How to Troubleshoot a Gas Cap That Won't Come Off

The gas cap on any car is one of the most important, yet least talked about, parts on a vehicle. In most circumstances, the gas cap can be removed and installed easily every time you fill up the fuel tank. However, there are some situations that occur where the gas cap is stuck and won't come off the vehicle's fuel tank. When this occurs, it can negatively impact the car owner's ability to put fuel inside their tank and may render the vehicle "shut down" until it's removed.

There are a few reasons why the gas cap can stick or is unable to be removed from the gas tank. The first and most obvious is that the gas cap is broken. In most cases, the broken component on the gas cap will be the lobes on the cap that secure onto the gas tank filler housing. The issue could also be caused by damage to the actual housing where the gas cap is inserted and secured.

Like any other mechanical component failure, there are a few things you can do to troubleshoot why the gas cap won't come off, as well as a few methods for removing it, so that you can replace the gas cap or repair damage to the gas tank itself. In the information below, we'll answer all three questions and provide some resources that will help you determine why that small plastic piece isn't cooperating.

Method 1 of 3: Understanding why the gas cap gets stuck and won't come off

The gas cap is designed to do two jobs. The first job is to prevent gasoline vapor or liquid from escaping from the tank. The second duty is to maintain a constant pressure inside the fuel cell. This pressure is what allows fuel to flow to the fuel pump, and eventually to power your vehicle. In the situation where the gas cap is damaged it loses its ability to maintain the seal on the fuel cell as well as reduce the amount of pressure inside the gas tank.

However, there are also situations where the gas cap fits on the fuel tank just fine – but won't come off.

In general, there are three common reasons why your gas cap might get stuck:

The gas cap is vacuum sealed: The fuel tank needs to have constant pressure inside the fuel cell in order to distribute fuel to the pump and eventually to the engine. Under many conditions, including excessive heat, dramatic changes in barometric pressure or altitude, or when the fuel tank is running very low, the gas tank can create a vacuum-like seal around the gas cap, which seals the cap tight. This high pressure situation creates a vacuum seal that makes it nearly impossible to unscrew your gas cap.

The gas cap is cross threaded: Similar in many ways to a cross threaded bolt or lug nut, a poorly threaded gas cap can also cause the gas cap to stick, making it nearly impossible to remove without tools or breaking the cap off.

Whether it's due to human error or a mechanical defect in the cap or the fuel tank housing, if the thread does not match up, you can sometimes force the cap on, just like you can force a glass jar lid on even when you’re not screwing it on correctly.

Locking mechanism is damaged: On some gas caps there is a locking mechanism which ensures that the cap can only be removed with a key. However, this lock is prone to damage due to the excessive exposure to gas vapors and excessive pressure inside the fuel tank under the situations described above.

If the lock is damaged, it can cause the gas cap to spin without coming loose.

There is debris in the threads in the fuel cell: If dirt, leaves, or any other form of debris contaminates the gas cap and gas tank threading, the cap can easily get stuck. This debris can get trapped in the threading.

When this is the situation, it creates extra pressure and friction, making the cap extremely tight which can make it very difficult to remove.

Typically, the only way to remove gas caps that experience the above issues is by breaking the cap or removing it with specialized tools. It's always a good idea to have a professional mechanic remove the broken gas cap in order to reduce the potential of damaging other components on the fuel cell.

Method 2 of 2: Inspecting the gas cap and filler housing for damage

Once you're able to remove the gas cap or a professional mechanic was able to complete this step on your behalf, it's important to determine why the problem occurred in the first place - before you buy and install a new gas cap.

In many circumstances, the problem that causes a gas cap to be stuck is caused by one of three things:

  • A mechanical defect in the gas cap
  • User error on installing the gas cap
  • A mechanical defect in the gas tank filler neck

Since there are three different causes of a gas cap that is stuck, you should complete due diligence to ensure all three areas are troubleshot for damage. This process should begin with an inspection of the gas cap itself.

Step 1: Inspect the gas cap for damage. Once the gas cap is removed from the vehicle either by yourself or a professional mechanic, inspect the a few individual components that may have caused the gas cap to stick or spin freely without being able to come off.

  • The pressure relief valve: This part is located inside the gas cap and is what allows small amounts of pressure to be released from the cap in cases where the tank is building excessive pressure. If the valve is stuck, it can cause excessive pressure to build up inside the fuel cell and make it difficult to remove.

To determine if this part is damaged, press the middle of the gas cap to see if the valve moves up and down when pressed. If it does not, this may have been the reason why the gas cap was unable to be removed without tools.

  • The gasket: Underneath the gas cap is a rubber gasket that is designed to create a seal between the base of the fuel filler neck and the gas cap. In some cases, the gasket can become stuck in between the gas cap and the filler housing. Inspect the gasket for tears, frays or other signs of being "pinched" to determine if this is what caused the gas cap to stick.

  • The gas cap threads: In many instances, the gas cap thread will be cross threaded onto the fuel cell filler housing. When this occurs, it may damage the threads. To determine if this was the case, inspect the threads for signs of scratching or any damage you'd commonly see on a cross threaded lug nut or bolt.

Step 2: Inspect the lock on a locking gas cap. On some vehicles the gas cap will have a locking mechanism that requires a key to unlock it from the fuel tank housing. If the lock is broken, the gas cap will spin freely.

When the key is inserted and turned, a set of "locks" that secure the cap to the threads on the filler housing will be released, causing the gas cap to easily be removed. When the lock is damaged, these "locks" or tumblers will fail to open; which is what commonly causes the gas cap to spin freely.

With the gas cap removed from the vehicle, insert the key into the locking mechanism and turn the key to the "unlock position." Look underneath the cap to determine if the locking bracket moves inward.

If you don't see any action on the bottom or internal parts of the gas cap, the lock was the culprit. Purchase a new locking gas cap or standard OEM gas cap and install on your fuel tank.

Step 3: Inspect the gas tank filler housing. There are some occasions when the filler housing where the gas cap is inserted may be the reason why it was stuck. Under this scenario, the filler housing will have damage or excessive debris that causes the gas cap to not thread correctly or smooth onto the gas tank.

The best way to determine if this part is the culprit is to follow these individual steps:

  • Inspect the tank filler housing for signs of excess dirt, debris, or scratch marks where the cap screws into the gas tank.

  • Look for any obstructions on the fuel neck. In many cases foreign objects like a twig, leaf or other object will find its way into the fuel neck and will obstruct the smooth installation of the gas cap. This can cause it to stick or unable to remove without tools.

If the gas tank filler housing is damaged, it will need to be replaced by a professional mechanic. This is highly unlikely but can happen in some rare cases.

Under most circumstances, replacing the gas cap on any car, truck or SUV is very easy to complete. But if you have a situation where the gas cap is too tight, spinning freely or you simply can't remove it, you might want to contact a professional mechanic for assistance. Have a professional mechanic, such as one from YourMechanic, complete a visual inspection to determine what contributed to the gas cap being stuck and repair what is damaged.

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