Ameer Randolph
Vacuum in the fuel tank is a common issue faced by many car owners. It can cause the fuel pump to malfunction, leading to the car stalling or even shutting down. While some temporary fixes include removing and refitting the cap or driving with the gas cap loose, these solutions may worsen fuel mileage and increase the risk of fuel vapour contamination, pump blockages, and fire hazards. To resolve the issue, it is important to identify the root cause, which could be a defective gas cap, a blocked pipe, or an obstruction in the EVAP lines or between the canister and vent valve. In some cases, replacing the gas cap with a stock emissions legal gas cap or fixing the vent may be necessary to alleviate the vacuum issue and prevent further complications.
| Characteristics | Values |
|---|---|
| Cause of vacuum in fuel tank | Weight of sand and low gas level |
| Gas cap is defective | |
| Blocked pipe | |
| Vacuum on the tank pulling a higher amp load on the fuel pump | |
| Poor/non-existent air intake | |
| Obstruction in the vapor-to-canister line or between the canister and vent | |
| Charcoal in the canister breaking down and getting sucked into the evaporative emission canister purge solenoid valve | |
| Solution | Remove and immediately refit the cap |
| Get a stock emissions-legal gas cap with a one-way valve | |
| Get a big hammer and bash a nail to ease the pressure | |
| Check and replace the vent hose, evaporator canister, and canister purge valve |
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What You'll Learn
A defective fuel cap may cause a vacuum in the tank
One of the first signs of a vacuum in the fuel tank is a noticeable pressure equalization when opening the gas cap. This pressure release may be accompanied by a hissing sound, indicating that the tank is under pressure. To confirm this issue, you can perform a simple test. Drive your vehicle for a short period, and then open the gas cap. If you notice a significant pressure release and hear air rushing into the tank, it is likely that a vacuum has formed.
The cause of this issue is often a defective fuel cap. A stock emissions-legal gas cap is designed with a one-way valve that allows air to enter the tank under vacuum conditions but prevents fuel vapors from escaping under pressure. If your fuel cap is damaged or defective, it may not properly regulate the pressure, leading to a vacuum forming in the tank.
To resolve this issue, it is recommended to replace the fuel cap with a new, original equipment (OE) cap from an authorized dealer. This type of cap will have the correct check valve to ensure proper pressure regulation. Additionally, you can inspect the vent lines and EVAP components for any obstructions or blockages, as these can also contribute to vacuum formation.
It is important to address a vacuum in the fuel tank promptly, as it can lead to several issues, including fuel starvation, pump malfunction, and reduced engine performance. Some temporary solutions include driving with the gas cap loose or removing and immediately refitting the cap to release the vacuum. However, these are not permanent fixes, and a replacement cap with a functional one-way valve is typically necessary to resolve the issue completely.
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A temporary fix is to remove and refit the cap
If you are experiencing a vacuum in your fuel tank, a temporary solution is to remove and refit the cap. This method can prevent a tank vacuum, stop the car from stalling, and prevent pump malfunction. However, it is important to note that this fix increases the risk of fuel vapour contamination, pump blockages, increased consumption, poorer performance, and a potential fire hazard.
- Locate the fuel tank cap and ensure the engine is turned off and cooled down.
- Remove the fuel cap slowly and carefully. As you do this, listen for any unusual sounds, such as a sharp intake of air or hissing, which could indicate a vacuum or pressure issue.
- Inspect the fuel cap for any defects. A defective cap may contribute to the vacuum issue. A stock emissions-legal gas cap should have a one-way valve that allows air to enter under vacuum but prevents vapours from escaping under pressure.
- If the fuel cap appears damaged or faulty, consider replacing it with a new one.
- Immediately refit the fuel cap securely onto the fuel tank. Ensure it is tightened properly.
- Start the engine and observe whether the vacuum issue has been resolved. You may hear a sound of pressure equalizing, indicating that the vacuum has been released.
- Monitor your vehicle's performance and fuel consumption over the next few drives.
While this temporary fix can provide some relief, it is important to consult a mechanic or a professional for a more permanent solution to the vacuum issue in your fuel tank. They can help diagnose the root cause and provide appropriate repairs or replacements to ensure the safe and efficient operation of your vehicle.
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A vacuum pump can be used to test the EVAP system
Removing a vacuum from a fuel tank can be done in several ways, with one temporary fix being to simply remove and immediately refit the cap to release the vacuum. However, this increases the risk of fuel vapour contamination, pump blockages, and fire hazards.
There are two different ways for an OBD system to leak-test an enhanced EVAP system: pressurization or vacuum. Vacuum testing is mechanically simpler, requiring only two solenoid valves and a fuel tank absolute-pressure sensor. However, because the vacuum comes from the intake manifold, the drive cycle is very precise, so there is no guarantee the test will run. Leak-testing with pressure requires a pump and extra plumbing, but the drive cycle is less exacting.
Chrysler has been leak-testing the EVAP system with air pressure generated by an on-board pump since 1996. This has the advantage of a simpler drive cycle because manifold vacuum is only used to power the on-board LDP, which is controlled with a solenoid valve. The PCM opens a vacuum solenoid valve inside the LDP, admitting vacuum pressure to the top of the LDP diaphragm. As the diaphragm moves, filtered air is drawn into the chamber below it. When the PCM closes the solenoid valve, the diaphragm is pushed back down, and the air is pushed through an exit check valve and into the EVAP system through the canister.
There is also a method of leak-testing an EVAP system using test conditions that are very easy to meet, called Natural Vacuum Leak Detection (NVLD). This method was introduced by DaimlerChrysler in 2002 and has been adopted by other manufacturers for new models. NVLD can detect small leaks with great accuracy because the test runs with the vehicle parked and the engine turned off.
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A poor/non-existent air intake can cause a vacuum
A poor or non-existent air intake can cause a vacuum in the fuel tank. This is a common issue faced by car owners, and there are several ways to identify and resolve the problem.
One way to identify a vacuum in the fuel tank is by observing the pressure equalization when opening the gas cap. If there is a hissing sound or a release of pressure, it indicates a vacuum or pressurization in the tank. This issue can be caused by various factors, including a blocked breather pipe, a faulty gas cap, or an obstruction in the EVAP system.
To resolve the issue, it is recommended to check the breather pipe for any blockages and clear or replace it if necessary. As a temporary solution, removing and immediately refitting the gas cap can release the vacuum. However, this increases the risk of fuel vapor contamination, pump blockages, and fire hazards. Therefore, it is advised to replace the gas cap with a stock emissions legal gas cap that has a one-way valve. This valve allows air to enter under vacuum conditions while preventing fuel vapors from escaping under pressure.
Additionally, it is important to inspect the EVAP system for any obstructions, particularly in the vapor-to-canister line or between the canister and the vent valve. Disassembling and cleaning the components, including the vapor line and fuel cut valve, can help identify and resolve the issue.
In some cases, the vacuum may be caused by a possible electrical short in the system, keeping the evap solenoid open constantly. It is recommended to test the system thoroughly and consult a professional if the problem persists.
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A blocked vent return could be the cause
Symptoms of a blocked fuel tank vent include poor performance, stalling, and difficulty starting the engine or filling up the tank. These symptoms are similar to those of a clogged fuel filter, as both issues prevent fuel from reaching the engine. However, you can distinguish between the two by checking both the fuel tank vent and the fuel filter. The fuel filter is usually easily accessible and can be examined for blockages.
To check for a blocked fuel tank vent, you must first locate the air vent line. Then, use a compressor to shoot air into it. If the vent is unblocked, you should hear a gurgling sound. If the vent is blocked, there will be no sound. You can also try blowing through the canister vent line that is attached to the gas tank. If there is a blockage, you will be able to detect it due to the short line.
If you suspect a blockage, you can try to clear it using compressed air or a wire or plastic tubing inserted through the vent. It is important to exercise caution when checking your fuel tank vent to avoid damaging the tank or allowing gas to escape into the air. Additionally, ensure that your vent is mounted vertically, as mounting it at an angle may cause the vent to shut off.
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Frequently asked questions
A vacuum in the fuel tank can be caused by various factors, including a defective gas cap, a blocked vent line, or a faulty evaporator system.
When you remove the fuel filler cap, listen for a sharp intake of air or a hissing sound, indicating pressure equalization. You may also experience starting problems, such as the engine cranking but not firing up.
A vacuum in the fuel tank can lead to fuel starvation, where the pump is unable to draw fuel. It can also cause the fuel pump fuse to blow due to a higher amp load. Additionally, a strong vacuum may damage the tank.
Here are some possible solutions:
- Check and replace the gas cap with a stock emissions legal cap that has a one-way valve.
- Inspect and clear any blockages in the vent line, EVAP lines, or hoses/lines related to the evaporator system.
- Replace the evaporator canister if it is faulty.
- Ensure the charcoal canister is functioning properly to trap gas vapors.
- Refer to specific car model forums, such as Nissan, IH8MUD, Car Talk Community, Chevy Impala SS Forum, or MotorhomeFun, for additional troubleshooting tips and common issues.
