Injector Leak Back Test

First of all, what is injector leak back?

Leak back is the release of excess pressure in the fuel injectors that return the extra fuel back to the fuel tank. In other words, it can be thought of like a pressure check valve on the top of the diesel injector.

Sounds like a safety feature, what’s the problem?

The problem is that over time, these “check valves” weaken releasing more than just the excess pressure and returning too much diesel to the fuel tank rather than into the cylinder. This causes the fuel pump to have to work harder than normal to maintain the proper pressure in the injectors and eventually can lead to issues up to, and including, failure to start.

Ok, so now what?

Fortunately this is a simple test to perform, even during routine maintenance. Personally, I was lead to this test with the guidance from Joel of Million Mile Sprinter based on the feedback from my MBII Code Scanner that I talked about in this post.

What is needed to do this test?

Time: ~30 minutes

Cost: ~$5



  • Wire snipper (or other cutting device to cut the tube)
  • Needle nose pliers
  • Vice grips
  • 5mm allen wrench
  • Measuring device
  • Container to hold the bolts/clips

Ok, cool. Lets do it.

The first step is to remove the 7 cap screws that hold on the plastic top cover. There are 4 on the drivers side and 3 on the passengers side.

Put all the bolts in the container so they don’t get lost.

Next, carefully remove the plastic cover and put it somewhere safe (not behind you where it will get stepped on by mistake)

With the cover removed, the top of the diesel injectors are revealed.

At this point, for the test to be accurate the engine should be at operating temperature. I suggest taking the cover off while the engine is cold so it’s easier to work around and then going for a quick drive to bring it up to temperature.

With the engine hot, grab the needle nose pliers and carefully remove the clips from the top of each injector.

They should be tight but pop off easily enough.

Put them in the same container as the bolts so they don’t get lost. Then put the container in the same safe place as the top cover as they won’t be needed until reassembly.

With the clips removed, carefully pull the plastic fittings out of the top of each injector. They can tolerate a little wiggling to help them out but go easy as they can break.

All fittings removed, clamp the hose before the 5th fitting with the vice grips to prevent diesel from leaking out.

At this point, cut 5 pieces of the 3/8″ clear tube roughly 4″ in length.

Then fit them over the top of the injectors. They should be easy enough to push on but fit snugly enough not to leak.

The last step before performing the test is to locate the camshaft position sensor toward the back on the passenger side of the 5th cylinder injector.

Simply unplugging won’t do any harm but it will prevent the engine from starting when performing the test.

Finally, jump in the drivers seat and hold the key in the start position for ~10 seconds. Afterwards, the amount of diesel leaking out of the injectors can be observed in the pieces of clear tubing. In this case, all 5 of these injectors will need to be replaced in the near future. Ideally, there will be very little extra fuel (if any at all) in the tubes. The second injector from the front is definitely the worst and wouldn’t be as big of a deal if the other 4 were in better condition, however, collectively this amount of leak back is most likely the cause of my intermittent check engine light, especially since the fuel filter (another culprit) was recently replaced.

After performing the test, reassemble everything in the reverse order. Don’t forget to plug the camshaft sensor back in and don’t forget the clips that hold the plastic fittings in the top of each injector. As a note, it’s a good idea to start it and check for any leaks before putting the plastic cover back on.

I also took the opportunity to clean all the dirt and grim off the plastic cover before reinstalling it, but this is just a preference.

Obviously this test ended up revealing an upcoming repair that will be needed. If you found this useful and would like to see the future repairs, feel free to subscribe to this blog or follow me on Instagram or Facebook so you don’t miss anything.



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