Is the gas cap warning light active on the dashboard?

The gas cap seals the gas vapors inside the gas tank filler neck to prevent leaking. But if you accidentally leave the gas cap off or the seal on it gets old or broken, those gas vapors can escape. When that happens, a warning light called the gas cap light will pop on the dashboard, telling you something is wrong with the gas cap.

Does your vehicle carry a strong fuel smell after you refuel it?

The fuel tank in modern vehicles is made of molded plastic. Over time, due to heat, the plastic fuel tank seams can become brittle and develop cracks. When these cracks occur on the top side of the tank and you fill-up the gas tank, it creates pressure that forces the gas vapors to escape through the cracks in the upper part of the tank. This is what causes the smell of gas when fueling your vehicle.

Is the vehicle difficult to start after refueling it?

The purge control valve has an important job: it controls how much vacuum is created in the evaporative emission system to remove the gas vapors that gather in the charcoal canister. But if the purge control valve gets stuck open while fueling the vehicle, it allows raw gas vapors to flow into the engine intake. This can flood the engine, making it difficult to start the car immediately after you've filled it with gas.

Does the vehicle leaves the garage smelling like fuel?

The charcoal canister is an essential part of the vehicle. It's like a container filled with special charcoal that helps to trap and hold onto the gas vapors that might leak from the gas tank. The canister is made of plastic and filled with charcoal and filters. The different parts are joined together by melting the plastic. However, these melted ligaments can develop cracks over time and with temperature changes. When that happens, the gas vapors can escape from the canister before they get absorbed by the charcoal. This is what causes the smell of gas in the garage.

Is the check engine light active on the dashboard?

The leak detection pump, a part of the vehicle's system, has an important job. It uses a unique diaphragm to check for any vapor leak in the system. This pump is connected to the charcoal canister and is sealed with a rubber seal. However, because of heat and the way the charcoal canister is made, it can sometimes get warped, making it difficult for the rubber seal to fit correctly. A leak in this seal, even a very small one, triggers the check engine light to turn on.

Do you notice a fuel smell by the capless fuel door?

The filler neck on modern vehicles uses a capless system, which means it doesn't have a traditional gas cap. Instead, it has a unique mechanism that can open and close to access the fuel tank. However, as time goes by and the vehicle is used, this system can develop issues that cause gas vapors to leak out. When that happens, you might notice a gas smell near the fuel door area.

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Home / Symptoms / Evap System Leak

Four common causes for a vehicle EVAP system leak and their related parts.

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Is Your Vehicle's Evap System Leaking?

The EVAP system leak is due to a problem with the gas cap; check if the cap is loose or cracked. EVAP stands for Evaporative Emissions Control Systems, and any vapor or gas leak in this system is picked up by sensors, leading to a warning light on the dashboard. Apart from a faulty gas cap, the issue might also relate to a crack in the fuel tank, charcoal canister, filler neck, purge valve, etc.

Paying attention to the signs and taking action can save you some headaches and money in the long run. So, if you catch a whiff of gasoline or that "Check Engine" light starts acting up, don't brush it off. Your Evap system might just be trying to get your attention.

Watch out: An Evap leak is when your vehicle's Evaporative Emission Control system, which is responsible for keeping fuel vapors in check, develops a minor breach. So instead of those vapors staying where they should, they sneak into the air.

The most common causes for a vehicle’s EVAP system leaking

The most common causes for the car’s EVAP system leaking:

Cracked fuel tank: An EVAP leak indicates anything in the system that’s allowing vapor and gases to escape, including fuel vapor. If the fuel tank has a crack, fuel and related vapors will leak out. The vehicle’s sensors pick that up and trigger a warning light.
Leaking purge valve: The purge valve redirects vapors to the engine’s combustion chamber at the right times to help with combustion. However, if the purge valve is faulty, instead of redirecting these vapors properly, it might allow leaks, causing vapors to escape.
Cracked charcoal canister: A charcoal canister, also known as an EVAP canister, stores fuel vapors from the fuel tank. It’s called a charcoal tank because it has activated charcoal, which is ideal for absorbing vapors. However, if the charcoal canister cracks, it’ll allow these vapors to escape through the gaps.
Leaking detection pump: A detection pump, known as a leak detection pump (LDP), constantly monitors the EVAP system searching for vapor leaks. It works by pumping pressure in the system and seeing if this pressure lowers quickly; a quick loss of pressure means a crack. However, if the pump fails, it might lead to an EVAP warning light without actually having a leak problem.
Leaking filler neck: The filler neck is the tube or pipe that connects the fuel cap area (where you insert the gas nozzle) to the fuel tank of a vehicle. However, if the filler neck is faulty, like if it develops cracks, fuel refill might not be as efficient. Additionally, fuel vapors will leak through these cracks, triggering an EVAP warning light.
Loose/poor gas cap: A loose or bad gas cap, although a simple thing to fix, can cause an EVAP warning light. This is because the gas cap is sealing the fuel vapor inside the gas tank. Therefore, if the gas cap isn’t sealing properly (if it’s loose or damaged), fuel vapors escape, triggering the EVAP light.

Frequently Asked Questions

Is the gas cap warning light active on the dashboard?

The gas cap seals the gas vapors inside the gas tank filler neck to prevent leaking. But if you accidentally leave the gas cap off or the seal on it gets old or broken, those gas vapors can escape. When that happens, a warning light called the "gas cap light" will pop on the dashboard, telling you something is wrong with the gas cap.
Does your vehicle carry a strong fuel smell after you refuel it?

The fuel tank in modern vehicles is made of molded plastic. Over time, due to heat, the plastic fuel tank seams can become brittle and develop cracks. When these cracks occur on the top side of the tank and you fill-up the gas tank, it creates pressure that forces the gas vapors to escape through the cracks in the upper part of the tank. This is what causes the smell of gas when fueling your vehicle.
Is the vehicle difficult to start after refueling it?

The purge control valve has an important job: it controls how much vacuum is created in the evaporative emission system to remove the gas vapors that gather in the charcoal canister. But if the purge control valve gets stuck open while fueling the vehicle, it allows raw gas vapors to flow into the engine intake. This can flood the engine, making it difficult to start the car immediately after you've filled it with gas.
Does the vehicle leaves the garage smelling like fuel?

The charcoal canister is an essential part of the vehicle. It's like a container filled with special charcoal that helps to trap and hold onto the gas vapors that might leak from the gas tank. The canister is made of plastic and filled with charcoal and filters. The different parts are joined together by melting the plastic. However, these melted ligaments can develop cracks over time and with temperature changes. When that happens, the gas vapors can escape from the canister before they get absorbed by the charcoal. This is what causes the smell of gas in the garage.
Is the "check engine" light active on the dashboard?

The leak detection pump, a part of the vehicle's system, has an important job. It uses a unique diaphragm to check for any vapor leak in the system. This pump is connected to the charcoal canister and is sealed with a rubber seal. However, because of heat and the way the charcoal canister is made, it can sometimes get warped, making it difficult for the rubber seal to fit correctly. A leak in this seal, even a very small one, triggers the check engine light to turn on.
Do you notice a fuel smell by the capless fuel door?

The filler neck on modern vehicles uses a capless system, which means it doesn't have a traditional gas cap. Instead, it has a unique mechanism that can open and close to access the fuel tank. However, as time goes by and the vehicle is used, this system can develop issues that cause gas vapors to leak out. When that happens, you might notice a gas smell near the fuel door area.

Typical fixes to address the cause(s)

These are typical processes and repair services that fix a check engine light issue

The most common cause: Faulty gas cap

The most common reason for a car EVAP leak is due to a faulty gas cap. The gas cap is a sealed “door” between the fuel inside your vehicle and the outside environment. Apart from protecting the liquid fuel, the gas cap also prevents the vapors inside the fuel tank from escaping the vehicle. These vapors are also needed for combustion; therefore, if the gas cap is loose or racked, allowing fuel vapors to escape, EVAP sensors shoot a warning light to the dashboard.

Symptoms it causes: Apart from the EVAP warning light you might notice longer stopping distances, squealing noises, vehicle pulling to one side, grinding sounds, etc.
Medium risk of priority: A loose, faulty, or missing gas cap allows fuel vapors to escape the vehicle, which can lead to bad fuel economy, combustion issues, and stress on combustion components.

How exactly does the gas cap go faulty?

It can happen for many and even random reasons. Most commonly, the gas cap can become cracked or loose due to age, since the seals around it can become dry and crackly over time. However, as has happened to the writer telling you this information right now, you might refuel the car in a fuel station and leave without the gas cap; this undoubtedly gets the EVAP light on.

Go safe: Ask for a mechanic to inspect your vehicle’s emission system to investigate if the gas cap is faulty due to surrounding components and if there’s further damage to check out. Many shops do transparent vehicle inspections and lay out your options after investigating your vehicle’s conditions.

What’s a vehicle inspection? It’s “detective work” on your vehicle, checking its systems to see if anything prevents the car from working as expected. In this case, mechanics examine different parts of your vehicle to track if anything is causing the gas cap to wear out. They’ll also check the condition of components during their inspection, noting which are causing the problem and which got affected by it, recommending to replace them in order of priority.

Transparency example in a loose gas cap service

An emission system inspection is the most transparent process to understand why your vehicle’s gas cap is worn. This is because many components surrounding the gas cap, fuel tank, and filler neck can have a say in the issue, indicating the real culprits that caused it all.

Let’s read an example of how this kind of inspection helps to service a loose gas cap.

When the customer dropped off their vehicle, they expressed concern about the check engine light being on. They mentioned one instance where they forgot to put the gas cap on, resulting in the light coming on. They quickly remedied the situation by securing the cap and having another shop clear the light. However, the light remained active despite the gas cap being correctly placed.

During the test drive, the technician verified the presence of the gas cap light. As part of the vehicle health inspection, the technician further confirmed that the gas cap was tightly secured.

A diagnostic scan was performed to investigate the issue, revealing a code indicating a detected problem with the evaporative emission system, specifically a p0457 code.

Upon receiving authorization for additional diagnostics, the technician removed the gas cap and utilized a filler neck tester to check for any leaks in the tank and vapor system. Next, a vehicle-specific scanner was employed to assess the situation further, and the vent valve was energized to seal off the vapor system.

The pressure was applied using a smoke machine, but no leaks were detected. Finally, upon closer inspection, the technician discovered that the gas cap's seal was damaged and needed replacement.

Loose/poor gas cap cusing sensors to detect an EVAP system leak and trigger the check engine light

In this case, the mechanic put the “Immediate Action” tag on the gas cap.

Similar Symptoms

Below are just a few examples of typical Symptoms and Fixes your car might be experiencing

Four common causes for a vehicle Leaking Oil and their related parts.

Is Your Vehicle's Evap System Leaking?

The most common causes for a vehicle’s EVAP system leaking

Frequently Asked Questions

Is the gas cap warning light active on the dashboard?

Does your vehicle carry a strong fuel smell after you refuel it?

Is the vehicle difficult to start after refueling it?

Does the vehicle leaves the garage smelling like fuel?

Is the "check engine" light active on the dashboard?

Do you notice a fuel smell by the capless fuel door?

Typical fixes to address the cause(s)

The most common cause: Faulty gas cap

How exactly does the gas cap go faulty?

Transparency example in a loose gas cap service

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