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Diesel Injector Cleaning

on the 1987 300D (OM603)

by Jeremy5848


300D Diesel Injector Cleaning (W124, OM603)

Here’s how I cleaned my diesel injectors. You can pay a shop to do the job for you but it’s actually fairly easy to DIY. Injector cleaning is a job that doesn’t get done as often as it should (at least every 100,000 miles, maybe more often). If you have a problem with rough idle, smoking, poor fuel economy, etc., one of the things that you might want to try is to clean your injectors. Folks who experiment with alternative fuels (WVO, etc.) should clean their injectors more often. Anyone buying a car that has run WVO should ask when the injectors were last cleaned.

This is one of the injectors from my 1987 300D Turbo (W124, OM603). My engine now has a #20 head with inclined injectors; the original #14 head had different injectors. Other diesel engines will have still other injectors but they will all be similar so that you can modify my instructions to fit your injectors. Some injectors have needles with holes in their tips; my injectors don’t have that so I don’t cover that part of the cleaning process. Diesel Giant has a good DIY on his site for OM617-type injectors. Warning: don’t swap parts between injectors.

First, pull off the little return lines. Don’t cut them off; you could damage the little barb connectors and cause a leak. Next, disconnect the metal hard line at the injector. In most Mercedes engines this requires a 14 mm wrench. A crowfoot adapter for your socket set allows you to get at the connector without damaging anything. Once you have the line free at the injector, use a cord to gently tie the hard line just far enough out of the way so that you can remove the injector.

The injector has two flats on the top half of the body and a hex on the bottom. Do not put a wrench on the flats; you might cause the top half to come loose. This gets done in the next step but now is not the time. Use a deep 6-point 27 mm or 1-1/16 inch socket and make sure that it will clear the little barbs for the return lines and not damage them. Mercedes has a special socket for this but I found one at a local store that worked fine. Once you have broken the torque the injector should come out easily.

Now is a good time to fish the old heat shield ("crush washer") out of the top of the prechamber and throw it away. You’ll need a new one for each injector. Stuff a rag in the hole where the injector came out so that dirt doesn’t get into the prechamber. If you put a rubber cap on the end of the hard line you won’t lose any fuel and the engine will be easier to start when you are finished.

Now turn the injector upside down (a little fuel may leak out) and put it in a bench vice. Use sheet metal to pad the jaws of the vice so they don’t damage the injector.

You can use either a wrench or a socket to separate the two halves of the injector. A torque wrench and socket will be needed to reassemble the halves. The wrench size is the same as used to remove the injector from the engine: 27 mm or 1-1/16 inch.

Once you have loosened the two halves (they are tight -- the torque spec is typically 70-80 NM or 55 foot-pounds) you should be able to unscrew the injector by hand. The good news here is that there is nothing inside to go "sproing" and send parts all over the garage. However, you should hold the injector horizontally over a box lid lined with a shop towel so that anything that might fall out is contained.

The above picture shows the two halves. All of the bits and pieces are still inside. You may be able to tip the halves over and the pieces will fall out but if the injector is badly clogged with French fry grease then you may have to do a little prying. Each half has three pieces in it. The one that I found hardest to remove was the shim (like a small fat washer, it puts tension on the spring to set the pop pressure of the injector). It tends to stick in the top half of the injector.

Here I have laid out in order the injector halves and all their internal pieces, before cleaning. This injector was actually pretty clean and really didn’t need cleaning but I had already decided to do all six. As expected, the shim was firmly stuck in the top half of the injector body. I left it there, knowing it would drop out in the acetone bath.

To clean all of the parts of the injector, I used acetone, because that was what I had in my cabinet. You could also use carburetor cleaner, gasoline, or some other solvent. Be careful using any of these chemicals indoors; consider doing this part of the process outside. Wear rubber gloves and eye protection. Keep the chemicals away from kids, pets, and fire. I put the parts and the acetone in a beaker and put the beaker in a small ultrasonic cleaner with water. If you use a plastic container, make sure your chosen solvent won’t dissolve it. You don’t have to use ultrasonic but it saves a lot of time. If you don’t have an ultrasonic cleaner, put the container of parts and solvent into a pan of hot water. Cover the container so the solvent won’t evaporate. Go away for an hour. When you come back, the solvent and the ultrasonic should have removed all of the crud from the parts. Take the parts out of the solvent and rinse them in alcohol, then allow them to dry. Without ultrasonic, you may want to soak the parts overnight.

My injectors each have four surfaces that must be lapped so that they will seal properly (there are no o-rings or other rubber parts in the injector). All of the parts are metal and the sealing is metal to metal. The picture above shows the surfaces that must be smooth: the top half of the injector body, the two sides of the "intermediate disk," and the top of the nozzle. These surfaces are bare steel and will corrode if water gets in the fuel. Corrosion causes an imperfect seal and the injector then will leak.

You can use a surface plate if you have one. I used 2000 grit wet/dry sandpaper made for fine finishing. I cut a small piece and taped it to a flat glass plate. A piece of window glass will be flat enough. Use a little diesel fuel as a lubricant and gently rub each surface around in circles on the sandpaper until it is clean and shiny. Rinse in alcohol and wipe dry.

The needle and nozzle need a little special attention. Use a brass wire brush to clean the tip of the needle. Be very careful not to damage the tip. The end of the nozzle will have some carbon on it from combustion in the prechamber. Any that doesn’t come off in the solvent can be gently scraped off with a razor blade. The inside of the nozzle (where the needle goes) may need a little scrubbing with a Q-tip and alcohol. When you are finished, dip the needle in filtered diesel fuel and put it in the nozzle. Once the fuel has wetted the surfaces, you should be able to lift the needle up a few millimeters and it will slide back into the nozzle under its own weight. If it sticks, clean it again.

Now you can reassemble the injector. The pieces, now clean, won’t stick together like they did before so be very careful handling them. Make sure everything is clean and lint-free, especially the four sealing surfaces. The spring and the shim can go in either way but all of the other parts must go in facing the same direction as when you took the injector apart. The metal "pressure pin" on the end of the spring is especially difficult to keep in place. Once each injector half has its three pieces, carefully bring the halves together and hand-tighten them. You should feel pressure as the spring begins to compress for the last couple of turns. Make sure the "pressure pin" doesn’t slip out of place and end up sideways.

Put the injector back in the vice and tighten the two halves using your torque wrench. The spec for my injectors is 70-80 NM or about 55 foot-pounds. Now the injector is ready to go back into the engine with a new heat shield or crush washer. The picture above shows the right and wrong way to position the heat shield relative to the injector. Of course you put the heat shield in the hole and screw the injector in on top of it. I photographed them on the injector because it’s easier to see that way.

Torque the injector to spec (most are 70-80 NM but a note in FSM for the OM602/603 engine says that inclined injectors should be torqued to only 30 NM, about 22 foot-pounds). This lower spec for inclined injectors has been questioned by some readers and needs further investigation. The lower spec is apparently because inclined injectors screw into the prechamber itself rather than the locking ring. Caution is advised.

Now reattach the hard lines (torque spec 20-30 NM) and the little rubber jumpers. Start the engine and check for leaks. If you haven’t lost much of the fuel in the hard lines the engine should start with only a little more cranking than normal. You may need to tighten the hard lines a little more if they leak.

That's all there is to it -- nowhere near as hard as I had imagined.


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