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What are the risks on the evaporative emissions system of your car when topping off your tank?

I've done a fair bit of searching, but this is the best I've found. Emission Control Systems: Evaporative Controls (http://www.familycar.com/CLASSROOM/emission.htm#EVAPORATIVE%20CONTROLS) briefly describes the concept of a charcoal canister (aka evaporative emission canister, fuel vapor canister, vapor storage canister, etc.).

Hazards At The Gas Pump (http://www.garage-pak.com/pat-goss-washington-post/1997-wp-hazards-at-the-gas-pump.html) is the best description I've found of why topping off is bad. I still see that some people do it here for the extra distance on a tank. What's to keep this from damaging the charcoal canister?

On my car, there's a vapor control valve on top of the tank, and vapors pass through this before reaching the canister. There's also an evaporative canister vent shut valve on the canister itself. I have a shop manual, but there's not any information on what activates either of these valves. I guess the fuel tank pressure is a factor, but that wouldn't necessitate two valves. I want to make sure topping off won't damage anything before trying it.

I know Wayne and others do this regularly and don't notice any negative performance impact, but how do you check for emissions impact?

Could there be a valve that closes when the fuel level rises above it, but opens when the fuel is below it, thus allowing just vapours to the charcoal? Any diagram i've seen of the system was just a schematic and didn't show much detail.

ollie

I used to top off with as much fuel as I could until I saw what was in the fill neck of my neon.
The fill tube on the Neon went through the wheel well. Open to all the sand and salt from the road. The tube had rusted through. When repairing the tube I noted the location of the evaporation line, just were the end of the filling nozzle goes to when filling.

Seeing the location of that hose, I could see why they tell you not to over fill.

There was some type of valve, and it may be there to try keep the fuel form flowing back down that vent tube. But why push it?
I’m going for MPG not Miles to tank. With that in mind I see no advantage to the top off fill, only a possible disadvantage.
In fact I am thinking not filling each time, only putting in say ½ a tank. Keeping the weight down.

If someone wants to over fill, I doubt there is much chance of damage, as I nevver damaged anythhing. so I never yell at anyone for doing it. just don't do it my self.