How to Tell if Your Gas Cap Is Bad?

A small part of the car that seems meaningless can actually cause a lot of engine-related problems. Each part of the car gas cap is there for a reason, and if it’s malfunctioning, it needs to be fixed as soon as possible. So how to tell if your gas cap is bad? Keep reading, and I will show you some of the most common symptoms of faulty gas caps and how to fix them.  

If you have a faulty gas cap, figuring out this problem is not complicated like replacing U-joints. If you follow my tips, you will soon know how to recognize and fix this issue. 

What Is a Gas Cap? 

A gas cap is a cover that is placed at the end of the fuel pipe through which you put fuel in the car tank. The whole purpose of a gas cap is to prevent air, debris, and dirt from coming inside the tank and prevent fuel from going out of it. In essence, a cap prevents the evaporation of fuel through the fuel neck, which can be the cause of serious hydrocarbon emissions. 

A faulty gas cap can emit 23 gallons of evaporated fuel released into the atmosphere per year. If you drive without a gas cap, the fuel will not leak immediately but in time. The biggest problem with driving without a gas cap is that dirt and debris can enter the tank and cause problems with the engine. 

Tank door opened
The gas cap is crucial for preventing air and debris from coming into your tank

Is the Gas Cap an Important Part of a Car? 

Like you can not start a car without a spark plug, the gas cap is also there for a reason. And this reason is to normalize the air pressure inside the tank – without it, the pressure can rise too much, which can cause leakage and other problems. And even though you can easily drive without a valve stem and gas cap, this is not recommended. Not only that you will fail your emission testing, but you will damage the engine injection system. 

Bad Gas Cap Can Be Dangerous 

As you can see, driving with a bad gas cap will not affect your car immediately, but if you leave this problem for too long, it may cause some serious and dangerous problems. Leakage of evaporated fuel can harm the environment, your health, and the car emissions system as well. A loose cap can cause rough idling, misfire, or even stall out.

However, if your car is misfiring, there can be other causes for this problem: bad spark plug, bad fuel pump, sensor problems, or transmission leak. But before you start checking other things, first check if your gas cap is good and sealed properly. Here are the main problems a bad gas cap can cause: 

  • Health issues – Leakage of evaporated gasoline can cause nausea, dizziness, brain damage, and heart-related problems. Driving with a gasoline smell can cause problems, and you may even faint during the drive. 
  • Environmental issues – Not only that you will not pass the emission test, but you can harm the environment with really bad gas emissions. All cars have catalytic converters and emissions systems to prevent these harmful gasses from being emitted into the atmosphere. If your gas cap is not working, these gasses will simply evaporate into the air. 
  • Mechanic issues – In time, loose gas caps can damage your vehicle’s fuel and emission system. Your car will pick up the wrong emissions readings and will try to adjust its systems to adjust using the wrong data. This can damage hoses, sensors, and other essential parts of the vehicle. 

It Can Affect Your Fuel Economy

Aside from car prices going up, we can see that the price of gas is also rising, so the last thing you need is to have a small part of your car that you can easily replace to affect your fuel economy and, most importantly, damage your car. A malfunctioning of the gas cap can influence the amount of gasoline you spend on a monthly basis.

A loose cap can also allow dirt and air to come into a tank which can lead to a reduction in power and performance problems for your engine. Also, hot air that leaks when fuel is evaporating will not only waste the fuel you put in the tank, but it can harm other expensive car parts.

Woman at the gas station
It can affect your monthly fuel budget

Symptoms of Gas Cap You Must Not Overlook

Now that you know all the problems a faulty gas cap can cause, let’s take a look at the most common symptoms you will experience if this cap is not working. Just keep in mind that some of the symptoms may be caused by other problems, so if the issue is not resolved once you replace the gas cap, make sure you pay a visit to your mechanic. Here are the most common symptoms of a bad gas cap. 

The Gas Cap Doesn’t Tighten Properly 

If you bought a used car, aside from checking the car battery, paint, used steering wheel, and other parts, you should also check if your gas cap is working properly. If the cap can not be tightened enough, it means something is wrong and that you will have leakage for sure. If this issue is not resolved immediately, you should also check emission and gas injection systems as well.

There Is a Gasoline Smell Coming From a Vehicle

We already mentioned that this can happen, and if this is the case with your car, you must address this problem immediately. Because breathing these gasses can cause serious health issues and even road accidents.

Check Engine Light Is On

It can happen that your check engine light turns on if you have a faulty gas cap. If the gasses start to leak, your car emission systems might catch that leakage, and this will eventually turn the check engine light on when the computer detects the leak. However, a lot of things can cause the check engine light to turn on, so make sure you eliminate any other issues as well. 

Emission Error Codes Appear on the Dashboard

Aside from turning on the engine light, some error messages can appear on your dashboard. If you have a check engine code reader and connect it to your car electronics, it will show you the exact codes and issues that are found. The best is to find a reader that reads your exact car model, but if you have the universal one, it will do the job. This will not fix your problem, but it will tell you what the problem is. If you don’t know how to connect a reader to your car, then take it to the mechanic store, and they will do the full diagnostics. Here are some of the common error codes in the Evaporative Emission System – P0456, P0452, P0441, P0440, and P0455.

The Car Won’t Start

No matter if you drive the best car for women or some of the best cars for off-road driving, faulty gas caps can prevent your car from starting or even shutting down during the drive, which can be dangerous. The problem is your car electronics and sensors. If your car detects leakage or emission of harmful gasses, the computer may block your engine from starting. This is usually done as a prevention measure in case you have forgotten to remove a filler nozzle at the pump.

Misfire and Idle Problems

When your air-to-fuel ratio is not balanced, you will feel something that is called insufficient combustion, which is caused by a bad gas cap. This can lead to misfires and the theft of your car. Also, a bad gas cap will lead to insufficient fuel pressure, which will, as a consequence, create problems with your car.

How to Check if Your Gas Cap Is Good?

The first thing you need to do is to physically examine your gas cap. Open the tank door on your car and then check if there are any leaks around the gas cap. Like when checking the oil or brake fluids, the best is to have white tissue you will use to wipe around the gas cap. If the tissue is dirty and wet, it means you probably have some leakage. Open your gas cap and see if there are any damages to the cap you can see with bear eyes. If you can not see any damage, maybe the problem is a bit deeper. This can indicate you have a problem with vapor hoses.  

How Long Does the Gas Cap Last?

Like anything on a car, the gas cap has an expiration date, and if your car is brand new, this date will come after 50,000 miles of driving. However, frequent use of the gas cap and the condition of the vehicle can reduce this time by far. If your car is exposed to moisture and humidity and low or extremely high temperatures, this can influence all parts of your car, including the gas cap. Also, if you have been in a car crash and this part of your car was damaged, it may be the cause of the faulty gas cap. 

car crash collision in urban street
Sometimes, it can be destroyed in a crash

How to Fix Gas Cap – DIY Version

If your gas cap is broken, the only course of action is to find a new one. When I say the new one, I don’t mean brand new – you can use the old gas cap as long as it is in good condition. However, there are things you can do before you are able to find a replacement for your gas cap, so do this only as a short time solution until you take your car to the mechanic. Here is what you can do: 

  • Replace the gas cap with another one – Measure your old gas cap to see the dimensions and try to find another similar to the one you wish to replace. If you don’t have a gas cap, you can measure the tank entrance where the gas cap is screwed in. The gas cap doesn’t need to be in the perfect measurements or fit as long as the entrance to the tank is somehow closed. 
  • Place an old rag – This is not highly recommendable, but if you don’t have any other solution, it will work. Take an old rag and place it at the beginning of the tank entrance. The tank hose has its own cap, and the rug is there to prevent debris from coming in.
  • Go to the junkyard and find a gas cap – If you don’t want to pay for a brand new gas cap, you can always visit a junkyard and find used ones. Make sure you write down the model of the car, and if possible, bring your old one to the yard to compare. 

Always Check with Mechanics Before Doing Anything Alone

As you can see, there are many things you can do alone. However, some problems with the Evaporative Emission System can be a lot deeper and hardly detectable by an inexperienced eye. My advice always is to try to eliminate some of the easy problems, and if this is not working, then take your car to the mechanic. This is the safest option you can do; otherwise, your car may get even more damaged if you try to fix it alone. Of course, if you know your way around the car and you have time to do this alone, there is no problem you can not solve.