Fuel Tank And Evaporative Emissions

Component Operation
Canister - Contains activated charcoal to absorb EVAP (Evaporative Emissions) generated in fuel tank.
Cut-off valve Located in fuel tank. - Valve floats and closes when fuel tank is 100% full.
Purge VSV (Vacuum Switching Valve) -Opens or closes line between canister and intake manifold. ECM uses purge VSV to control EVAP purge flow. In order to discharge EVAP absorbed by canister to intake manifold, ECM opens purge VSV. EVAP discharge volume to intake manifold is controlled by purge VSV duty cycle ratio (current-carrying time) (Open: ON; Closed: OFF).
Refueling valve - Controls EVAP pressure from fuel tank to canister. Valve consists of diaphragm, spring and restrictor (diameter: 0.08 inch). When fuel vapor and pressure inside fuel tank increase, valve opens. While EVAP is purged, valve closes and restrictor prevents large amount of vacuum from affecting pressure in fuel tank. Valve is opened while refueling.
Roll-over valve Located in fuel tank. -Valve is closed by its own weight when vehicle overturns to prevent fuel from spilling out.
Soak timer. - Built into ECM. To ensure accurate EVAP monitor, measures 5 hours (+-15 min) after engine switch is turned off. This allows fuel to cool down, stabilizing EVAP pressure. When approximately 5 hours have elapsed, ECM is activated.
Canister pump module. - Consists of (a) to (d) below. Canister pump module cannot be disassembled.
(a) Vent valve. - Vents and closes EVAP system. When ECM turns valve ON, EVAP system is closed. When ECM turns valve off, EVAP system is vented. Negative pressure (vacuum) is created in EVAP system to check for EVAP leaks by closing purge VSV, turning on vent valve (closed) and operating leak detection pump.
(b) Canister pressure sensor. - Indicates pressure as voltages. ECM supplies regulated 5 V to canister pressure sensor, and uses feedback from sensor to monitor EVAP system pressure.
(c) Leak detection pump. - Creates negative pressure (vacuum) in EVAP system for leak check.
(d) Reference orifice. - Has opening with 0.02 inch diameter. Vacuum is produced through orifice by closing purge VSV, turning off vent valve and operating leak detection pump, to monitor reference pressure. Reference pressure indicates small leak of EVAP.

Fuel Tank Venting - Over Filling

So how much fuel do you put into your tank? Do you stop when it first clicks, two clicks, or do you wait till the gasoline comes shooting out? Are you possible damaging your car, ie Evaporative Emissions System, that might result in an expensive repair? Myth or Truth? http://www.club-tc.com/forums/general-discussion/8744-overfilling-gas-tank-good-bad-idea.html
Straightforward answer is "No", atleast not for the modern Toyota brands. It will not damage your motor, evaporative emissions, or clog the canister. It is still not a good idea to overfill your tank as the fuel will end up splashing back out of the fuel filler soaking you and your paint job. This portion of the article is meant to shed light on mechanical workings of the Evaporative Emissions System and how it is designed to prevent such calamities.

Here is the detailed explanation according to the Scion Technical Repair Manual.

fig. 1

Component Operation
Canister Contains activated charcoal to absorb EVAP (Evaporative Emissions) generated in fuel tank.
Cut-off valve Located in fuel tank. Valve floats and closes when fuel tank is 100% full.
Purge VSV (Vacuum Switching Valve) Opens or closes line between canister and intake manifold. ECM uses purge VSV to control EVAP purge flow. In order to discharge EVAP absorbed by canister to intake manifold, ECM opens purge VSV. EVAP discharge volume to intake manifold is controlled by purge VSV duty cycle ratio (current-carrying time) (Open: ON; Closed: OFF).
Refueling valve Controls EVAP pressure from fuel tank to canister. Valve consists of diaphragm, spring and restrictor (diameter: 0.08 inch). When fuel vapor and pressure inside fuel tank increase, valve opens. While EVAP is purged, valve closes and restrictor prevents large amount of vacuum from affecting pressure in fuel tank. Valve is opened while refueling.
Roll-over valve Located in fuel tank. Valve is closed by its own weight when vehicle overturns to prevent fuel from spilling out.
Soak timer Built into ECM. To ensure accurate EVAP monitor, measures 5 hours (+-15 min) after engine switch is turned off. This allows fuel to cool down, stabilizing EVAP pressure. When approximately 5 hours have elapsed, ECM is activated (refer to fig. 3).
Canister pump module Consists of (a) to (d) below. Canister pump module cannot be disassembled.
(a) Vent valve Vents and closes EVAP system. When ECM turns valve ON, EVAP system is closed. When ECM turns valve off, EVAP system is vented. Negative pressure (vacuum) is created in EVAP system to check for EVAP leaks by closing purge VSV, turning on vent valve (closed) and operating leak detection pump (refer to fig. 1).
(b) Canister pressure sensor Indicates pressure as voltages. ECM supplies regulated 5 V to canister pressure sensor, and uses feedback from sensor to monitor EVAP system pressure (refer to fig. 2).
(c) Leak detection pump Creates negative pressure (vacuum) in EVAP system for leak check.
(d) Reference orifice Has opening with 0.02 inch diameter. Vacuum is produced through orifice by closing purge VSV, turning off vent valve and operating leak detection pump, to monitor reference pressure. Reference pressure indicates small leak of EVAP.

fig. 2

The cut-off and roll-over vents close once the once the fuel tank is at 100% capacity. This prevents fuel from backfilling up the vent to the charcoal canister. Additionally, there is a refill vent valve that opens to let fuel vapors into the Canister Filter (Atmosphere) which let's the excess vapors to vent.

fig. 3

You can see the refill bleeding vent located on the inside, top of the filler neck. It's purpose is to allow the air displaced by the incoming fuel into the tank equalize and prevent "burping".

Note: It does not vent to atmosphere, but rather back into the fill neck where it either joins the flow of fuel into the tank or is captured by the vapor recovery system of most fuel station pumps (where applicable by law).

The fresh air line is connected to the top of the filler neck itself. It is always open to atmosphere even with the fuel cap on. The small hole just below the filler neck directs any spilled fuel (coming out of the pump nozzle/refill line) in the refill compartment to drain overboard. There is a notch at the bottom edge of the surround to drain any liquids, under normal conditions as in water intrusion from rain or carwash. As you can see from it's small aperture, it is not meant to accommodate large amounts of gasoline nor is the Purge VSV meant to pass any appreciable amounts of liquid fuel as it is introduced into the intake manifold as unmetered fuel causing a temporarily rich condition.

Note: Modern fuel pumps at gas stations have vapor recovery systems that capture this vapor where it condenses back to fuel in the storage tank. You will notice this as the boot on the gas pump filler nozzle.

This posted from Member mtnbk is from a more updated manual than mine. It provides a clearer picture. For the sake of time, I have just included his post.