So how do you change the fuel filter? It is in theory simple, but in practice it can be quite messy and also tricky to fully access in the rear end fuel delivery system on the w123 chassis. You also need to make sure you have as low fuel as possible or drain the fuel tank dry.
WARNING : There is a high explosion and fire risk when dealing with petrol fumes, so don’t light up your cigarette and avoid sparks!
Fuel delivery assembly rubber bushings – I highly recommend to change these
Main tank hose
High pressure hose
Low pressure hoses
Fuel hard line and fittings
You need to access the rear end fuel delivery system on the rear right end of the car. It consists of the fuel pump, fuel filter and the fuel accumulator. They are suspended from 4 rubber bushings secured in a weird metal clamp. It is partly hidden behind a plastic cover near the right rear wheel.
Start by removing the battery, trust me, you dont’ want sparks while working under the car with petrol fumes! And don’t smoke!
Then jack up the rear end of the car up on jack stands and block the front wheels from rolling. As extra safety measure I always leave the jack in raised position on the same side as I will be working.
Remove the rear right wheel and you have better access to the fuel delivery system. Locate the plastic cover which is held by a few screws, use a long thin socket extension to remove these.
Now you have to drain the fuel from the tank to avoid petrol spills and excessive fumes. This can either be done with using a vacuum pump with a hose down into the filler neck, or by disconnecting the main hose to the fuel pump. It will also leak fuel from the delivery line to the engine, and this can be blocked from continuously dripping with a hose clamp on the high pressure hose. Make sure the clamp is of high quality with no sharp edges, since a new high pressure hose is quite expensive.
When the fuel is drained. You can start by loosening the 2 lines that connects to the fuel filter. The left side (from rear) is the high pressure line and the right is the hard line. Notice the position of the two copper washers on the hard line fitting. A lot more fuel will now drain out from the filter so take care. It is possible to remove the filter from the cage without removing the whole assembly.
If you need to also change the rubber bushings, then change them one by one and you don’t have to take the whole assembly down. Alternatively to get better access you can secure the assembly with zip ties while changing them.
Installation is pretty much reverse of removal, but you have to remember to use the new copper washers that followed with the new filter. Also I found that you need to torque down the screw between the hard line and the filter for it to not leak.
After everything is put together and you have put some fuel in the tank, try to start the car, it should not take long for the engine to fire up after you crank it. The fuel pump is pretty quick pushing fuel through. When the car is running, go back and inspect carefully if fuel is leaking and check for any wetness.
If it’s dripping or you can see wetness, shut off the car. Tighten the bolts, wipe off the fuel so it’s completely dry and try again. I had to redo this step twice before it was properly sealed, and the leak was indeed between the new copper washers and the hard line. The copper washers will actually start to seal better when exposed to moisture and some small corrosion will start forming in the gaps.
Be sure to check for any wetness under the rear end of your car after you parked it the first times, since leaking fuel is bad for your wallet and is a potential fire hazard.
Ever felt that the engine is not making as much power as it used to? And that the fuel economy has been slowly decreasing? The first thing you should check is the condition of your injectors. Unless you have a pressure tester for the injectors, you would have to guess out from the age of the injectors. If they are more than 10 years old I would suggest to change them, that said if the injectors are relatively cheap since they vary a lot in price between the models. They are still relatively cheap for the M110 engine. E.g. If you have the rarer M100 engine, it will cost you nearly a new house to buy new injectors!
I will describe how you change the injectors with new ones. Refurbishing the old ones is a bit more cumbersome and it needs a pressure tester which you can clean them with and test for release pressure and spray pattern.
Changing the fuel injectors is not difficult and you don’t need much preparation except acquiring new injectors, gaskets and o-rings. There are two gaskets per injector you will have to change or you will end up leaking fuel which is expensive. You can change the injectors easily in an afternoon. I also recommend to change the fuel filter at the same time to not clog up your new injectors.
Start with opening your hood and disconnecting the battery. Sparks can make explosions from gasoline vapor when you are working with the fuel system, and we don’t want that to happen.
Remove the air filter housing to get access to the fuel delivery system.
The four front injectors you can now access without removing more stuff, but to gain access for the last two, you have to remove throttle linkages, fuel hard lines and a fuel regulator. We’ll come to that later.
Two and two injectors are held in place by a single metal plate, to remove the metal plate loosen the injector hard lines first. The injector hard lines should not sit on very hard unless they corroded over time or they have been over tightened. Use two spanners, one 14mm to loose the line nut and one 12mm on the injector itself to to keep it from rotating.
When two hard lines are loosened you can bend them a little to the side to get more access and then remove the injector hold-down plate.
After the plate is removed you can pull out the injectors with you fingers, they sit loose in there. The injector housing will be left, This plastic housing you can just pull out straight too, but be warned, do not use excessive force and pull only the housing straight out with you fingers, don’t use tools or you might damage it. It most likely have become quite brittle by heat over the years. If they are in such a bad condition they are cracked, you have to replace them.
Most likely some of the injector housing gaskets do not come out with the housing, but is still sitting in the injector hole, use a small flat screwdriver and pry it carefully out.
After removing the first four injectors, the two back injectors are left, now the big job starts. See in the picture below that these things blocks access to the fuel injectors.
Start with removing the throttle linkage. Remove the pin and the c-clip along the ball joint. I use a wide flat screwdriver to pry the ball joints off. Remove some more linkages and then you are ready to remove the regulator.
Warning: Get a small collecting can of some sorts and tilt it towards the injector line shown in the picture below. Loosen it and fuel with shoot out with high pressure. Wait until the fuel stops flowing, then loosen the second fuel line connected to the regulator.
Also loosen the vacuum lines before removing the fuel regulator, it is fastened to a bracket with two small bolts.
Now you should have clear access to the last two injectors.
Now is the time to inspect the old injectors and look at those old brittle gaskets. It is a reason why your car was using excessive fuel…
My old gaskets were super hard, especially the round outer ones. No wonder why there was a lot of dirt around the injector area by the engine, they have probably been leaking a while. This job should have been done years ago. I didn’t notice it when the engine performance still was very good, but obviously not optimal when looking at this leaking issue. The old injectors I don’t even bother testing, I will just throw the old ones and put in new ones.
This is how you should assemble the injector with the gaskets:
Pop on the big rubber gasket on the injector itself.
Pop on the little rubber ring on the injector plastic holder.
Pop the injector holder into the engine block until you hear and feel a nice pop.
Slide all the injectors into the holders. Put the hold-down plates over them and tighten.
The injectors will now have a good seal and no fuel will leak. It is also a good idea to clean up the area around the injector holes in the engine before assembly. Also try to be super clean with the new injectors, keeping the plastic cups on as long as possible before fitting them.
No you also have the perfect opportunity to lubricate all the throttle linkages, such as the pivot points and ball joints, I know they have been neglected for years and we are all guilty as charged. However now is the perfect opportunity for redemption. Remove all the linkages and pivot plates, clean them and apply new synthetic grease that will last a while and hold up to water and heat from the engine.
When everything is bolted back together, there is no such thing as dreadful bleeding like in a diesel. Just crank the engine over a few times and the engine should start, it will run rough for a little while until the pressure have settled, and you might experience hard start the first couple of times before the first drive. This is the time when you realize if the electric fuel pump needs replacement or not, mine is some years old and still going strong, although it have been replaced a couple of times the last 15 years.
The fuel delivery system on w123 consists the fuel tank and a collection of components until it reaches the combustion chamber. This article will focus on the rear components until it reaches the CIS fuel injector assembly. The CIS system is pretty much a black box for me still. I will however go through how you change the fuel injectors and gaskets in another article.
I had to do a re-haul of the entire rear fuel delivery system when an entire tank worth of gasoline (~80 liters) had leaked out after my 280CE was parked for a while, totally delayed my plans for cruising in the summer by almost a week. This made me furious and is the reason for why I wrote this article.
It is up to you how many part you want to change. I would recommend to change all the rubber fuel hoses and fuel filters as a minimum!
New fuel tank filter (the first one)
New main fuel tank delivery hose
New fuel delivery filter (the second one)
New high pressure flexible fuel hose
Two other fuel hoses and clamps (see pics)
4 fuel assembly rubber mounts
[New fuel accumulator?]
[New electric fuel pump?]
So why bother changing the fuel accumulator? Well it can lead to problems such as warm start problems (yeah the M110 usually have warm start problems more than cold starts), this is due to the inability to hold fuel pressure after shutting down the engine and starting it before it gets cold. There are separate start up procedures either if engine is cold or warm. I thought a broken fuel accumulator was the problem with my car’s warm start problems, but it was in fact the old injectors and brittle gaskets that was the cause since they could not maintain fuel pressure.
Why bother changing the electric fuel pump? This has to be the number one reason why people are stranded in their cars with M110 engine. After some time (around 7 years if in daily use) it will with certainty stop working. The part is however quite expensive and I would suggest to not change it if is still working and not obviously very old. A maintenance tip instead of investing in a new part is to remove the corrosion on the electrical connections.
Pretty straight forward, just change all the fuel hoses and refit… OK I will guide you through it like Gandalf in the mines of Moria. Smelling fuel is usually an indication of leaking hoses.
Before you even start you have to disconnect the battery to avoid explosions. Then completely drain the tank, either by sucking up the fuel from the gas filler tube or by unloosening the main tank outlet hose from under the tank. I would suggest you drive the car nearly empty before undertaking this task. Make sure the area you are working in is very well ventilated since gasoline fume is highly explosive!
After the tank is drained you can remove the plastic cover start by removing the thick fuel hose connected to the tank. Just have a look at this one in the picture under which caused all the fuel in my car to drain out.
A massive 45mm socket is needed to remove and fit the fuel tank filter. Remember to replace the gasket as well.
Now you can remove the flexible hoses connecting the high pressure and the return hard lines. If you see rust on the connection then you have to be extremely careful not to round off the relatively soft metal on the hard line unless you want to go through with replacing the entire line. Believe me this is a shit job (my old hard lines were rusted and leaked fuel).
Disconnect the two electrical wires to the fuel pump and when all the fuel lines are disconnected from the body, you can go on and loosen the 4 bolts holding the bracket for the pump, fuel filter and accumulator. Beware this bracket can be pretty rusted and you need either to get hold of a new one or remove the rust and repaint the one you have. The rubber mount bushings are probably dried up or broken so you should replace all 4 of them.
On a work bench you can now go on with the job of replacing the components and the old fuel hoses. Look at the picture below on comparing new and old hoses.
Assembly is just opposite of removing. The main thing you have to consider is to make sure there are not any leaks before you start driving. Look at the picture below to see how the components should fit together in the assembly.
The best thing is that there is no need of special fuel bleeding or start up procedure when starting up the car after fitting all the lines together. Just refill the tank and crank the engine and it will start right up. Nothing difficult as with the diesel.
Then mount the assembly and fasten the hose to the fuel filter. Don’t forget to fit the electrical wires to the fuel pump and mount check for leaks when starting the car. When no leaks are detected, mount back the plastic mud cover to protect the components. Hopefully you will have no leaky days or being stranded by a malfunctioning fuel pump.
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 mercedessource.com, 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.
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.
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.
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!
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.
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.
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.
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.
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.
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.
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.
After cooking and not eating, the hard labour of cleaning the small injector parts can begin.
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.
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.
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.
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.
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.
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.
When every injector is tested, adjusted, checked for leaks and balanced between each other, they are ready to be placed back into the car.
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.
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.
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!