What Does It Mean If Your Car Has A Yellow Gas Cap?

Gas tank caps: They play an important part in your car's fuel system, even setting off a check engine light on the dashboard if they go missing or aren't screwed on correctly after refueling. Yet somehow, they're one of the least inspired-looking parts on a car. They're just utilitarian and black. Unless they're yellow -– then they have a special meaning.

Yellow gas caps serve as an alert, letting drivers or gas station attendants (looking at you, New Jersey) know that the vehicle's tank can be filled with E85, also known as biofuel or flex fuel. Yellow caps often have E85 written on them, or you'll see E85/Gasoline, as a reminder that the vehicle can run on regular unleaded gasoline, as well. 

That's extra useful to remember because not every gas station carries flex fuel in the U.S. However, the stations that do will have a corresponding yellow-handled pump that dispenses the E85 mixture, alongside the black-handled pumps with other grades of gas. 

Don't have a yellow gas cap? Put that yellow-handled pump down and back away slowly. Here's why flex fuels only belong in flex-fuel vehicles.

The yellow caps and pump handles serve to distinguish this class of fuel and its vehicles from black-gas-capped vehicles that only run on typical unleaded gasolines. What you don't want to do is accidentally put E85 in an engine that's not designed to operate on flex fuels.

E85 is typically a mix of 15% gasoline and 85% ethanol, though for a variety of reasons, that mix can range to as low as 51% ethanol. Ethanol is an alcohol made most often from plant parts such as corn, although scientists have found ways to use straw and other nonfood feedstocks. In the right car, the benefits of flex fuel include more horsepower and higher torque than the engine would generate from regular unleaded. The fuel also produces fewer greenhouse gas emissions, making it more climate friendly. And, it's less likely to cause engine knocking.  

In the wrong car, that higher alcohol content from ethanol can cause problems for your engine, including corroding the fuel system. Also, regular cars aren't tuned with the correct combustion timing and air-to-fuel ratio necessary to burn E85 effectively. This means those cars won't get the benefits of flex fuel. 

So if your car gas a black gas cap, always use the black pump handle at the gas station. If you have a yellow gas cap, go for the yellow or black pump handle. Just stay away from that green one — it's for diesel.