How To Find Air Leak In Diesel Fuel System

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If you have an air leak in your diesel fuel system, then it calls for lots of concern as this can affect your engine over time. Sadly, it can be somewhat difficult to establish where the precise point of leakage is and how best to handle it, but that’s why we’re here for you.

In this post, you’re going to learn how to find air leak in diesel fuel system. Aside from that, we’ll also talk about how to bleed air out of diesel fuel system.

Sometimes it’s hard to even know if there are leaks because diesel fuel doesn’t evaporate like gasoline does; instead, it can just pile up in your system before seeping out.

Let’s get started!

Why Causes Air Leak In Diesel Fuel System?

The diesel fuel system uses pump to send diesel from tank to injector. If there is an air leak in the line, it will be difficult for the pump to push liquid into the engine. This can cause a clogged filter because of too much pressure.

The most common reason behind an air leak are cracks on lines near bends or at welding joints. If you notice that your engine is struggling to run, then the air leak could be caused by a faulty filter or a worn gasket.

The other reason for an air break can come from inadequate fuel supply (i.e., insufficient tank pressure).

There are many symptoms of diesel fuel system leaking air, which include:

  • Red smoke out of exhaust
  • Black smoke out of exhaust
  • Engine losing power
  • Etc.

What Can Happen When There’s Air Leak In Diesel Fuel System?

Why Causes Air Leak In Diesel Fuel System

A leaking diesel fuel system can cause a great deal of trouble to you because it will decrease performance and eventually lead to an expensive repair bill.

if you have these issues, it’s important to act promptly and not ignore potential air leaks in your diesel fuel system, as this can lead to a variety of problems.

These include:

  • Inferior performance by the vehicle being driven
  • Reduced gas mileage and economic efficiency because of misfuelling
  • Engine damage potentially leading to expensive repairs or replacement down the line.

While engines may not be damaged by air leaks in the diesel fuel system, they can still be harmed over time.

In order to avoid these problems in the future, you should be checking your vehicle’s air leak from time to time. So, how do you find air leak in diesel fuel system?

Let’s quickly find out!

How To Find Air Leak In Diesel Fuel System

Let’s now look at how to find air leak in diesel fuel system. A common way to find air leak in diesel fuel system is by using a “bubble test.”

Follow the steps below:

  • Fill the tank with fresh diesel.
  • Start the engine and let it idle for about an hour or more, depending on how long you want to wait.
  • Check fuel lines from pump to filter screen for bubbles rising up through them (usually near the filter screen).
  • Check engine for bubbles on surface or inside (usually near manifold) – this indicates that diesel is escaping from system.

If the air leak can’t be found, you have to remove fuel lines and remount them in a new configuration. Take care not to create any other leaks during the process as they may become obscured by the air leak.

Check for bubbles in newly configured lines as before and then examine engine for any new leaks that may have been created during this process.

If no other leaks are found, return fuel lines to original configuration and resume checking for leak with bubble test or visual inspection of pressure characteristics (deteriorating vacuum).

One possible reason why you might not have encountered an air leak is that you did not pressurize the system.

If there is any doubt, pressure can be added to the test line by temporarily installing a connector from one of the auxiliary fuel tanks and turning on all engines for about ten minutes before releasing them.

This should ensure adequate pressures in every segment of pump line.

It is also possible that you have an air leak elsewhere in your system which may require additional diagnosis.

How To Bleed Air Out Of Diesel Fuel System

Watch the video below to see how to bleed air out of diesel fuel system:

Now, below is a step-by-step instruction for bleeding air out of diesel fuel system:

  1. Check the fuel filler cap to see if it is loose or missing.
  2. If it looks ok, make sure no one has put air into your tank recently.
  3. Start the engine and let it idle for a few minutes; turn off the engine once you start hearing the hissing sound of air escaping through the valve on top of the injector.
  4. Repeat the process if you still hear a hissing sound after a few minutes of idling and turn off the engine again.
  5. Continue this until no more air can be heard escaping through the injector valve or see bubbles coming out from the fuel filler neck area on top of the tank during idle periods.

How Do You Fix An Air Leak In Diesel Fuel System?

1. Seal the leak

The first step to fixing air leak in diesel fuel system is to seal the leak through a process called machining.

This is done by carefully penetrating and then resealing any hole found in the tank with specialized tools, such as oxy-acetylene torches or other specially designed equipment to heat up areas of your fuel system where air has leaked inside.

2. Plug it up

The second step to deal with an air leak is simply plugging it up. This can be done through a process called welding, which is an operation that uses extreme heat to seal any hole found in the tank by fusing metal together on either side of the leak point.


The air in your diesel fuel system can lead to mechanical problems, which is why you must do everything possible to find and get rid of it.

We have outlined the necessary steps on how to find air leak in diesel fuel systems and how to bleed air out of diesel fuel system.

Now, take your time and apply all you’ve learned – and if you found the steps challenging, we’d suggest you take your car to your auto mechanic for the best possible diagnosis and fixes.

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