Mercedes W123 Glow Plug, Pencil Style Replacement


  1. Mercedes W123 Glow Plug, Pencil Style Replacement, Glow Plug, Pencil Style: crwdns2935265:01crwdnd2935265:01crwdnd2935265:03crwdne2935265:0 Mercedes W123 Glow Plug, Pencil Style Replacement, Glow Plug, Pencil Style: crwdns2935265:01crwdnd2935265:02crwdnd2935265:03crwdne2935265:0 Mercedes W123 Glow Plug, Pencil Style Replacement, Glow Plug, Pencil Style: crwdns2935265:01crwdnd2935265:03crwdnd2935265:03crwdne2935265:0
    • It is highly recommended to use glow plugs made by the original OEM supplier, Bosch.

    • Some brands of glow plugs have been known to fail prematurely; others have been known to fall apart inside the pre-chamber leading to much more work to remove them.

    • When using Bosch, it's good to keep in mind that fraudulent reproductions exist on the market. Look for the proper authenticity marking. Be careful where you buy them, and look to see what country they are made in.

    • If you want to be sure, Bosch offers a way to check the code on the package against their code database.

    • There are other brands that can be used instead, but be sure to do your research.

  2. Mercedes W123 Glow Plug, Pencil Style Replacement: crwdns2935265:02crwdnd2935265:01crwdnd2935265:01crwdne2935265:0
    • To make this job much easier, consider removing your injector hard lines. It makes it easier to get the correct tools on the glow plugs for removal. For guidance on this, see the injector hard line removal guide.

  3. Mercedes W123 Glow Plug, Pencil Style Replacement: crwdns2935265:03crwdnd2935265:01crwdnd2935265:01crwdne2935265:0
    • Begin by removing the small nut that holds the wire to the top of each glow plug. Use an 8mm wrench.

  4. Mercedes W123 Glow Plug, Pencil Style Replacement: crwdns2935265:04crwdnd2935265:01crwdnd2935265:03crwdne2935265:0 Mercedes W123 Glow Plug, Pencil Style Replacement: crwdns2935265:04crwdnd2935265:02crwdnd2935265:03crwdne2935265:0 Mercedes W123 Glow Plug, Pencil Style Replacement: crwdns2935265:04crwdnd2935265:03crwdnd2935265:03crwdne2935265:0
    • You'll now be able to pull the wire off the end of each plug.

    • Not that it's likely the wire ends will be corroded or dirty or both.

    • Wipe them down with a shop towel, and then consider rubbing them with a fine grit sandpaper, steel wool, or emery cloth to remove any corrosion. You should end up with shiny metal. This will help reduce resistance and make the system work better.

  5. Mercedes W123 Glow Plug, Pencil Style Replacement: crwdns2935265:05crwdnd2935265:01crwdnd2935265:01crwdne2935265:0
    • Now loosen and remove each of the five glow plugs with a 12mm wrench.

    • Work your way from front to back, with the knowledge that you're saving the hardest for last; the glow plug on cylinder number 5, furthest at the back of the engine, is difficult to reach. Be patient...

  6. Mercedes W123 Glow Plug, Pencil Style Replacement: crwdns2935265:06crwdnd2935265:01crwdnd2935265:01crwdne2935265:0
    • Once removed you can inspect and test the glow plug to see if it was the one that was faulty.

  7. Mercedes W123 Glow Plug, Pencil Style Replacement: crwdns2935265:07crwdnd2935265:01crwdnd2935265:02crwdne2935265:0 Mercedes W123 Glow Plug, Pencil Style Replacement: crwdns2935265:07crwdnd2935265:02crwdnd2935265:02crwdne2935265:0
    • While the glow plugs are out, it's now time to ream the openings in the pre-chambers.

    • Reaming them removes excess carbon which will make the new glow plugs work better, and will help prevent premature failure.

    • It's recommended you use a reamer made for the job; it threads directly in to the glow plug opening and makes this job quick and easy.

  8. Mercedes W123 Glow Plug, Pencil Style Replacement: crwdns2935265:08crwdnd2935265:01crwdnd2935265:01crwdne2935265:0
    • To ream the cylinders, begin by coating the blades of the reamer with some sort of oil like automatic transmission fluid or gear oil. This will help the reamer pick up and pull out the excess carbon.

    • Now push the reamer in to the cylinder gently while turning it. Thread it by hand as far as you can in to the pre-chamber hole. Then, use a 12mm wrench to continue to thread it. You'll feel resistance come and go as it carves out the carbon.

    • Insert the reamer until it stops turning against the head, then back it out. Clean it between cylinders with some brake cleaner.

  9. Mercedes W123 Glow Plug, Pencil Style Replacement: crwdns2935265:09crwdnd2935265:01crwdnd2935265:01crwdne2935265:0
    • Here is what the reamer pulled out of the #1 pre-chamber at the front of the engine.

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  11. Mercedes W123 Glow Plug, Pencil Style Replacement: crwdns2935265:011crwdnd2935265:01crwdnd2935265:01crwdne2935265:0
    • Install the new glow plugs once reaming is complete.

  12. Mercedes W123 Glow Plug, Pencil Style Replacement: crwdns2935265:012crwdnd2935265:01crwdnd2935265:01crwdne2935265:0
    • Just like any project on your W123, sometimes things don't go as planned. On this car, two of the glow plugs had been rounded off by a previous owner or mechanic.

    • They then gave up and left these old plugs in there. As expected, one of the two rounded off plugs was the bad one.

    • It just took patience and some vice-grips to convince these to come out. There was so much carbon build up the "squeaked" as they rubbed on it on the way out. Reaming them made the new plugs go in easily.


To reassemble your device, follow these instructions in reverse order.


Nicolas Siemsen


38,695 crwdns2915208:0crwdne2915208:0



Master Techs crwdns2935289:0Master Techscrwdne2935289:0





I didn't know that I needed a Glow plug prechamber Reamer for this Job! I'll have to go back to the Motor-Store shop to do this job properly & to have Patience! Thank you for a clear Step by Step Guidance, much appreciated!! :)

Phillip Shaw - crwdns2934203:0crwdne2934203:0

Thanks this helped a lot.

Removing the fuel lines was key to making the job easier. Also a good ratcheting box wrench was necessary. I picked up a nice Husky brand from Home Depot, black anodized, fine ratchet.

Also cleaning out the carbon with the boring tool is another step you don't want to skip, I was surprised at how much build up came out. The build up actually made it difficult to remove the glow plug.

I had 3/5 bad and the car would start after a long crank, now I know I have 5/5. Replaced at 170K, not sure if they had been replaced before.

Jim Lewandowski - crwdns2934203:0crwdne2934203:0

Thanks for the step by step guide. All in all this was a pretty simple operation that flowed very smoothly. I did however run into an issue removing the nut and wire on the #1 glow plug at the front of the engine. The nut turned very easily, but seems to be fused to the wire, therefore it wants to twist the wire around with it and it wont break loose. I don't want to twist too much because I'm afraid I'll break the wire. I tried WD-40 and it wouldn't work, I even tried holding the wire with a pair of needle nose pliers to no avail. Any tips, tricks or ideas on what I might try??

Thanks again!

Josh Gilreath - crwdns2934203:0crwdne2934203:0

For anyone that might be reading this now or in the future. WD-40 sucks for applications like this.I would personally grab some of CRC’s Freeze Off.But if that doesn’t work use a propane or map torch. Remember not to touch the bolt.

Abdul Kelani -

Sehe gute Beschreinung.

Sie hat mir gut geholfen.

Mein 200D läuft wieder und Starten wie früher.

Vielen Dank.

Artur Pauls - crwdns2934203:0crwdne2934203:0



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