For cars with a returnless fuel system, not having the cap would cause a check engine light because of the drop in fuel pressure. Not sure what car you have, but the older cars didn't have this issue. Next would be the lighter fractions of fuel evaporating from your fuel and leaving through the filler hole, thus making your fuel burn with less energy and making fuel consumption worse. Then, you have the risk of water entering the fuel tank. If you look at how the fuel filler lid is positioned, rain water can enter between the gap and then splash down into the filler hole. Finally, if you let your car sit in the sun and have the fuel evaporate out your tank, someone standing next to your car lighting a ciggie could start a fire.




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