Introduction

The secondary, larger, spin-on fuel filter on the W123 diesel engines does the heavy lifting when it comes to filtering. It keeps small particles in the fuel from damaging the injection pump and the injector nozzles.

It does a good job of this, but over time it can be come clogged. Eventually, fuel flow will be limited to the point where it affects power output. Change it before that happens. Consider changing it at least every other year, depending on how much you drive. And definitely replace it after you perform a diesel purge.

Shown here is the large secondary fuel filter on a W123 diesel. It looks similar to oil filters you may have seen on other cars.
  • Shown here is the large secondary fuel filter on a W123 diesel. It looks similar to oil filters you may have seen on other cars.

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Begin by loosening the large bolt on the top of the filter housing. There are two styles. This one is a later style, that has a 24mm bolt. An earlier style uses a 22mm bolt. You'll need to verify yours before beginning.
  • Begin by loosening the large bolt on the top of the filter housing. There are two styles. This one is a later style, that has a 24mm bolt. An earlier style uses a 22mm bolt. You'll need to verify yours before beginning.

  • Once the bolt begins to loosen, you'll need to use your other hand to hold the filter.

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Once the  bolt is fully removed, you can now carefully remove the old filter. Be aware that fuel is at the very top of the filter and may spill if you tip it.
  • Once the bolt is fully removed, you can now carefully remove the old filter. Be aware that fuel is at the very top of the filter and may spill if you tip it.

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Inspect the bolt. On the newer style, as pictured, there are two different sizes of o-rings to seal the bolt. One the earlier style, there is only one o-ring. On this bolt, the o-rings are old, brittle, and flattened. They need to be replaced, there are o-rings in most metric o-ring kits that will fit.
  • Inspect the bolt. On the newer style, as pictured, there are two different sizes of o-rings to seal the bolt. One the earlier style, there is only one o-ring.

  • On this bolt, the o-rings are old, brittle, and flattened. They need to be replaced, there are o-rings in most metric o-ring kits that will fit.

  • On the early style housing there is also a copper crush washer that goes under the head of the bolt, in place of the second o-ring. This should be replaced with every filter change.

  • See the new o-rings installed here.

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Fill the new filter with fuel. Diesel, or anything else that will burn safely in a diesel engine, such as virgin motor oil, virgin vegetable oil, clean automatic transmission fluid, or some diesel purge.
  • Fill the new filter with fuel. Diesel, or anything else that will burn safely in a diesel engine, such as virgin motor oil, virgin vegetable oil, clean automatic transmission fluid, or some diesel purge.

  • Doing this will reduce the amount of fuel system priming that is necessary.

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Re-install the filter, following this guide in reverse order.
  • Re-install the filter, following this guide in reverse order.

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Prime the system using the primer pump until the fuel pressure release valve opens. You'll be able to tell by the squeaking noise from the injection pump.
  • Prime the system using the primer pump until the fuel pressure release valve opens. You'll be able to tell by the squeaking noise from the injection pump.

  • Now is also a good time to replace the clear plastic primary fuel filter. If you'd like to do this, and need guidance, see the primary fuel filter replacement guide.

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Conclusion

After finishing, run the engine to ensure it was properly primed. If it doesn't start after 30 seconds of cranking, stop and prime some more.

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Nicolas Siemsen

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3 Comments

Using Two people One can operate the starter while the other operates the hand pump until the engine begins to run---then close off the hand pump and starter switch. gary

gvonneida - Reply

This is a very clean and concise manual. Nice Work!

To gvonneida and the general public

You could do that but it would be unnecessary as this a gravity fed injection pump, and there isn't any low pressure/lift fuel pump. So there is nothing gained from the 2nd person/engine cranking. Just use the hand pump to prime the high pressure injection pump, the engine will start no problem.

Dirk McGirt -

Dirk, just to correct something, there is a low pressure lift pump on these engines. That is what the primer pump is sitting on top of. However, you are correct that generally it is unnecessary to crank the engine to help with starting after you replace the fuel filters especially if you pre-fill the secondary large filter. Just use the pump until you hear the bypass opening (the squirting/wooshing type sound) and you know your fuel system is full of fuel.

Nicolas Siemsen - Reply

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