from Goss’ Garage by Pat Goss

Cars are moving targets; nothing stays the same on them. There are hundreds of things we’ve done for years that are now forbidden and hundreds of new things we’ve never done before that we’re supposed to do now.

Even the basic task of filling a gas tank has become an exercise in restraint. As long as I can remember it’s been normal to top-off gas tanks. Topping-off crammed more gas into the tank which meant more miles between fill-ups which meant fewer trips to the gas pump. Fewer gas station visits was welcome in a busy lifestyle but you can kiss that goodbye. Fuel system changes can make topping-off expensive and even dangerous.

To understand this you have to first understand how fuel systems have changed. Today’s cars are equipped with an EVAP or Evaporative Emissions control system which includes a device called a vapor storage canister. The storage canister, as its name implies, stores fuel tank vapors (fumes). Before EVAP and storage canisters, fumes from evaporating gasoline were expelled directly into the air producing millions of tons of hydrocarbon pollution every year. As more cars were built the quantity of these emissions escalated dramatically so the Environmental Protection Agency passed laws to control them.

Now vapor canisters collect the fumes from the evaporating fuel in the car’s fuel tank and hold them until the engine is started. At the appropriate time after the engine starts the car’s computer opens a valve on the canister and lets the fumes go into the engine. Inside the engine the fumes are burned with the gasoline used for combustion. But how does this relate to topping-off?

To get fuel out of or to put fuel into your gas tank it has to breathe. In other words air has to enter the tank as the fuel level drops and leave the tank as you pump fuel into it at the gas station. This breathing occurs through the EVAP system and vapor canister. In essence these parts are the link between the fuel tank and outside air.

Topping-off is a bad idea because it can force liquid gasoline through the vent system and into your vapor canister. Although canisters work fine with gasoline vapors, liquid gasoline usually kills them. Force a couple tablespoons of liquid gasoline into the canister and you’ll probably experience things like; like rough idle, hesitation, stalling, or an illuminated check engine light.

Canister flooding can be difficult to diagnose, so crudely translated a flooded vapor canister means a big repair bill. Replacing a flooded canister and related parts can set you back several hundred dollars.

As if the inconvenience and expense isn’t bad enough, topping-off may also create the potential for fire or explosion. Depending on the location of the canister and the condition of the rubber hoses connected to it, the same liquid gasoline that floods the canister may shoot out of a broken hose right onto hot engine or exhaust parts. As you might guess hot parts and gasoline are not the ingredients of a barbecue but more likely fire or explosion. So you’re busy topping-off your gas tank at the rear of the car and feeding a growing fire on the other end of the EVAP system.

Because there’s no way to know when or to what extent any given car will be affected it’s best not topping-off your tank! If you persist in topping-off, sooner or later you’ll probably destroy a vapor canister or be front and center at an unanticipated fire.