How do I......
 Do-It-Yourself Guides
1. How do I change the glow plugs in a Mercedes 300D, 300SD or 240D.
2. How do I change the fuel injectors in a Mercedes 300D, 300SD or 240D.
3. How do I change the alternator in a Mercedes 300D, 300SD or 240D
How do I change the glow plugs in my Mercedes 300D? (240D, 300CD, 300TD wagon, 300SD)

Ok, first lets make sure it's the glow plugs and not the relay or fusible link or......

What is the car doing or not doing?

The engine turns over as fast a usual, starts on the second or third try, starts right up if I cycle the glow plugs two or three times. After its warm, starts right up.

Sounds like glow plugs, lets get started. We always replace the glow plugs as a full set, even if you only have one or two that don't work, replace them all because the others aren't far behind.

Parts list:

1. Glow plugs. Make sure you have the correct quantity and type for your car. 240D,TD-4 units, 300D,CD,TD and 300SD-5 units thru 1985. We only use Bosch or Monark brand plugs. Off brand plugs don't last as long and straight edge plugs like AC Delco can swell at at the tip making them fail and almost impossible to remove. The Bosch are inexpensive and last as long as anything we've used.

Series style glow plugs thru 1979 (except SD only to 1977) will have a thick wavy solid bare metal wire connecting the plugs together. These have looped tips and a larger thread with a 17mm head.

Parallel style 1980 thru the 90's will have a standard looking insulated wire with connectors on the ends. These are pencil style fast glow type with a smaller thread and a 12mm head.

Note: A Bosch upgrade kit is available to modify the older style series system to the newer more reliable parallel system with a large thread pencil type.

2. Anti seize lubricant


Pencil style Fast Glow 12mm thread
bosch loop style glow plug
Loop style Series 17mm thread
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Pencil with 17mm thread
Tool list:

1. 12mm combo wrench - a 12mm ratchet box wrench will speed this job up quite a bit but is not required to do the job. Change these wrenches to 17mm if working with large thread style glow plugs.

2. 17mm combo wrench

3. 8mm combo wrench

4. Needle nose pliers or needle nose vice grip pliers if possible. (this is to allow you to hold the insulator at the rear of the 8mm nut to avoid damaging the wire if it wants to spin too far.)

5. Glow plug pre chamber reaming tool. 

Removal :  Read this through before you start.

1. Remove the fuel injector hard lines with the 17mm open end. Depending on the body of your wrench you will have to tilt the wrench at an angle to break it loose. Keep the lines together as a set and place them in a clean area. Cover the injection pump outlets with a towel or tape them over to keep debris out.

2. Remove the 8mm nuts that hold the wires to the glow plugs. The wires have a tendency of spinning when you turn the nut. If they spin too far the wires can become damaged. Avoid damaging the wire and connector by holding the insulator and connector with the needle nose pliers while you break the nut loose. Don't lose the nuts.

With the series style plugs make notes of how the resistance wires and insulators go together so you wont have any questions when you have to put them back on. Lay them out on the bench like they were on the car. Incorrect assembly can cause the fuse to blow and the plugs to heat improperly. 

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17mm at pump 17mm at injectors
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8mm with pliers at glow plug
3.Remove the glow plugs by turning them counter clockwise with the 12mm (17mm for series) box wrench, this is where the ratcheting box wrench speeds things up.

4. Pull out the old glow plugs. Usually the old glow plugs will pull right out with a little effort, however if the carbon build up is too great they can become stuck. Grab the plug with a pliers or vise grip and twist as you pull, use a little leverage if you need too, but make sure you have the plug turned all the way out so you don't damage the threads.

5. (Optional) If you have a reaming tool clean the excess carbon from the pre-chamber using the reamer and brush. Insert the reamer in until it bottoms out then remove it and run the brush in the hole. This step will extend the life of your glow plugs and help them heat the chamber more efficiently.


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12mm box at glow plug
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12mm ratcheting box wrench at glow plug

1. Apply a small amount of anti-seize compound to the threads of the glow plugs before you install them. Start the threads by hand to avoid cross threading and turn them in as far as you can, should be two or three turns. Finish the tightening sequence a wrench, cinching them down about as tight as you would a spark plug, do not over torque.

2. Now install the wires, the injector lines and anything else you removed in the process. Note: don't lean on the hard vacuum line that goes back to the brake booster while your working in the engine compartment. Most of the lines are 30+ years old, brittle and easily damaged.

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Glow plug with Anti-Seize
 note small amount on plug
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3. Tighten the injector line nuts at the pump, if you tightened the nuts at the injectors, back them off a turn or two so we can bleed the air out of the lines.

4. Get in the car, hold the fuel pedal down and crank the engine over until you can see fuel coming out at the injectors (30 seconds at a time then let the starter cool down between cranks). Once you have fuel coming from all lines tighten the nuts at the injectors. Start the car and  keep it running while the rest of the air clears out of the lines.

If your car started easily it should start fine tomorrow morning when everything has cooled off. If your car doesn't start easily we have to look a little deeper, but that's for another installment.

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17mm at pump
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17mm at injectors