Recently, a local TV station (NBC Oklahoma City) ran an article advising drivers how to conserve gas, and topping off your car gas tank was one of the things not to do. The report said if you top off, the extra gas goes into a carbon filter that will, after time, cause it to stop working, which will cause your gas mileage to drop.
The TV station was dead-on accurate, Merle. Topping off your gas tank after the pump automatically turns off can harm a vehicle emissions system that re-burns fuel vapors. The pump at your local gas station is designed to turn off when the fuel reaches a certain level, leaving room in the tank that is needed for the emissions system to operate correctly.
When that air dome is reduced from topping off your tank after the pump automatically clicks off, raw gasoline can enter one of the system's important components that's only supposed to carry vapors: the carbon filter — also called the charcoal canister.
When raw gasoline enters the canister/filter, it can spill on the ground (meaning it's not going into your engine — both a waste of gasoline and an environmental no-no) and it can disrupt engine operation, causing bad mileage, poor emissions and overall decreased engine performance. An Automotive Service Excellence representative we talked with says except for rounding to the nearest dime, never fill the fuel tank once the automatic shutoff clicks at the gas pump.
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