Why Fuel Tanks Pressurize: Understanding The Science

There are several reasons why a fuel tank may become pressurized. In cars, fuel tanks are pressurized to control and capture gasoline vapors, preventing them from being released into the atmosphere. This is achieved through the EVAP system, which filters fuel vapors to prevent the gas tank from becoming overly pressurized, which could lead to hazardous situations, including fires. Additionally, the temperature of the gasoline affects the pressure in the tank, with higher temperatures resulting in higher pressure. While some pressure is normal in a fuel tank, excessive pressure can be caused by a faulty or clogged EVAP system, leading to issues such as gas leaks and engine problems.

Gasoline vapours

Gasoline is made up of over 500 different hydrocarbons, which vaporize when running, creating pressure in the tank. This pressure is entirely normal and is caused by the vaporization of the fuel. Car fuel tanks are pressurized to control and capture these gasoline vapors, preventing them from being released into the atmosphere. This system helps to reduce the amount of fuel vapors that escape, contributing to environmental protection.

The amount of pressure in the gas tank depends on the temperature of the gasoline—the higher the temperature, the more pressure there will be. The EVAP system in cars filters the fuel vapors and prevents the gas tank from becoming overly pressurized, which could result in hazardous situations, including fires. The EVAP system contains a charcoal canister to absorb excess gas from the fuel system. Over time, the charcoal canister can become clogged with excess buildup, preventing gas fumes from entering or exiting the canister, and causing a pressure buildup within the tank.

Another cause of pressure buildup in the EVAP system is when the purge valve becomes stuck open, creating a vacuum leak that affects the engine. This results in a large quantity of air entering the engine, changing the car's air-to-fuel ratio. Additionally, if the fuel vapor line becomes kinked or there is a failure in the purge control or vent solenoid, the vapor pressure in the tank will not be able to move through the EVAP system, leading to increased pressure in the gas tank.

Heat

The temperature-dependent pressure buildup in fuel tanks is usually managed by the evaporative emission control system, often referred to as the EVAP system. This system filters fuel vapours and prevents the tank from becoming dangerously pressurized, which could lead to hazardous situations, including fires.

The EVAP system can become compromised in several ways, leading to a pressure buildup in the fuel tank. One common issue is a clogged charcoal canister. Over time, the charcoal canister, which absorbs excess gas from the fuel system, can become clogged with buildup, preventing gas fumes from entering or exiting the canister. This obstruction leads to a pressure increase within the tank.

Another cause for concern is a stuck-open purge valve, which creates a vacuum leak that affects engine performance. A large amount of air entering the engine through this leak alters the air-to-fuel ratio, causing the engine to run too lean or too rich, resulting in potential misfires and reduced engine performance.

Additionally, issues with the fuel vapor line, such as kinking, or problems with the purge control or vent solenoid, can impede the movement of vapour pressure through the EVAP system. This obstruction raises the pressure within the fuel tank, leading to potential safety hazards if left unchecked.

Failed EVAP system

A failed EVAP system can cause a fuel tank to pressurize. The EVAP system, or Evaporative Emission Control System, is designed to prevent fuel vapors from escaping into the atmosphere from your gasoline tank. It captures and stores fuel vapors from the fuel tank and fuel system, which are then burned off as part of the combustion process. This system is monitored by the vehicle's on-board diagnostic system, which triggers a malfunction indicator light (MIL) to alert the driver of any issues.

There are several symptoms that can indicate a malfunction in the EVAP system, including:

• Decreased fuel economy
• Hard starting or rough idling
• Failed emissions test
• Strong smell of gasoline around the vehicle

When the EVAP system fails, it can cause the engine to start hard or run rough. This is often due to a malfunctioning purge valve that regulates the flow of fuel vapors to the engine. If the purge valve is stuck open or closed, it can cause the engine not to start at all.

Additionally, a failed EVAP system can result in higher emissions from the vehicle, which can lead to a failed emissions test. This is because the EVAP system is responsible for regulating fuel vapors and preventing them from escaping into the atmosphere.

There are several reasons why the EVAP system may fail, including:

• Loose or worn gas cap
• Faulty purge valve
• Leak in a fuel vapor line
• Foreign objects such as dust, dirt, and bugs in the EVAP system
• Faulty fuel tank pressure sensor
• Software issues in the vehicle's control module

Clogged charcoal canister

A charcoal canister, also known as the EVAP canister, is a part of the EVAP system, which is designed to control and capture gasoline vapours, preventing them from being released into the atmosphere. It is located under the vehicle, often near the fuel tank, and is responsible for absorbing fuel vapours. While it is not common for the charcoal canister to fail, issues can arise, such as a clogged charcoal canister, which can lead to a pressurised fuel tank.

A clogged charcoal canister can cause difficulties when filling up the fuel tank at a gas station. The fuel fumes will struggle to pass through the canister, resulting in high pressure in the fuel tank. This can cause the fuel hose nozzle to shut off, leading to fuel spills. Additionally, a clogged charcoal canister can result in a hissing noise when opening the fuel tank cap due to the buildup of pressure in the tank.

Another sign of a clogged charcoal canister is the check engine light illuminating on your dashboard. The EVAP system is a closed system that needs to maintain a specific pressure. If the engine control unit detects a leak or a clog, it will trigger the check engine light. To identify the exact issue, a diagnostic scanner is required to check the trouble codes.

Furthermore, a clogged charcoal canister can cause issues with emissions testing. The canister may be misfuelling the engine, leading to failed emissions tests. To diagnose this issue, you can try blanking off the fuel canister's vent pipe to observe any changes in the vehicle's emissions.

Excessive smoke from the exhaust may also indicate a problem with the charcoal canister. Although it is uncommon, a faulty canister can cause the engine to misfuel, resulting in excessive smoke. Blanking the vent pipe from the charcoal canister can help determine if it is the source of the issue.

To prevent issues with a clogged charcoal canister, it is important to avoid overfilling the fuel tank. Stop fuelling at the first dispenser click to avoid potential damage to the vehicle and the environment. Regularly checking and cleaning the charcoal canister ports is also crucial to ensure proper venting of the fuel system.

Stuck open purge valve

A stuck open purge valve is a common issue that can cause a fuel tank to pressurize. The purge valve is a crucial component of a vehicle's Evaporative Emission Control (EVAP) system, which helps to control emissions by redirecting fuel vapors from the fuel tank back to the engine. When the purge valve is stuck open, it creates a vacuum leak, allowing excess air to enter the engine and affecting the air-fuel ratio. This can lead to several issues, including:

• Rough idling: The engine will not run smoothly, especially when stopped or at low speeds.
• Misfiring and reduced performance: The excess air in the engine can cause misfires and make it difficult to accelerate or maintain speed.
• Increased difficulty in starting the engine: The vacuum leak caused by the stuck open purge valve can make it challenging for the engine to ignite.
• Poor fuel efficiency: With the purge valve failing to close properly, fuel vapors intended for combustion are released into the EVAP canister and the atmosphere, leading to increased fuel consumption.
• Failed emissions test: A stuck open purge valve can result in increased emissions, potentially causing the vehicle to fail emissions tests.
• Fuel system issues: The pressurization caused by a stuck closed purge valve can lead to fuel system problems.

To identify if the purge valve is stuck open, look for symptoms such as rough idling, misfires, reduced engine performance, and difficulty starting the engine. It is important to diagnose and address a stuck open purge valve promptly, as it can affect the vehicle's performance and emissions. Regular maintenance and inspection of the EVAP system can help prevent issues with the purge valve and ensure the vehicle runs efficiently and emits fewer harmful pollutants.

Frequently asked questions