Vacuum And Pressure: Fuel Tank's Unseen Forces

The presence of a vacuum or pressure in a fuel tank is a common issue faced by vehicle owners, and it can cause problems with the engine's performance. When a fuel tank is under vacuum, it can lead to issues such as bubbles in the injection pump, causing the engine to misfire or stall. On the other hand, excessive pressure in the fuel tank can also lead to problems. Understanding and resolving these issues is crucial for optimal vehicle performance and safety.

Blocked tank vent port

A blocked tank vent port can cause a range of issues with your vehicle, and it is important to be able to recognise the symptoms of this issue. The purpose of a vent is to relieve system pressure and equalise the pressure with the surrounding environment. Fuel is constantly expanding and contracting as it warms up and cools down, and a blocked vent port can cause a build-up of pressure or vacuum in your tank.

Some symptoms of a blocked tank vent port include reduced fluid fill volume, reduced power levels at high throttle, pump starvation, a loud fuel pump, and in extreme cases, the tank may collapse, or the fuel pump may not start or prime. You may also experience difficulty filling up your tank, or your car may stall or not start at all.

To check if your tank vent port is blocked, locate the air vent line and put a compressor into it. An unblocked vent will emit a gurgling sound, whereas a blocked line will result in 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 tell as it is a short line.

Another common problem is that wasps sometimes build nests in fuel vents, which can cause a clog. It is recommended to arm yourself with wasp spray and gloves if you suspect this may be the case.

If your tank vent port is blocked, you will need to clear the blockage and then drain and dry the tank before reconnecting the vent hose and fuel line.

Faulty fuel cap

A faulty fuel cap can cause a vacuum to form in the fuel tank. This is because the fuel cap is responsible for venting the fuel tank and preventing a vacuum from forming. If the fuel cap is faulty, it may not be able to vent the tank properly, leading to a vacuum.

There are several ways to identify if you have a faulty fuel cap. One way is to check for vacuum in the fuel tank. If you remove the fuel cap and hear a hissing sound or notice air being sucked into the tank, it indicates that there is a vacuum present. Additionally, if your vehicle is displaying issues such as fuel pressure problems, trouble starting, or poor fuel mileage, a faulty fuel cap may be the culprit.

To resolve the issue of a faulty fuel cap, there are a few options. One solution is to modify the existing fuel cap to improve its venting capabilities. This can be done by drilling a small hole in the cap, allowing air to enter and escape the fuel tank. However, it is important to note that drilling a hole may lead to fuel leakage when the tank is full. Another option is to replace the fuel cap with a new one that is specifically designed for your vehicle's fuel system. This could be a vented cap or one with a larger vent built-in, ensuring proper ventilation and preventing a vacuum from forming.

It is important to address a faulty fuel cap issue promptly as it can lead to significant problems with your vehicle's performance and fuel efficiency. A vacuum in the fuel tank can cause the fuel pump to malfunction, leading to engine issues and even stalling. By taking the necessary steps to correct the faulty fuel cap, you can ensure the optimal performance of your vehicle and prevent further complications.

Collapsed fuel tank

A fuel tank can collapse due to a vacuum pressure build-up. This can occur when the vent valve malfunctions or the charcoal canister gets clogged, resulting in the tank collapsing due to the suction power of the fuel pump.

In some cases, the wrong fuel filler cap can be used, which can also cause a vacuum to form in the tank. This is because diesel fuel caps are designed to hold a small amount of pressure but not a vacuum. If there is a vacuum in the tank, simply loosening or drilling a small hole in the fuel cap can alleviate the issue.

To fix a collapsed fuel tank, it is recommended to replace the vent valve, which is located underneath the fuel tank inlets/outlets. Additionally, ensuring proper ventilation is crucial to prevent the tank from collapsing again.

It is also important to check for any leaks in the fuel lines, as this can cause air to enter the system and lead to vacuum-related issues.

If you suspect that your fuel tank has collapsed, it is best to consult a professional mechanic to diagnose and repair the issue properly.

Blocked breather pipe

A blocked breather pipe in a fuel tank can cause a range of issues, from reduced engine performance to auto shutoff problems when fuelling the vehicle.

The breather pipe is an essential component of the fuel system, allowing air to escape from the tank and preventing a vacuum from forming. When this pipe becomes clogged, it can lead to increased pressure in the tank as vapour builds up. This, in turn, can affect the fuel pump's ability to supply adequate pressure to the fuel rail, resulting in the engine running lean and experiencing issues such as jerky, twitchy throttle and strange behaviour at certain RPM ranges.

In some cases, a blocked breather pipe may cause the vehicle to constantly shut off while fuelling due to back pressure. Additionally, the presence of a vacuum in the fuel tank can lead to air getting into the system, causing misfires and affecting engine performance.

To resolve these issues, it is necessary to unblock the breather pipe. This can be done by using compressed air to blow through the line, dislodging any blockages. Alternatively, a small-gauge plastic tube or heavy wire can be inserted into the vent pipe to check for and clear any physical obstructions, such as a wasp nest. In some cases, it may be necessary to drain the tank, dry the interior, and then retry the compressed air blow to ensure the line is clear. If the problem persists, the issue may lie with the valves, and replacement of the canister assembly or related valves may be required.

It is important to note that modifying the fuel cap to improve venting, as suggested by some, may not be a viable long-term solution and could potentially lead to fuel leakage. Therefore, ensuring the breather pipe is clear and functioning properly is crucial for optimal engine performance and fuelling.

Blocked EVAP lines

One of the most common problems with the EVAP system is a blocked or faulty purge valve, which can cause the Check Engine light to turn on and prevent the vehicle from passing an OBD II plug-in emissions test. Another common issue is a blocked vent line, which can lead to symptoms such as fuel starvation or a collapsed fuel tank. If you suspect a blockage in the vent line, it is recommended to check the tank venting by removing the gas cap and disconnecting the gas tank vent line from the EVAP canister. If the system is clear, you should be able to blow through the vent line into the fuel tank.

In some cases, a blockage in the vent line may be caused by a faulty liquid-vapor separator, which is located on top of the fuel tank or as part of the expansion overflow tank. The liquid-vapor separator prevents liquid gasoline from entering the vent line to the EVAP canister. If the liquid-vapor separator becomes clogged with debris or if the vent line becomes blocked or crimped, it can lead to issues with the fuel tank venting.

To fix a blocked EVAP line, it may be necessary to drop the fuel tank and inspect the lines for any signs of damage or blockage. In some cases, it may be possible to clear the blockage by blowing through the vent line with compressed air. If the lines are damaged, they may need to be replaced. It is also important to ensure that the gas cap is the correct type (vented or non-vented) and is tightly sealed, as a loose or missing gas cap can also trigger fault codes related to the EVAP system.

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