Why you shouldn't over-fill your gas tank
First let's talk about what your EVAP system is, why car manufacturers introduced it, what it does and how it works...
What is an EVAP system?
EVAP is the automotive industry abbreviation for evaporative emissions systems. Designed to control the release of raw fuel vapour into the atmosphere.
In the past before emissions laws started playing a major role in how cars and trucks are manufactured, excess fuel vapour in your fuel tank would have just been vented into the air. In the 80's car manufacturers started introducing EVAP systems to help control that. If you have had a car older than 1994 you have probably seen what looks like a metal or plastic coffee can underneath your hood.
These were the major component of early design EVAP systems, known as charcoal canisters. Newer cars are much more advanced in the way they control fuel vapour emissions and they have created much more pleasant looking EVAP canisters that are usually placed under the vehicle near the fuel tank.
Modern day EVAP systems have several components to them. We will describe what each of them do as we explain how the system works. Your fuel system is a sealed system, some of you may have experienced a check engine light coming on because your fuel cap was not tightened or installed properly, that is because of a fuel tank pressure sensor that is constantly looking to have a slight negative pressure (or vacuum). If your fuel cap isn't sealing the system, it will most likely turn your check engine light on.
During normal operation in a properly sealed EVAP system excess fuel vapour that is in your fuel tank will be drawn into a charcoal canister. This is accomplished by opening a canister vent valve. The vent valve opens when there is excessive fuel vapour pressure in the fuel tank. When the canister vent valve is opened it draws fuel vapour into the charcoal canister, where it is stored.
So what happens to the stored fuel vapour? Easy! There is a hose between the EVAP canister and the intake for your engine. Intake manifolds are under constant vacuum while your engine is running, so there is another valve in your EVAP system called a purge valve. The purge valve opens when there is higher engine load (not idle) and uses the vacuum in your intake manifold to suck the stored fuel vapour out of your charcoal canister, which is then mixed with the other air coming into your engine and burned inside of your engine during normal operation.
I know what you're thinking...
WHAT DOES THIS HAVE TO DO WITH OVER-FILLING YOUR GAS TANK!!!
When you're at the fuel pump and the pump clicks off... STOP!
If you continue to try and get that extra couple of dollars into your tank, instead of fuel vapour being absorbed into your charcoal canister you can actually fill your charcoal canister with liquid fuel. The problem with that is, your EVAP system is not designed to handle liquid fuel (only vapour). This can cause a variety of problems in your EVAP system, and also can cause your engine to run rich, or set other engine codes that are very time consuming to diagnose.