Introduction

Diesel fuel is often a bit dirtier than gasoline. Especially at less traveled fuel stations, there's a good chance of picking up particulate matter. For this reason, the diesel W123's are equipped with a primary fuel filter with a screen inside to remove all but the smallest particles before the fuel is drawn in to the lift pump. This protects the lift pump from damage and also prevents the larger secondary filter from clogging on large particles.

Be sure to replace this if it looks dirty inside. They are clear for easy inspection. If the filter clogs, it will stop fuel flow to your engine and can leave you stranded. This is one easy item to keep a spare of in your trunk or glove box.

Here is your clear fuel primary filter. It is located directly next to the fuel primer pump.
  • Here is your clear fuel primary filter. It is located directly next to the fuel primer pump.

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Begin by loosening this hose clamp, at the top of the primary filter, nearest the engine.
  • Begin by loosening this hose clamp, at the top of the primary filter, nearest the engine.

  • Pull the primary filter up and out of this hose. A small amount of fuel will leak out, consider putting a towel, plastic bag, or other appropriate catch underneath.

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Now loosen the clamp on the bottom of the primary filter, further from the engine.
  • Now loosen the clamp on the bottom of the primary filter, further from the engine.

  • You will now be able to pull the filter out the rest of the way. The fuel in it will drain out, catch it with towel or something similar.

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Here is the old primary filter, removed from the car. Note the large chunks of debris in the filter.
  • Here is the old primary filter, removed from the car. Note the large chunks of debris in the filter.

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You can see here the new primary filter. Note that the filter is usually marked with an arrow noting the direction of fuel flow. It should be installed with this arrow pointing towards the engine.
  • You can see here the new primary filter. Note that the filter is usually marked with an arrow noting the direction of fuel flow. It should be installed with this arrow pointing towards the engine.

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Install the new filter by retracing the steps so far, just backwards.
  • Install the new filter by retracing the steps so far, just backwards.

  • This is a good time to consider replacing the large secondary filter. If you would like guidance on this, see the secondary fuel filter replacement guide.

  • Then, pump the primer pump on the car until the primary filter fills with fuel. It's normal for there to be a small air bubble in the primary filter but it should be mostly clear.

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Conclusion

When finished, run the engine to ensure it's been primed properly.

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Nicolas Siemsen

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