before exiting next to the fill hole.
In summary, the difference is that in the early systems the car was allowed to purge hydrocarbons with the tank air and it would be recaptured by the station’s system. In newer systems, the car itself must scrub the air of hydrocarbons before release. It uses the charcoal canister to do this.
In my car, I even discovered a charcoal bed in the induction system to prevent hydrocarbons from finding their way out after the engine is turned off. There’ll always be an intake valve open to a cylinder that was pulsing fuel but had not yet fired, and even that has to be captured. The carbon filter in the induction system is not shown anywhere on the parts lists or parts drawings… unless you have access to the Scion drawings direct from Toyota, which I used to. One of our campuses had an educational agreement with Toyota and received higher level drawing access than is publically available as well as cars, sub assemblies, and parts. Car manufacturers provide fantastic support to colleges teaching repairs of their vehicles through educational partnerships. Obligations come with these agreements, but they’re worth it IMHO.
I’m not sure if anybody else thinks so … lol. …but I find this an interesting discussion.
A few further comments/questions about the early 90’s Corolla evap design. If that hole adjacent to the fill opening is to allow air to exit the tank during re-filling, with the idea that the HC’s in the air will be captured by the pump/bellows, what is it that turns off the pump?
The shop manual doesn’t describe that hole’s purpose as venting air during re-fueling, instead it says that hole is to allows gasoline to go back into the tank during a small over-fill.
The shop manual doesn’t say so explicitly, but it seems to imply that during re-fueling at least some of air in the gas tank which gets displaced goes into the canister, and out the canister’s air port after getting scrubbed by the charcoal. So I’m currently thinking re-fueling air exits the tank two ways, the small hole next to the fill opening, and the canister air port. The confusing thing is that the drawing of the canister implies the purpose of the air port is to allow into the canister, not out.
Also the shop manual says that the top of the gas tank contains what appears to be some version of float-valve in the air breather tube port which prevents liquid gasoline from flowing out that tube and into the charcoal canister if the tank is over-filled. So now I’m curious why all cars don’t have that valve, b/c if they did it seems like the canister couldn’t get saturated with gasoline and cause all the troubles reported here by various posters. If the owner overfilled the tank, the gasoline would presumably just spill out onto the ground.
They do have that float valve, but they’re not all well designed.
I’ve attempted to attach some photos of pages describing my own system in the hopes that it will help. Let me know if you were able to open and read them.
Thanks for posting TSM, interesting. Your engine has a more sophisticated version of evap. The Corolla doesn’t have the air pump or pressure sensor business, and whatever valves it has are not controlled by the ECM, but by coolant temp in the case of the purge valve and simple pressure differences in the case of the canister check valve, whose purpose is to prevent fumes stored in the canister from getting sucked back into the tank. And my Corolla’s evap system doesn’t know when refueling is taking place, so it can’t directly handle the re-fill problem as well your engine. I presume during re-fueling some of the air goes through the canister and get cleaned and out, but some un-cleaned air goes directly out via the tank overflow hole. The English description your manual provides is considerably more clear than in my car’s FSM. I’ll try to post some of the bizarre things they say here in this thread in the future, verbatim, as it appears in the manual. Should provide a chuckle at least.