Overfilling the gas tank -- bad?

I vaguely recall reading on an automotive forum someplace that it is not good to overfill (top off) the gasoline tank. Something about overfilling can damage some kind of "gasoline vapor recovery system" resulting in the car not being able to pass the emissions inspection test. All my operator's manual says is don't overfill because overfilling can allow gasoline vapors to escape into the air and harm the environment. It doesn't say anything about harm to the car's emission control system.

Would appreciate it if someone can shed some light on this matter.

 

If you tried to fill the tank to the top of the fill tube you would risk running fuel into the evap line that runs to the carbon cannister that controls the escape of fuel vapors = emissions control.

Normally the fuel tank needs about 5% to 10 % percent of its total capacity left empty for the expansion of fuel as it goes from ground temperature (about 50 degrees to current outside temperature). If that buffer was filled with fuel, the expansion of fuel would force a significant amount into the evap system.

How much it would take to do damage, I don't know.

Hope this helps,

Bob

 

I don't know if it is bad, but I do it all the time.

 

As long as the filler nozzle is all the way in, it's not likely you will be able to 'overfill' the tank, as the vent is at the very top of the filler opening.

 

If you force gas back up the line into the vapor cannister it can end up in the atmosphere. It is called a Vapor cannister not a gas recovery cannister. If the manual says not to do it there is a reason. RW

 

You will know if you overfill the gas tank because it will trigger the check engine light.

 

Doesn't always trigger the light, but it IS a bad idea. Professionally I love seeing people do it at the gas stations.