Why Does My Gas Tank Hiss When I Open It?

Suppose you are standing beside your car one sunny afternoon. Then, you are ready to fill up the gas tank. But suddenly, you hear a strange, hissing sound as you unscrew the gas cap. I guess that would be a confusing thing. In such a situation, you may think, “Why does my gas tank hiss when I open it?” Is your can wanting to communicate with you regarding something?

You may hear this sound for several reasons. Well, they can be a negatively pressurized fuel tank, an open bleeder valve, or a fixed liquid level gauge. Sometimes, hot weather can make gasoline or diesel hotter than their optimal temperature. So, when you open the tank, the excess pressure release causes hissing.

Usually, hissing is a normal occurrence – and you can stay calm. I am here to explain all possible causes of gas tank hissing. You may also learn about the indication of hissing sounds once you complete reading this blog.

Gas Tank & Role of Pressure & Gas Cap in It

A gas tank is like a human stomach. You fill it with fuel and use it as a vehicle storage unit to power the engine. However, these tanks meet the regulations for minimizing unburnt hydrocarbons released into the atmosphere (source). This is why manufacturers design them to remain sealed. Also, they ensure a low-pressure environment inside tanks.

The pressure inside the tank is an important matter. When you drive by consuming fuel, the pressure inside the tank decreases. Then, air gets drawn into the tank to equalize the pressure as the air pressure outside is higher. This is the moment or process to create a hissing noise. Usually, it ceases after 10 to 15 seconds (source).

Another essential part of a gas tank is its gas cap. It maintains the pressure in the tank by sealing it tightly. However, a leaking or broken cap can cause fuel vapors to leak from the tank. It can cause low fuel economy and environmental issues (source). Thus, excessive pressure in the gas tank can happen due to this faulty gas cap.

So, you see a strong relationship between the gas cap, internal pressure, and gas tank. If you hear hissing while opening the tank, this relationship may have issues.

Why Do Gas Tank Hiss When You Open It: Reasons & Indications

As I said, hissing sound is typical, but it may indicate different issues in your vehicle. In other words, those issues cause a hissing sound when you open the gas cap.

The pressure difference between the inside and outside of the tank is the reason for hissing. The internal pressure equalizes through the hissing. Besides, the venting system of a gas tank also changes the pressure. It cannot let the fuel vapors go outside, though it releases excess pressure.

Let’s see other causes for the gas tank hissing while releasing the pressure.

Air Rushing to Balance Internal and External Pressure

Typically, the pressure inside the gas tank is maximum when you fill it. When you drive the car, it consumes fuel and reduces the pressure inside the tank.

On the other hand, the air outside remains at a higher pressure. It rushes in the tank to equalize the pressure inside and outside. While the air passes through, you may hear hissing, which is absolutely normal.

Hot Weather

Temperature has a significant effect on gas tank pressure. So, hot weather can be a natural cause of a hissing sound. It can raise the temperature of gasoline or diesel in the tank to rise noticeably. And according to science, increased temperature makes the atoms move faster.

As a result, the fuel expands and creates excess pressure inside the tank. When you open the tank, this excess pressure goes out. And you already know that it causes hissing noise (source).

Zero Pressure Fuel Storage Systems

You may find many fuel storage systems to operate at zero pressure. The internal tank pressure drops when you burn the gas by driving the vehicle. Then, air is drawn into the tank to maintain this zero-pressure status. This process can cause a hissing sound as the air rushes in.

Evaporative Emission Control System (EVAP)

EVAP prevents gas vapors from going outside the tank into the atmosphere (source). Hence, this system performs routine checks to ensure no leaks in the fuel system. During these checks, it pressurizes the fuel system. It can sometimes cause hissing sounds when you open the gas tank.

Vapor Tube Issues

You will hear the hissing noise if there is any problem with the vapor tube or its hoses. It happens as these components can become dirty over time. Also, they may not work well and fail to remove vapors from the tank. It can lead to increased pressure inside the tank. And you will hear the noise (source).

Defective Gas Cap

The cap you use to cover and seal the gas tank can be another reason for the hissing sound. Suppose you haven’t closed the tank with the cap. Or, if the cap cannot seal the tank properly, it will cause fuel vapor to escape. It will trigger the EVAP and generate a hissing sound (source).

Additionally, a damaged or worn gas cap seal can cause fuel vapors to leak from the filler neck. You will start smelling the gasoline odor (source).

Loose Gas Cap

When the gas cap leaks, it can cause the hissing sound to release the excess pressure. It can also initiate the Check Engine Light (source). Besides, a loose gas cap can cause various issues. They can be problems with the emissions system, idling problems, misfires, and stalling (source).

Clogged Gas Tank Vent

It can be another reason! But a clogged gas tank can cause other problems. They can be difficulty filling the tank, a strong gasoline odor, reduced fuel efficiency, and even stalling or engine misfires. So, the overall thing is that if you hear hissing, it can be a sign of all those problems.

One more thing, clogged tank can increase the internal pressure of the gas tank (source).

Low Fuel Level in the Gas Tank

Did you check the level of fuel inside the tank? Even without fuel, it can still create a hissing sound. The empty space can hold the vapor that may pass through outside when you open the tank.

Is the Hissing Sound Normal?

To be precise, hissing from the gas tank while opening it is normal. But sometimes, it can be an indicator of other issues happening in the vehicle. Earlier, I discussed the reasons for this hissing sound. Now, the thing is, “do you need to worry after hearing the hissing from gas tank?”

For sure, a brief hissing sound is normal. It indicates that the systems are functioning as they should. And it is a part of the regular operation of your vehicle. So, there is generally nothing to be concerned about.

If you hear the hissing sound consistently, you may think about it. It can also be unusually loud or an indicator of other problems. Again, you may experience other symptoms like a fuel smell or decreased fuel efficiency. Some symptoms may indicate problems like a faulty venting system, a clogged vent, or gas cap.

If things remain persistent, you should call the mechanic or professional to inspect the issue.

How to Prevent Hissing When Removing the Gas Cap

You can follow some instructions to fix the hissing sound coming from the gas tank.

Regular Maintenance

You should not leave your gas tank alone! Yes, like every other part you care about, you should also take care of the gas tank. However, the maintenance includes taking care of the venting system, gas cap, and other components.

Check the Gas Cap

Of course, you must keep the gas cap in good condition. After all, it keeps the fuel inside the tank and protects the evaporation into the outer atmosphere. If it leaks, you may invent other problems in your vehicle. Again, replace the faulty gas cap immediately to prevent further issues.

Avoid Overfilling the Tank

I hardly suggest you fill the tank maximum. Overfilling the gas tank can cause excess pressure and lead to hissing sounds. So, you should follow the recommended fuel-filling guidelines for your vehicle.

Typically, you should stop fueling when the pump nozzle automatically shuts off. It allows for proper expansion space within the tank. Also, it helps to maintain the correct pressure balance.

Checking the Fuel Filler Flap and Hoses

Always check for any damage or leaks in the fuel filler flap and its connecting hoses. If you find any problem, repair, or replace the damaged components as necessary.

Be Mindful of Temperature Changes

Extreme temperature changes, especially in hot weather, can affect the pressure inside the gas tank. You can spray the tank surface with cool water from a garden hose as a temporary solution. Why? Well, the cool water can lower the temperature of the tank. Eventually, it will reduce the pressure, potentially alleviating the hissing sound.

Suppose you park the vehicle for extended periods in direct sunlight or icy conditions. It will increase the pressure inside the gas tank. So, if possible, park the car in shaded areas or use sunshades to minimize temperature fluctuations.

Final Words: Why Does My Gas Tank Hiss When I Open It?

I guess I have discussed many reasons for the hissing problem when you open the gas tank, isn’t it? You have learned about so many things relevant to this topic. Besides, I have instructed you well to prevent the hissing noise.

Still, get professional assistance if you cannot figure out what is happening to your fuel tank. They will take care of everything with their expertise and tools.

Hopefully, you have enjoyed reading this blog. Thanks for coming!

Have a nice day!

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