Mercedes Mullins
Air bubbles in a fuel line can be caused by a variety of factors, including pinhole leaks in the low-pressure lines from the tank to the pump, a leaking seal in the injection pump, or a poorly-balanced propeller causing vibrations and the clunk in the fuel tank to bounce and suck in air. To get rid of air bubbles in a fuel line, one can try shortening the fuel line, replacing it with a fresh piece of tubing, or using a feed pump to remove air when replacing a diesel filter.
How to get rid of air bubbles in the fuel line
| Characteristics | Values |
|---|---|
| Cause | Pinhole leaks in the low-pressure lines from the tank to the pump, a leaking seal in the injection pump, a hole in the flexible line connecting the fuel filter to the rigid line, a loose stopper in the end of the tank, a leaking fuel filter, a leaking valve on the pump, a leaking fuel tank drain plug, a split line, a stuck clunk, high-pitch vibration of the needle valve, poorly-balanced propeller |
| Prevention | Ensure the fuel line is not more than a year old, ensure the screws fixing the tank are not too tight, ensure the stopper in the end of the tank is tight enough, use a filter that doesn't come apart, use a check (a.k.a. non-return) valve near the high-pressure pump, use a remote needle valve on back of motor, use a balanced propeller |
| Solution | Nip off one inch from each end of the fuel line, replace the fuel line, replace the leaking seal in the injection pump, install a fresh piece of tubing between the filter and the hard line, use the feed pump to remove air when replacing a diesel filter, tighten the clamps on the fuel line, double-check connections on the charcoal canister and the return line to the tank, check all rubber lines where they connect to the hard lines, check the hoses starting at the tank and working forward |
What You'll Learn
Check for leaks or holes in the fuel line
When checking for leaks or holes in the fuel line, you should begin by inspecting the fuel pipes. Disconnect the battery to prevent stray sparks from igniting petrol or petrol vapour. Use a torch instead of an inspection lamp or naked lights, which could start a fire if the bulb is accidentally broken. Check the condition of all the fuel lines, connections, and seals on the filter. Pinhole leaks in the low-pressure lines from the tank to the pump can cause air bubbles, as the pump sucks in air through the hole instead of fuel. Listen for a quiet hiss when priming the line, which could indicate a hole.
You can also check for leaks by pressurising the fuel tank to about 4-5 lbs and then inspecting for leaks. Alternatively, install a fuel vacuum test tool and prime the fuel system to 10 PSI, then check for external leaks and repair if necessary. If the pressure drops quickly, remove the fuel outlet line from the filter and cap it, then remove the ignition relay and crank the engine. If air gets into the system, it will cause a false/low reading.
To check for splits or cracks, bend flexible pipes between your fingers. Push clips and metal joints to see if they are loose. Feel around joints to see if they are wet, as petrol may run along a pipe before it begins to drip. Kinks and dents in the line can also cause blockages, so check for these by blowing through the pipe, not by poking anything through it. If you suspect a blockage, free both ends of the pipe and blow through it gently with your mouth, not with a high-pressure air line, which may be dangerous and cause damage.
You can also try taking out your tank and using a fresh fuel line. Dunk the tank in a bucket of water, pinch one end of the fuel line, and blow into the other length. If you see bubbles, you have a crack in the tank.
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Ensure the fuel tank stopper is tight
Ensuring that the fuel tank stopper is tight is crucial to prevent air bubbles in the fuel line. A loose stopper can cause air leaks, allowing air to enter the fuel system and form bubbles. This can lead to reduced fuel pressure and engine performance issues, such as difficulty starting the engine when it's warm.
To prevent this, it is essential to periodically inspect the fuel tank stopper and ensure it is securely tightened. This simple maintenance step can help avoid air bubble-related problems and maintain optimal engine performance.
Additionally, it is worth noting that while a tight fuel tank stopper is essential, over-tightening the screws fixing the tank in place can also cause issues. The tank should be allowed to 'wobble' freely on its mounts to prevent bubbles in the fuel line. Therefore, finding the right balance between a secure and overly tight stopper is crucial.
Furthermore, it is recommended to regularly inspect the entire fuel system, including the fuel lines, connections, and filters, for any signs of damage or leaks. This proactive approach can help identify potential sources of air leaks and prevent air bubbles from forming in the fuel line, ensuring a smooth and efficient engine performance.
By paying attention to these details and maintaining the fuel system, you can help ensure the reliable operation of your vehicle and reduce the likelihood of air bubble-related issues.
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Check the fuel filter
If you have air bubbles in your fuel filter, there are several things you can try to resolve the issue. Firstly, check the age of your fuel line. If it is more than a year old, it is recommended to replace it. If it is relatively new, you can try trimming about an inch from each end of the line and then reconnecting it. This is because the stretching that occurs when the line is pulled on and off can weaken the tubing, leading to air bubbles.
Another potential cause of air bubbles is the inlet and outlet lines of the primary filter being at the top rather than the midsection. If there is a screw at the top, you can try pouring fuel into it after shutting off the fuel line, then re-tightening the screw and reopening the fuel line. You can then work upstream to pump fuel and any air to the secondary filter.
If you have a pinhole leak in the flexible line connecting the fuel filter to the rigid line, this can also cause air bubbles. To diagnose this, use a hand diesel primer pump and listen for a quiet hiss. If this is the case, you can fix it by installing a fresh piece of tubing between the filter and the hard line.
If you have recently replaced your diesel filter, ensure that you use a feed pump to remove air from the system. If you do not do this before the filter is full, the system will suffer from air bubbles.
Finally, if you are using a glass fuel filter, be aware that these are banned by all racing associations due to safety concerns. One user reported that their glass fuel filter broke, causing extensive damage to their vehicle.
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Shorten the fuel line
Shortening the fuel line can be an effective way to get rid of air bubbles in your fuel system. Long fuel lines can make it difficult to eliminate air bubbles once they form. By shortening the line, you can improve fuel flow and reduce the likelihood of air bubbles causing issues.
If you decide to shorten your fuel line, it is recommended to trim about one inch from each end. This helps to remove any weakened sections of tubing that may have been stretched or damaged over time. It is also suggested to inspect the entire fuel line for any leaks, cracks, or split ends, as these can be a source of air bubbles.
When shortening the fuel line, it is important to ensure that it is still long enough to function properly. The recommended length for a fuel line is generally around 8 to 9 inches. Additionally, consider using an inline filter or reservoir, as this can help to minimize the occurrence of air bubbles.
While shortening the fuel line can be beneficial, it may not always solve the problem. In some cases, air bubbles may be caused by other factors, such as a faulty pump, loose connections, or cracks in the tank. Therefore, it is important to thoroughly inspect your entire fuel system and address any potential issues to effectively eliminate air bubbles.
If you are unsure about shortening your fuel line or unable to do so, it is recommended to seek professional assistance. A qualified mechanic can assess your vehicle's specific needs and make the necessary adjustments to ensure optimal performance and minimize air bubbles in the fuel system.
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Check the screws fixing your tank
To check the screws fixing your tank, start by locating the fuel sending unit. This is usually found near the fuel tank and can be identified by its wires and connectors. Check if the unit is seated correctly in the tank and inspect it for any signs of damage or corrosion. If the top of the unit appears dirty, clean the connectors to improve conductivity.
Next, you will need to check the resistance performance of the fuel sending unit using a multimeter. Set the multimeter to Ohms and remove the unit from the fuel tank by following the instructions in your vehicle's repair manual. Once the unit is out, manipulate the float arm while measuring the resistance. Refer to your service manual to determine the specific resistance or Ohms value that the unit should be achieving.
If you need to replace the fuel sending unit, be sure to use the included washers to prevent leaks and tighten the screws in a star pattern. This will ensure that the gasket is properly aligned between the fuel sending unit and the tank. It is important to note that you should always work with fire safety in mind when dealing with fuel-related issues. Have a fire extinguisher nearby and avoid any potential sources of sparks.
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Frequently asked questions
If you have a small leak at the fuel tank drain plug, this could be the cause of air bubbles in your fuel line. Check all fuel lines for any leaks or split ends, and check the connections on the charcoal canister and the return line to the tank. If the fuel line is more than a year old, replace it. If it's fresh, nip off one inch from each end of your fuel line and try again.
If you have a diesel filter, the air should be removed by using a feed pump. If you don't apply the pump until the filter is full, the system will suffer from air.
Make sure the screws fixing your tank are not too tight. If the tank cannot 'wobble' freely on its mounts, it can cause bubbles in the fuel line.
You could have small holes in the fuel lines in the tank. The stopper in the end of the tank may not be tight enough, and it can draw in air there. Some filters leak air.
If you have a remote needle valve on the back of the motor, try attaching the needle valve to the firewall.
