Ameer Randolph
Gas tanks are pressurised due to the high vapor pressure of gasoline, which causes it to easily vaporise and build up pressure when enclosed. This pressure is released when the gas cap is opened, resulting in a hissing sound and the escape of petrol fumes. While this may be alarming to some, it is a common and normal occurrence, especially in older cars. Modern cars are equipped with an EVAP system that stores fuel vapours in a canister to be burnt off in the engine, reducing emissions. However, factors such as high ambient temperatures or a faulty EVAP system can contribute to increased vapour formation and pressure build-up.
| Characteristics | Values |
|---|---|
| Noise | Hissing sound |
| Fumes | Visible vapour haze |
| Cause | Accelerated fuel evaporation due to hot ambient temperatures |
| Solution | Get the EVAP system checked for any kinks or blockages |
| Solution | Check the ECU for any error codes related to the EVAP purge valve |
| Solution | Replace the gas cap |
| Solution | Check the charcoal canister vent valve |
What You'll Learn
- The hissing sound is a normal phenomenon
- It is caused by pressure differences inside and outside the tank
- Modern cars have systems to avoid the emission of fuel vapour
- The fuel tank vapour system prevents harmful fumes from being released
- A faulty fuel level sensor can cause the gauge to show full all the time
The hissing sound is a normal phenomenon
The hissing sound is caused by air rushing into the fuel tank due to the low pressure caused by the use of fuel. This is perfectly normal and happens with just about all cars. The noise and vapours are caused by the evaporation of the fuel. The vapours are stored in a canister and then burnt off in the engine.
The hissing sound can also be caused by the fuel pump, which gets quieter when the tank is full. It is a sign that the fuel pump is tired. New or like-new fuel pumps are very quiet and never hiss.
To keep vapour formation and vapour loss at bay, it is ideal to keep the fuel tank more than half full.
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It is caused by pressure differences inside and outside the tank
Gas has a relatively high vapour pressure, which means it tends to vaporize easily and can build up pressure when enclosed in a tank. This is why you may hear a hissing sound when opening your fuel tank—it's caused by pressure differences inside and outside the tank.
Modern cars are designed to avoid the emission of fuel vapour into the environment. They allow some overpressure in the tank, which also reduces the rate of evaporation. If the pressure is too high, it is released through an active carbon filter. This is a normal phenomenon and indicates that your fuel tank vapour system is working properly.
The pressure difference can be caused by various factors, such as the volume of air inside the tank changing when hot and cold, the amount of fuel in the tank, and the temperature of the fuel. For example, a small volume of fuel in a hot tank will vaporize more quickly, leading to increased vapour pressure. Additionally, when the engine sucks petrol from the tank, air replaces it, sometimes causing a delay and a small vacuum, resulting in negative pressure.
If you notice a strong hissing sound or an excessive amount of vapour when opening your fuel tank, it could indicate a problem with your EVAP system. This system routes fuel vapours back to the engine to be burnt off instead of escaping into the atmosphere. A plugged or pinched vapour hose or faulty EVAP purge solenoid could be the cause.
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Modern cars have systems to avoid the emission of fuel vapour
Modern cars are equipped with evaporative emission control (EVAP) systems to prevent the emission of fuel vapours into the environment. This system allows some overpressure in the tank, which reduces the rate of evaporation. If the pressure is too high, it is released through an active carbon filter.
The EVAP system comprises the fuel tank, a vapor storage canister, valves, hoses, and the fuel tank gas gap. The canister, which is the focal point of the system, contains activated charcoal, an excellent medium for trapping fuel vapours. The canister absorbs and confines the vapours until the vehicle's engine starts. Once the vehicle is running, the storage canister releases the vapours through a purge valve, which is opened by the vehicle's powertrain control module (PCM). The vapours are then burnt along with the air-fuel mixture inside the engine.
The PCM is responsible for assessing the integrity of the EVAP system by running an EVAP diagnostic monitor to detect evaporative leaks. It will set a diagnostic fault code and illuminate the check engine light if leaks are detected. The PCM also monitors engine and vehicle operating conditions, fuel level, cycle times, and ambient temperature to adequately meter the stored vapour back into the engine.
The EVAP system is an active system that requires no maintenance. However, faults can develop, and the PCM monitor runs self-checks to verify airflow from the canister to the engine and check for leaks in the fuel tank, canister, fuel lines, and vents. The system will trigger a fault code and illuminate the check engine light even if a leak smaller than a pinprick is detected.
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The fuel tank vapour system prevents harmful fumes from being released
Modern cars are designed to avoid the emission of fuel vapours into the environment. The fuel tank vapour system, also known as an Onboard Refuelling Vapour Recovery (ORVR) system, is a vehicle fuel vapour emission control system that prevents harmful fumes from being released into the atmosphere.
When a fuel tank is opened, the pressure in the tank changes and vapours may be released. The fuel tank vapour system controls this pressure and captures the vapours, preventing them from escaping. This is achieved through a closed venting system that connects the storage tank to a vapour recovery unit. The vapours are then either returned to the process or destroyed, usually by oxidation.
The ORVR system captures volatile organic compounds (VOCs) during refuelling and delivers them to the vehicle's activated carbon-filled canister. These vapours are then disposed of by adding them to the engine's inlet manifold and the stream of fuel supplying the engine during normal operation. This process ensures that the harmful fumes are not released into the atmosphere.
The fuel tank vapour system is an important component of a vehicle's emission control system. It helps to reduce noxious and potentially explosive fumes and pollution. In some places, such as California and Australia, vapour recovery is mandatory by law to reduce environmental pollution.
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A faulty fuel level sensor can cause the gauge to show full all the time
It is normal to hear a hissing sound when opening the gas cap of a car. This is due to the pressure built up in the fuel tank, which is a result of the evaporation of gasoline. Modern cars are designed to allow some overpressure in the tank to reduce the rate of evaporation and prevent the emission of fuel vapors into the environment.
Now, regarding the issue of a faulty fuel level sensor causing the gauge to show a full reading all the time, here is some detailed information:
A faulty fuel level sensor, also known as a fuel sending unit, can cause the fuel gauge to malfunction and display incorrect readings. The fuel sending unit is located inside the fuel tank and is responsible for measuring the fuel level and relaying this information to the fuel gauge on the dashboard. The most common problem that causes the fuel gauge to read incorrectly is a faulty sending unit.
The fuel sending unit consists of three main components: a float, a metal rod or arm, and a variable resistor. The float sits on top of the fuel and is connected to the metal rod, which pivots as the fuel level changes. The movement of the metal rod adjusts the wiper inside the variable resistor, altering the electric current sent to the fuel gauge. This, in turn, results in the fuel level displayed on the dashboard.
When the fuel sending unit malfunctions, the gauge may exhibit erratic behavior, such as quickly changing its position or giving inaccurate readings. One specific issue is the gauge being stuck on full, even when the tank is not full. This usually occurs when the resistor sends a faulty signal to the cluster, leading to an incorrect reading.
To diagnose and fix this issue, several tests can be performed. Firstly, check for any error codes related to the fuel level sensor using a compatible scanner. Inspect the fuses and ensure there is no damage or corrosion in the wiring. Additionally, perform a self-test on the electronic fuel gauges, if possible, to determine if the issue lies with the gauge or the sending unit. If the problem persists, further inspection and repair of the fuel sending unit may be required.
In summary, a faulty fuel level sensor can indeed cause the fuel gauge to show a full reading all the time. To resolve this issue, it is important to perform diagnostics and, if necessary, replace the faulty components to ensure an accurate reading of the fuel level.
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Frequently asked questions
Yes, this is a common occurrence caused by pressure differences inside and outside the tank. It happens with almost all cars.
Gas has a relatively high vapour pressure, which means it tends to vaporize easily and even builds up pressure when enclosed in a tank. Modern cars are designed to avoid the emission of fuel vapour into the environment, so they allow some overpressure in the tank, which also reduces the rate of evaporation.
The gas cap is vented, meaning it has a check valve that only lets air into the tank, not out. The check valve has a spring that holds it closed, so it takes a little bit of vacuum in the tank before the check valve opens to let air in.
Excess pressure in the tank could be due to clogged vent lines or a clogged charcoal canister. The charcoal canister filters the fuel vapour out of the air, and the clean air is freely vented out of the other side.
If you notice a lot of pressure, check for clogged vent lines or a clogged charcoal canister. You can also try getting a new gas cap with a proper one-way vent, as a faulty cap could be the issue.
