Your Cars Gas Cap, Is It Really That Important?

Final Words

Apart from providing a top to the fuel tank, a vehicle gas cap performs many operations to ensure a car runs smoothly. A fuel cap maintains the fuel system vacuum pressure seal allowing the pump to feed fuel to the combustion system efficiently.

A fuel cap also seals deadly pollutants and fumes in the tank and protects the environment. The EVAP processes protect vehicle passengers and the environment from the negative effects of gasoline fumes.

Finally, the gas cap protects the gas tank contents from external impurities, dirt, and water during rainy weather. When you don’t have a clean gas tank because of debris or impurities, your vehicle will experience clogging, which spoils injection systems, the fuel filter, and the pump tank. These problems cause engine malfunction and diving difficulties.


2. Fuel smell from the vehicle

Another symptom of a possible issue with the fuel cap is a fuel smell from the vehicle. If the gas cap seal becomes damaged or worn it may cause fuel vapors from the gas tank to leak from the filler neck. This may result in a gasoline odor from the vehicle as a result of the escaping vapors.

Benjamin Hunting View All

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.

What Causes A Loose Gas Cap?

A loose gas cap can be caused by a number of factors, including:

  • Loose screws or bolts on the gas cap
  • A worn-out gasket on the gas cap
  • Debris or dirt caught in the threads of the gas cap
  • Incorrect installation of the gas cap
  • Loose or corroded electrical connections on the gas cap


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.

1. Smell of fuel from the outside of the vehicle

A bad gas cap can allow fuel vapors to escape from the vehicle as it no longer seals the tank as it should. This will lead to the smell of gas around the outside of the vehicle and sometimes inside.

There are many other (more serious) reasons why fu

There are many other (more serious) reasons why fuel vapors are escaping from the fuel delivery system of your car. Here’s a full article explaining why your car smells like gas that goes into more detail.

Next Steps

If it’s obvious that the gas cap is damaged or is not tightening properly then you should start by replacing the cap with a new OEM replacement. Gas caps are cheap and you should always replace one with a genuine OEM gas cap from a dealership.

If the gas cap doesn’t look worn out and is tightening properly, then you probably have a problem elsewhere in the fuel system. If the vehicle is old (a classic with a metal gas tank and gas cap) then it’s always worth replacing the gas cap to begin with. Older metal caps don’t seal as well as newer plastic versions, so it may not be obvious if there are problems with it.

If you have a newer vehicle, it’s unlikely a bad gas cap will cause major problems with how the engine performs, and it is less likely that the gas cap will become damaged to begin with.

Why is Gas Cap Location Not Standardized?

No US government regulations address on which side of the car the fuel door should be positioned. With no legal issues, and preferences typically low on a consumer’s wish list, there’s no marketing motivation to standardize a gas cap location. This means car companies are free to design vehicles with gas tanks, and therefore gas caps, on either side of the vehicle.

Irritated motorists take note: fuel door position isn’t a random choice. If engineers need to place a mechanism on the left side of the car for the best performance, the fuel tank and door will shift to the right. Gas cap location isn’t a high enough priority to second-guess other features just to make sure the fuel door ends up on the left side. Plus, there’s not enough room – not to mention no demand – for dual gas caps.

Don’t forget about car inspections

A technician works on paperwork | John Gress/Corbi
A technician works on paperwork | John Gress/Corbis via Getty Images

Depending on which state you live in, you may have to have your car inspected every so often to ensure its compliance with your state’s laws on what makes a car roadworthy. Many things can trip an inspection and make you fail. Not having a gas cap will make your car fail inspection in some states. With some newer cars, leaving the gas cap off may trigger the check engine light, which can also affect your inspection. 

Many modern cars will light up the engine light if you drive off without securing your gas cap. Some new cars will light a “gas cap” light so that owners don’t panic over a check engine light. 

How Stuff Works points out that in some cases, the check engine or check gas cap lights might not shut off immediately after securing your gas cap. HSW says that it may take up to 100 miles of driving for your gas tank to pressurize enough to shut off the light. 


Today, there are a lot of choices out there when it comes to purchasing a new fuel tank filler cap. The only problem is that not all universal ones will fit perfectly as many people would have you believe, especially if your car has a unique design or if it doesn’t follow any tried & true standards.

So make sure to take good measurements before buying one online and remember that these caps should always be treated with care.

You may also find more tips in these articles:

Why Does The Gas Cap Light Warning Come On In My Jeep?

How To Siphon Gas Out Of A Newer Car

Why Is Gas Leaking From the Bottom of My Car?