Sylvie Willis
A pressurised fuel tank is a common feature of modern cars, designed to control and capture gasoline vapours to reduce emissions. This vapour pressure is a natural occurrence, as the movement of the car causes the vaporisation of the fuel in the tank. The vapour pressure is dependent on the temperature of the gasoline, with higher temperatures leading to higher pressure. While some pressure is normal, a pressurised fuel tank can be dangerous if the pressure becomes too high, potentially causing fuel leaks, fuel pump problems, and even fire hazards. This can be caused by a faulty EVAP system, which controls the vapour pressure in the tank.
| Characteristics | Values |
|---|---|
| Pressure in the fuel tank | Normal |
| Reasons for pressure build-up | Vaporization of gasoline due to the movement of the car; higher temperature of gasoline; failed, clogged, or restricted EVAP system |
| Consequences of high pressure | Gas tank bulging; fuel line leaks; fuel pump problems; lack of fuel for combustion; dangerous conditions when opening the gas tank filler cap; fire hazards |
| Solution to pressure build-up | Relieve pressure by unscrewing the cap; use a fuel pressure regulator; disconnect the negative battery cable; locate and use a hand-operated vacuum pump on the fuel pressure regulator |
| Fuel tank pressurization | Helps control emissions by preventing gasoline vapors from being released into the atmosphere |
| Fuel vapors | Should be filtered by the EVAP system to prevent the gas tank from becoming overly pressurized, which can lead to hazardous situations, including fire |
| Pressure in the gas tank | Dependent on the temperature of the gasoline; higher temperature leads to higher pressure |
| Pressure when opening the gas cap | Indicates that vapors from the gas tank may not be routed back to the engine |
| Overpressure | Can occur if the EVAP system is not working correctly, i.e., it fails, becomes clogged, or restricted |
| Pressure release | Occurs when filling the tank, which depressurizes the system |
| Fuel caps | Some cars have pressure vacuum fuel caps, which vent certain air into the car gasoline track line |
| Fuel injectors | Not typically damaged by high fuel pressure unless there is debris in the gas tank |
| Fuel pressure regulator | Controls the fuel pressure in the fuel rail; a bad regulator can lead to low fuel pressure, causing an unresponsive throttle, difficulty starting the car, engine stalling, misfires, low performance, and a check engine light |
| Fuel filter | A dirty or clogged fuel filter can cause low fuel pressure, resulting in a lean fuel condition, engine misfire, poor fuel mileage, and a check engine light |
What You'll Learn
Gasoline vaporizes during movement
Gasoline vaporizes at a very low temperature of -40° Fahrenheit, which is known as its flashpoint. This vapor is heavier than the air around it, and it can collect in low places, creating a fire hazard if left in an unventilated area.
During movement, the fuel in your tank sloshes around, and this movement can cause gasoline to vaporize and build pressure in the tank. This is a common problem, and there are several potential causes. One possible cause is a clogged vapor vent tube in the tank or in the tube leading to the vapor canister. Another potential cause is a problem with the vapor lines, which may be plugged or pinched off, preventing the vapor from venting properly.
Additionally, the fuel tank is designed to run at a specific pressure, typically around 2 psi, to help prime the fuel pump. If the pressure in the tank exceeds this level, it can indicate an issue with the vapor recovery system, such as a clogged charcoal canister or a problem with the vapor lines.
It is important to address high vapor pressure in the fuel tank to prevent potential safety hazards, such as fuel leaks, fuel pump problems, and fire risks. Regular maintenance and inspection of the vapor recovery system can help identify and resolve any issues that may be causing the buildup of pressure.
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Vapor pressure builds from heat
Vapor pressure is a measure of the tendency of a material to change into a gaseous or vapour state, and it increases with temperature. The gasses released from liquid gasoline in a sealed container (the gas tank) are directly affected by the temperature of the gasoline; the higher the temperature, the more pressure builds in the container.
The gasoline in your car's fuel tank contains a mixture of over 500 hydrocarbons. The movement of the car allows gasoline to vaporize in the tank, and the higher the temperature, the more vapour is created. This vapour, or gas, builds pressure in the tank.
The temperature of the gasoline, and therefore the tank, is directly affected by the temperature of its surroundings. On a hot day, the temperature of the gasoline will increase, and so will the vapour pressure. This is why fuel tanks build pressure.
Reducing the temperature of the gasoline and the car's gas tank will reduce vapour pressure. Vapour molecules will condense and pressure will reduce.
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Failed EVAP systems build pressure
A fuel tank is designed to build some pressure under normal operating conditions. However, if the EVAP system is not functioning properly, it can lead to excessive pressure build-up in the fuel tank. The EVAP system, or Evaporative Emission Control System, is responsible for preventing the release of fuel vapors into the atmosphere. It captures and stores fuel vapors from the fuel tank and fuel system, which are then burned off during the combustion process.
When the EVAP system fails, it can cause a range of issues, including decreased fuel efficiency, hard starting or rough idling, and engine performance problems. One of the most common symptoms of a faulty EVAP system is a strong smell of gasoline around the vehicle. This is caused by fuel vapors escaping from the fuel tank and entering the atmosphere. In addition, a failed EVAP system can result in higher emissions, which may cause the vehicle to fail emissions tests.
There are several potential causes for a failed EVAP system. One possibility is a malfunctioning purge valve, which regulates the flow of fuel vapors to the engine. If the purge valve is stuck open or closed, it can affect engine performance and cause decreased fuel efficiency. Another potential cause is a clogged vapor vent tube in the tank or the tube leading to the vapor canister. This can restrict the flow of vapors and cause pressure to build up in the fuel tank.
In some cases, the issue may lie with the fuel tank pressure control valve, which is supposed to open when vapor pressure in the fuel tank exceeds a certain level. If this valve fails to open, it can lead to excessive pressure build-up. Other potential causes of EVAP system failure include burned or damaged control lines, a plugged vapor canister, or a faulty vent valve.
It is important to note that a properly functioning EVAP system is crucial for maintaining the integrity of the fuel tank and preventing fuel leaks. Excessive pressure can lead to fuel tank bulging, fuel line leaks, and even damage to the environment. Therefore, it is essential to have a qualified mechanic diagnose and resolve any issues with the EVAP system to ensure the safe and efficient operation of the vehicle.
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Evaporation of gasoline creates vapor pressure
Gasoline is highly volatile, and its vapors are flammable. When gasoline evaporates, it quickly generates a large amount of vapor from its surface. This vapor mixes with the air, forming a flammable gas zone in the immediate surroundings. If ignited, this zone can cause an explosion and extensive damage.
The vapor pressure of a liquid is the equilibrium pressure of a vapor above its liquid. The vapor pressure of a liquid varies with its temperature. As the temperature of a liquid increases, its vapor pressure also increases. Conversely, vapor pressure decreases as the temperature decreases.
The vapor pressure of gasoline is so high that its flashpoint is considerably lower than room temperature. The flashpoint of gasoline is expected to rise with the evaporation of low-boiling-point components.
The evaporation rate of gasoline is given by the equation:
N = k * A * P * e^(RT)
Where N is the evaporative molar flux (mol/s), A is the evaporative area (m^2), k is the mass transfer coefficient (m/s), P is the vapor pressure of the bulk liquid (N/m^2), R is the gas constant (8.314 J/mol K), and T is the environmental temperature (K).
In a car's gas tank, some pressure build-up is normal, but if it's enough to vent out of the gas cap, it's too high. This can be caused by a clogged vapor vent tube in the tank or a problem with the purge system.
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EVAP system failure
The EVAP (evaporative emission control system) is designed to prevent fuel vapours from escaping into the atmosphere from your gasoline tank. Fuel vapours are composed of hydrocarbon molecules (HC) which, when exposed to sunlight and oxides of nitrogen (NOx), result in photo-chemical smog.
The most obvious symptom of an EVAP system problem is the Check Engine light coming on. A common cause of this is a loose fuel cap. If the Check Engine Light remains on after checking the fuel cap, there may be a bigger problem.
Other common EVAP system issues include:
- Faulty O-ring seals
- Defective leak detection pumps
- An inoperative purge valve
- A leaking vacuum hose or vent
- Leaks in the EVAP canister
- Breakage or leakage somewhere else in the system
If you have a P0440, P0455, or P0457 code, the gas cap is likely the issue. For any other code, schedule a mechanic visit as soon as possible to address other potential causes.
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Frequently asked questions
It is normal for a fuel tank to build pressure as vapours from the liquid gasoline are released into the tank. This is caused by the movement of the car and the temperature of the gasoline.
Yes, it is normal to have some pressure in your fuel tank. Gasoline is made from over 500 different hydrocarbons, which get vaporized when running. This vapour then creates pressure in the tank.
If there is pressure when you open your fuel cap, it means that the vapours from the fuel tank may not be routed back to the engine. This could be due to a faulty EVAP system.
To release the pressure from your fuel tank, you can carefully push the valve inside the fitting using a rag and a screwdriver. Ensure the rag is there to catch any fuel that may be released. Alternatively, you can simply unscrew the cap when filling your tank.
Your fuel tank will experience a pressure build-up as vapour pressure increases from heat. Liquid gasoline will release gases into the tank, the amount of which is changed by the temperature of the gas. This is usually stabilized with the evaporative emission control system, known as the EVAP system.
