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Jun 16, 2025 - 04:49 PM
Replacing the fuel injectors and related parts on a Mercedes-Benz 300SE (W126, M103 engine) is a doable DIY job if you’re moderately handy. Here’s a complete walkthrough to guide you through it safely and effectively--
Parts You’ll Likely ReplaceFor a thorough job, replace these:
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Fuel injectors (x6)
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Injector seals/O-rings (upper and lower)
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Injector holders/sleeves (if brittle or damaged)
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Fuel distributor lines (optional, if leaking or rusted)
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Injector clips (if yours are corroded or missing)
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17mm open-end wrench (for injector lines)
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10mm socket & ratchet
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Torque wrench (optional but ideal)
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Needle-nose pliers
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Pick tool (for O-rings)
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Vacuum or shop towels (to catch fuel drips)
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Penetrating oil (for stuck parts)
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Safety glasses & gloves
1. Disconnect Battery
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Fuel can spray—disconnect the negative terminal to be safe.
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Loosen the fuel distributor lines slightly (engine cold!) to bleed pressure, or crank the engine with the fuel pump relay pulled.
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Unclip and remove the entire air filter assembly to expose the intake manifold and fuel lines.
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Use your 17mm wrench to loosen the metal fuel lines leading from the distributor to each injector.
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Be gentle—lines can be brittle with age.
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The injectors are seated in plastic holders that push into the intake.
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Use pliers or a small pry bar to gently pull the holders out.
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If brittle or cracked, replace them.
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Pull the injectors out of the holders.
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Some will come out with the holder; others may stick.
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Be careful not to drop anything into the intake ports.
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Install new O-rings and seals on the new injectors.
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If using new holders, pre-lube the seals with a little engine oil or fuel-safe grease for easier insertion.
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Press the injectors into the holders (or holders into the intake first, then injectors).
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Ensure they seat flush and evenly.
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Carefully rethread the metal fuel lines onto the injector tops.
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Do not cross-thread!
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Snug them up but don’t overtighten.
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Reverse your earlier steps, reconnect everything.
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Prime the fuel system by turning the key to ON (not start) a few times.
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Check all six injectors for any sign of leaks.
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It's often a good idea to replace all 6 injectors at once—if one’s bad, the others aren’t far behind.
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The job usually takes 2–3 hours, or a bit more if you're new to it.
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Watch out for cracked plastic injector holders—many 300SEs still have the originals.
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Keep a fire extinguisher nearby just in case (working with fuel).
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