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Mercedes W123 Diesel Fuel Filter, Secondary Replacement

Wat je nodig hebt

  1. Mercedes W123 Diesel Fuel Filter, Secondary Replacement, Fuel Filter, Secondary: stap 1, afbeelding 1 van 1
    • 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.

  2. Mercedes W123 Diesel Fuel Filter, Secondary Replacement: stap 2, afbeelding 1 van 1
    • 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.

  3. Mercedes W123 Diesel Fuel Filter, Secondary Replacement: stap 3, afbeelding 1 van 1
    • 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.

  4. Mercedes W123 Diesel Fuel Filter, Secondary Replacement: stap 4, afbeelding 1 van 2 Mercedes W123 Diesel Fuel Filter, Secondary Replacement: stap 4, afbeelding 2 van 2
    • 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.

  5. Mercedes W123 Diesel Fuel Filter, Secondary Replacement: stap 5, afbeelding 1 van 1
    • 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.

  6. Mercedes W123 Diesel Fuel Filter, Secondary Replacement: stap 6, afbeelding 1 van 1
    • Re-install the filter, following this guide in reverse order.

  7. Mercedes W123 Diesel Fuel Filter, Secondary Replacement: stap 7, afbeelding 1 van 1
    • 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.


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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7 opmerkingen

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 - Antwoord

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 - Antwoord

Do you mean that you can install the secondary filter dry and pump the primer to fill it up?

Angel Jon Camama -

Hiya! I’m about to change both the primary and secondary fuel filter on my 300D and was curious if it would be a problem to use the priming pump to fill up the secondary filter. Thanks for this guide!

Angel Jon Camama - Antwoord

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