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 fuel injectors in my 1983 300D? (240D, 300CD, 300TD wagon, 300SD, 300SDL)

Ok, first lets make sure it's the fuel injectors and not just a clogged fuel filter. We'll also try to rule out timing, fuel injection pump issues, valve adjustment  and......

What is the car doing or not doing?

Does the engine run rough at idle, smoke, sound like the injectors are nailing (loud clacking sound) when accelerating, lack of power, shift hard or too early. Ok, these could be symptoms of many different issues but we are leaning towards injectors. 

Here is a rough test to isolate the issue. Get a 17mm wrench and start the car. Once the car is warm, loosen the fuel injector line at the top of each injector one at a time. If the car gets rougher when you loosen the line, that injector is still working, we don't know how well but at least it is spraying some fuel into the cylinder. If you loosen an injector and there is no change we know that the injector is either clogged or squirting instead of spraying.

You can replace just the offending injector but if one went bad the others are probably close behind.

We usually use Monark (German) or Bosio (Italian) injectors, we figure the cost is close to the Bosch replacement so we might as well get some added performance for our investment and time. Install them in sets, don't mix with the Bosch injectors, your car may still run rough but for a different reason.

The Monark and Bosio injectors offer a more accurate spray pattern with better atomization which translates to a more complete fuel burn and more power. You will notice a Wider power band, less smoke, smoother idle, quicker response, better fuel mileage, the list goes on. Advance your timing and increase the fuel mixture and you'll swear you have a whole new car.

Note: if you have a non-turbo engine these injectors will give you a broader more useful power band which is really needed in the non-turbo versions. This will let you move your shift points a little later in the rpm range and really improve the overall performance of you car. If you have a turbo version the improvements are even more dramatic.


Parts list:

1. Fuel injectors. 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.

2. Heat shields/crush washers. One for each injector.

3. Fuel return line. One meter cut properly will give you enough to replace the line between the injectors, the end plug and the return to the fuel tower.


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Monark fuel injectors with crush washers
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Fuel return line
Tool list:

1. 17mm combo wrench.

2. 1/2" drive x 27mm or 1-1/16" deep socket.

3. 1/2" drive Breaker bar.

4. 1/2" Ratchet

5. 1/2" x 3 to 5" Extension.

6. Wire cutter.

7. Pliers

8. Hook tool - if you don't have access to a heavy duty hook tool a flat screw driver with a blade width that will press partially into the hole diameter of the heat shields will work.

9. Torque wrench - click type preferably.


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. Use the pliers and pull the return lines off of the injectors. If you cant pull them off use the wire cutter and clip them off as close to the tip of the connector as you can without damaging the connector.

Note: Save the hose plug at the last injector so you can use it when you install the new return hose.

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17mm at pump 17mm at injectors
Pliers and wire cutter at return lines
3. Break the fuel injectors loose. The injectors should come loose fairly easily with the breaker bar. If this is the first time your injectors have been changed in 25 or 30 years it might take a little extra effort. A longer breaker bar or a pipe fitted over the end of the handle to give you more leverage may be needed.

Remove the fuel injectors by turning them counter clockwise by hand or with the 27mm socket.

4. Remove the crush washers. Check the old injectors, the washers may already be stuck to the bottom of them. If not, use the hook tool to pull the washers out of the injector holder section. Depending on the quality of your hook tool don't apply so much pressure that the tip of the hook breaks off in the pre-chamber.

If the washers don't come out easily use the screw driver to break the washer free, insert the blade of the driver into the hole in the middle of the washer and twist the driver.

Note: Do not mistake the entry hole into the pre-chamber for a stuck crush washer and damage the pre-chamber, be sure that's a washer your prying out..

Breaking fuel injectors loose with breaker bar, extension and 27mm socket.
Removing injectors

1. Start by cleaning out the base of the opening where the new crush washer will sit (nozzle plate). Use a paper towel or good shop towel to clean out any carbon or debris. Try not to let anything drop into the pre-chamber.

2. Insert the crush washers bevel side down, flat side with groove towards injector.

3. Start the threads by hand to avoid cross threading, complete with the ratchet  if needed, don't over tighten we want to use the torque wrench for the final set.

4. Set your Torque wrench to 59 ft-lbs. Torque all five injectors. Do not over tighten, 59 ft-lbs is the max, tighter is not better.

5. Replace the hard fuel lines. Start the threads by hand to avoid cross threading. 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.

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This is the bottom of the washer and should point down.

6. Install new return line hose. Cut new hose to the approximate length of the hose you removed. plus the part left on the injector if any. Install the plug at the last injector.

7. 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.

8. Check for leaks at the hard lines, injector to head and the return lines.

Note: Enjoy the ride. Now that you have installed new fuel injectors (preferably Monark or Bosio) there are a few performance tips you can use to take advantage of this high performance upgrade. Well cover these tips in a later installment.