How to overhaul injectors on older Mercedes Diesel engine

New Monark nozzle tips

Wow that was a long title! And this topic is even longer. However it is the considered ed as the most effective way to improve performance and fuel economy. There is no point of doing this job unless you did the valve adjustment prior (check out How to Adjust the Valves on a Diesel 617-engine).

When the injectors are getting old, the spray pattern will not be optimal and it will also release pressure at the wrong pressure. This impairs performance and fuel economy. In the long run the time and investment overhauling the injectors will quickly surpass fuel expenses and frustration over poor performance. I got my OM617 engine down to using only 6.5L/100km after replacing the nozzles to a new Monark type and overhauling the injectors. That’s absolutely incredible for a 3.0L diesel engine with technology dating back from the 70s. I also got a noticeably better performance, especially on the higher gears going uphill and during acceleration.

The funny thing about these diesels engines is that they can run even if they have worn out injectors, wrong pressure release, bad timing, tight valves etc. So you will have a poor fuel economy and poor performance, but the car will still run. This can be a good and a bad thing. It will not leave you stranded since the car is still working, but it might lead you to think the engine is just getting old and beyond repair. Even people that have had these cars for 15+ years don’t really notice the gradually decline of power, but sure they must notice the mileage and decide to buy a new car. For us enthusiasts out there, this is the most performance enhancing job you can do to your car.


Quite some preparation is needed, most notably is the need to acquire special tools along with new parts. Don’t be alarmed though, it is not too expensive and you will make the profit back in saved fuel quite fast.

Of the special tools needed for the job, the most significant for testing the release pressure and spray pattern, is the fuel injector pressure tester, this tool is so awesome so it can even be used on Mercedes gasoline injectors. Other tools needed are a torque wrench, a big socket, steel brushes, brass brushes, brass pick tools for cleaning old nozzles, a small pick, a good digital dial caliper for measuring ultra thin shim sizes, also a wise with aluminium protective covers and fine grit sandpaper (1000,2000). You will also need some very clean extra Diesel, half a liter should be enough.

I will by far recommend the pressure tester from, they have specially developed this tool for us DIYs and they also provide all the other tools and parts for overhauling injectors. They even have the Monark nozzles which are better than the Bosch ones. You should also read their guide on the matter which I consider more detailed, however I think more references are always good and you should keep reading to gain more confidence.

The parts needed for doing this kind of job are:

  • A set of new injector nozzles if plan on just replacing the nozzles instead of trying to cleaning the old ones (I recommend to upgrade to Monark nozzles from the old Bosch ones for better performance)
  • A collection of adjusting shims of various sizes (these are super thin shims that adjusts the release pressure in the injector)
  • New crushable gaskets/heat shields for each of the injectors, these have to be replaced every time the injectors are taken out of the engine.
  • New return hoses and the little return end plug on the last injector.
  • New fuel filter and pre filter.
Injector tools and parts for diesel
Injector tools and parts for overhauling the injectors. Most notably is the pressure tester. You will also need a shim set for adjusting and brass cleaning tools.

Before changing the nozzles, a fuel system purge should be done. Use special diesel engine fuel cleaner by disconnecting the fuel line to the tank and running the engine straight from a little container with the fuel return line pouring it in to this container. Run the engine until the diesel cleaner is used up. Change both the diesel filter and the prefilter.

Injector overhaul can be done in one day, but I used two days for avoiding the stress and did things very thorough. I removed and cleaned the glow plug tips and reamed the glow plug holes too, since the injector lines had to be removed anyway and gives access to the glow plugs.


Start by pulling off the return fuel braided hoses, be sure to have some new spare ones on hand.

fuel return lines om617
Remove the old diesel return lines, always replace with new ones to prevent leaks!

Then you have to take off all of the injector hard lines, use a 17mm spanner. Remember that the fuel system has to bled from air every time you open the hard lines on a Diesel, this is different from gasoline engines. This also applies to changing the fuel filter which I think is annoying.

diesel hard lines OM617
Remove the Diesel hard lines

After the Hard lines are removed, you have access to the glow plugs. They don’t have to be removed, but I recommend taking them out for inspection and cleaning the glow plug hole for excess carbon with a reamer. If you are planning to take out the prechamber too, the glow plugs must always be removed or you will damage the engine!

glow plugs OM617
Take out glow plugs for inspection and cleaning

Careful not to loose the little nut and washer on the glow plug cable, you will never find them again down in the engine bay.

Inspect the glow plugs, if you have no record of how old they are, you should replace them safer than sorry.

dirty glow plug OM617
Dirty glow plug full of carbon
Clean glow plug OM617
Glow plug cleaned







Now it is time for using those muscles with a big socket and a long bar for cracking those injector loose, they may sit a bit hard if it’s been long since they were maintained. Remember to crank them open in the right direction.

Engine without injector hard lines
Fuel hard lines removed from injectors

Use an deep over sized socket which can fit over the return hose nipples without damaging them and the socket is still tight enough to fit snugly over the lower fitting. Before taking them out you have to organize a system so you know which injector fits for which cylinder, please do not confuse them with each other. I used a box where I marked each injector with a note. Also ow is the time to take car of the nozzle tips, do not ever bang them into metal otherwise they will be permanently destroyed.

injector removal OM617
Remove injector with the deep socket and long breaker bar

There are crushable washers down in the injector hole. They have to be taken out with a small pick and thrown away since they cannot be reused.

crushable washer under injector OM617
Remove the crushable washers with a pick. Note the direction of the washer!

When all the injectors are out and you hopefully remember or organized which ones went where, it is time for pressure testing the injectors BEFORE you start overhauling them. Why? Because you will know the condition of the nozzles and what kind of performance increase you can be expecting. Also it is used to predict the adjustment when you assemble the injectors. Even if I had in mind to just replace the nozzles I was still curious to look at the spray pattern and pressure reading for getting an idea of the condition of the injectors. This car had Bosch nozzles and the pressure readings was around 1500PSI (which is not too bad though the pressure should be around 1700PSI when optimal), and even the spray pattern was in a decent state. The injectors  was in a better state than I feared and the old nozzles was more than usable, but I still decided to change for new Monark nozzles for better performance and prevent me for doing this same job in some years. I can then have the old Bosch ones as backup nozzles with good confidence for the future, of course after cleaning them.

Do a pressure test, note down spray pattern and pressure release for each injector before dismantling, the spray should be a fine mist making a fan shaped pattern. Be sure to wear a gas mask like the ones used for paints to prevent inhaling harmful Diesel fumes, this is very important! Also place the injector inside a clear bottle to prevent fumes spraying all over. Lastly do this job outside in order to not be breathing Diesel the entire following week!

The pressure is built up steadily with a slow pump on the tester, when the injectors squirts, do some repetitive pushes to note the release pressure.

pressure testing setup
Ready for pressure testing, not that you should use a clear bottle with the injector inside it when doing the actual testing to prevent harmful fumes.

After testing each injector, you can open them/ separate the injectors in a wise with aluminium protection grips by placing the flat upper half down into the wise while using the same socket over the lower half. Use the out most care to not touch the nozzle tip with any metal. Muscles are usually needed to open them the first time due to corrotion.

opening the injector for mercedes Diesel
Separating the injector halves with use of a wise with aluminium grips

When you cracked it open, be sure to open it over a box to catch all the components coming out.


Now over to the fun part, cleaning the injector parts! This is by far the most time consuming part. You need to use quite a lot of time on each injector at a time and be sure to not mess up the internal parts with each other, since they will be near impossible to adjust and might leak fuel.

The easiest way is actually to cook each injector in a kettle with soapy water for some minutes, before cleaning each part with steel and brass brushes. The sediments will come much easier off then.

cooking injector parts
Cooking car parts in the kitchen, typical weekend meal.

After cooking and not eating, the hard labour of cleaning the small injector parts can begin.

cooked injector parts
Cooked injector parts, careful when hot. Note I have marked the injector number so I can keep track.

Use only brass brushes on the inside parts to avoid scoring the metal! On the outside of the injector housing you can use a steel brush. Use small brass picks to clean the crud out of the internal hard to get corners and inside the injector housing.

Cleaning inside injector parts with brass brush
Cleaning inside injector parts with brass brush

When you have cleaned all the parts, and not messing up the parts with other injectors, the adjusting can begin. This is the most fun part.

It is very important to do a thing called lapping before assembling the injector parts after cleaning. It involves flattening the internal pieces that goes together in the joint between the injector halves on very fine sandpaper on flat a piece of glass to make the surfaces exceptionally super flat. As you might have noticed the injectors do not have internal rubber gaskets and the super flat metal surfaces in contact with each other makes the seal between the injector halves.

lapping the injector halve pieces
Lapping on a flat piece of glass and fine grit sandpaper, you will need to polish this part and the lower edge of the upper injector housing.

Start by using the 1000 grit paper if you have some bigger scratches, then switch over to the 2000 grit to get the perfect polish. I only used 2000 grit on my parts.

Adjustment shims for injectors
Selection of adjustment shims

Use the notes from the initial pressure readings to “predict” the new pressure readings. The adjustment is done by adding or removing adjustment shims to obtain the right pressure. The pressure increases with approximately 100PSI with adding 0.05mm shims and decreases by the same 100PSI by reducing the thickness by 0.05mm. It may take some tries to get the wanted pressure settings. I aimed for 1800PSI on my non turbo engine, The factory manual says 1668-1780PSI, but you can safely go 50PSI over because the new nozzles will wear and the pressure will go down. Several shims can be added on top of each other for obtaining the right overall thickness.

Dial Caliper to measure shim sizes
Use the Dial Caliper to measure shim sizes

Adjustment Example:

Initial test: 1500PSI
Original shim: 1.45mm
Aimed pressure: 1800PSI
Adjustment shims extra required: 1 x 0.10mm + 1 x 0.05mm ~ +300psi

Note that having similar release pressures between all five(or four) the injectors within a maximum difference of 50PSI are more important than trying to aim single injectors to be dead on 1780PSI for example. This is to ensure smooth engine operation and reduced vibration and noise. If All your injectors are within 1750 and 1800PSI the result will be satisfactory. Some injectors are almost impossible to get the exact value you want due to age and spring wear. I didn’t have problems with getting them quite accurate at around 1800PSI so it’s not impossible.

Also now it the time for replacing new nozzles if you plan to do so. Be ultra careful of the new nozzle tips by  not smashing them into other metal.

Old Bosch nozzle tips
Old Bosch nozzle tips, note the little hole on the tip.
New Monark nozzle tips
New Monark nozzle tips. Note that the Monark nozzles does not have the little hole in the tip

Assemble the injectors the right way with using your selected adjuster shims. This part is iterative, since you have to assemble the injector, tighten the halves with a torque spanner, test for leaks and spray release pattern. Make notes as you do the testing, and open up and adjust the shim sizes if necessary, you may need to do this several time to get the pressure right.

Be very careful not to touch the nozzle tip during assembly and disassembly, take good time and care. Especially the Monark nozzle tip is quite pointy and you should take care to not even touch it with your finger.

Cleaned injector
Cleaned injector, as good as new

When every injector is tested, adjusted, checked for leaks and balanced between each other, they are ready to be placed back into the car.

fuel injectors OM617
Cleaned fuel injectors, adjusted and with new Monark nozzles.

Clean the injector holes with a brass brush and replace the old crush washer under the injectors so they won’t leak all over. Tighten them down to 75- 80Nm.

Cleaning fuel injector hole with brass brush
Cleaning fuel injector hole with brass brush

Clean the glow plug holes for excess carbon build up with a reamer and reinsert the glow plugs and connect the glow plug wires. Too much carbon in the glow plug hole might lead to shorting of the glow plugs, making cold starts nearly impossible.

Cleaning excess carbon with reamer
Cleaning excess carbon with a glow plug hole reamer

Attach all the fittings and injector lines and new braided rubber return lines as well as the end plug on the fifth injector. Do not fully tighten the hard line fittings at the injector end.

Now the air has to be bled out of the fuel system. Start by pumping the air bleeder pump at the fuel pump until fuel comes out of the hard line end. Now you should get a helper to observe if fuel are coming out while you are cranking the engine. The bleeding has to be done while cranking the engine while flooring the accelerator pedal, again make sure the hard lines are not fully tightened, leave 2/3s of a turn. When fuel starts leaking out of the lines, stop the cranking, tighten the nuts and start the engine normally. It might cough and run rough until all air in the system is gone. This is especially the case if you have a bad bleeder pump. Check for leaks while the engine is running and then take a test drive until engine is warm, recheck for leaks and do necessary work if leaks are evident. If diesel is coming out from the halves of the injector, then it must be lapped again. If fuel is leaking from the bottom of the injector, it is probably the prechamber seal, this is a bigger job to get out, but leaking fuel is expensive and should be dealt with.

Happy injectors, delivering more power for less money. Cheers!


Author: Robs

Passionate about vintage Mercedes Benz where driving, maintaining and learning new things and teaching others on the black arts of car maintenance.

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