A lost gas cap is one of those issues you always discover miles down the road. You pop the fueling cover open, notice that the cap is missing and your mind starts racing back to the last time you filled up. How many miles have you been driving without a cap? Where could it be?
Don’t panic. There’s little to no chance that you did any damage to your car, and it’s quick and easy to replace a missing gas cap.
What Does the Gas Cap Do?
It might seem obvious, but the primary task of your car or truck’s gas cap is to keep fuel from sloshing out of the tank. It also helps your vehicle maintain the proper fuel system pressure by ensuring a seal at the very top of the filling hose. These two tasks are typically interlocked, because a pressurized fuel system is more likely to spill gas out of the filler hose when rounding a corner or going up or down a hill.
It typically takes some fairly reckless driving for a lost gas cap to actually lead to fuel leaking out of the top of the tank and through the filler. A more common indication that your fuel cap is missing comes in the form of a check engine light, which shows up on your dash once your car’s computer has determined that fuel tank pressure isn’t being properly maintained. Modern systems are sensitive enough to tell that the cap is missing. If you see this light appear a few miles down the road after filling up, it’s a good idea to stop and make sure that your gas cap is still there and tightly closed. Taking the time to plug in an OBD scanner will usually show a P0457 trouble code which means the computer has detected a large evaporative system leak. If this is the case, the check engine light should go out once you have replaced the gas cap and driven it a few days.
Dealing with a lost gas cap is very simple. If you find yourself miles from your nearest auto parts store, where fuel caps are available to purchase, you can quickly stop up the fuel filler with a rag. This will prevent both gas and gasoline fumes from escaping your tank until you can buy a replacement. Try to drive as calmly as possible until you have a new cap in place to keep gas from splashing around the tank and potentially up the filler neck. Sometimes, gas stations will also have a collection of caps that have been left behind by other motorists, which you may be able to fit onto your own vehicle as a temporary measure.
Lastly, you may buy a new car (or a new to you car) and be surprised at your first fill up to find that there is no gas cap. Don’t panic! Some manufacturers (like Ford and Chevrolet) have moved to a capless fuel filling system. The fuel filler port uses a small spring-loaded flap that seals the system instead of a traditional twist-on cap. You simple insert the gas station fuel filler nozzle into the hole and gas up like normal. Once you remove the nozzle, the internal spring-loaded cap seals itself. Simple.
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Photo courtesy of Wikimedia Commons.
Having been bitten by the car bug at a young age, I spent my formative years surrounded by Studebakers at car shows across Quebec and the northeastern United States. Over ten years of racing, restoring, and obsessing over automobiles lead me to balance science writing and automotive journalism full time. I currently contribute as an editor to several online and print automotive publications, and I also write and consult for the pharmaceutical and medical device industry.