Mercedes Diesel Injector Seal Replacement – Notes

In a recent Mercedes Sprinter engine replacement (Used engine fitment) it was noticed at the time of purchase that there was an issue with No.2 Injector seal leaking. Obviously this needed attention and there was more chance of being able to remove the stuck injector once the engine was fitted, than try to do the work on the engine prior, as it would simply move around the floor with the physical effort involved!

What I did do before fitting the engine was to get rid of all the carbon ‘Black-Death’ build-up due to the leaking injector. This involved careful chipping and clearing the cables, connectors and pipes that were buried beneath the charcoal coating. If you want a really clean and sparking job, I can recommend a none-acidic oven cleaner for a final dressing, but in this case it was cleaned to a ‘practical degree’ that fitted with the age and condition of the vehicle.

Mercedes Diesel Injector Removal and Fitting 1

Without a doubt, if you are able to start the engine and get it warm, even loosen the clamp bolts five or so millimetres above the shoe clamp then rev the engine, what often happens is the injector gets ‘blown out’ a little under engine compression and once the carbon seal is initially broken, it can make subsequent extraction a great deal easier.  Sometimes it works – sometimes not!

In my case I knew that the engine would probably not start as the seals were in such bad condition. As I turned the engine over clockwise using a 27mm socket/breaker-bar on the crank nut, you could hear the escape of air past the injector seal on each rotation. This was confirmed by localised spraying of WD-40 onto the suspect injector and watching it bubble/vaporise as air from the combustion chamber was forced by the injector during a manual rotation of the engine. I had to proceed without heat or hot engine – always makes for interesting removal!

Mercedes Diesel Injector Removal and Fitting 2

Obviously you will have to remove the inlet manifold upper half and also the engine cover if fitted before you can gain access to the injectors. It is important to plug all the intake tracts to prevent items such as the bleed off pipe spring clips finding their way down there etc…

Once the engine was fully installed, the injector power plug was removed, the bleed off return pipe unclipped and moved well out of the way. The steel diesel supply pipe and the injector union connection were removed with a 14mm spanner and 13mm socket respectively. As soon as the union was removed the hole was plugged with a small section of kitchen towel pushed into the tapped hole. This simply prevents any bits getting into the open hole while working to extract the injector.

Rarely it is possible to clamp a pair of good mole grips to the injector solenoid fixing nut and rotate the injector ten or twenty degrees back and forth to loosen it in its bore, more often it takes a great deal more work!  Using a searching penetrant or diesel itself is a good aid to getting things moving. This twisting method can be successful, though what usually happens once you start to get going is that the fixing nut undoes slightly and puts a stop to using that method of extraction, as it simply rotates upon the head of the injector body.

What you have to do in this eventuality is grip the injector just below the solenoid nut and attempt to move the cast head part – rotationally back and forth. As the diesel supply union is completely removed, you will get the maximum chance of realising this important few degrees of movement, that will in time result in the total removal of the injector body.

If you have a 24mm open end spanner or its imperial equivalent this is your key tool. If your area of work is the rear three injectors there is a little technique that may work for you. If you lay a 19mm combination spanner along the top of the duct that carries the injector wiring, interlocking the open end into the rear intake manifold support bracket, this will provide a sturdy supporting surface on which to lay the ‘operating’ slightly slack fitting, 24mm spanner onto whilst moving it back and forth. As you will probably only be able to shift the injector a few degrees back and forth at first, it is important to keep the area well irrigated with penetrant. As your movements get more dynamic, begin to press the back of the operating spanner down onto the 19mm rider. Keep pushing as you rotate back and forth. This induces a small lifting force under the solenoid nut from the 24mm spanner and as you move it, it will in time begin to raise the injector out from its bore. If your injector is at the forward end of the engine you will have to locate the 19mm ‘rider’ spanner’s open end somewhere up-front to fully enable this method, but it is not impossible!

Sprinter Diesel Injector

Mercedes CDI Diesel Injector

Eventually you will extract the injector – a feeling of achievement will appear as you take an inspection mirror and torch to peer into the blackness from where it came. If you were lucky then the copper washer/seal will have accompanied the injector out of the hole.  If not, a ‘rat-tail’ round file of suitable size, pushed into the hole will retrieve the seal without issue.

Now you can begin to inspect the seal itself, giving up many clues to the demise of its capability to hold pressure. Clean it with a rag and take a close look at its copper surfaces. If there are any scored lines, cuts or deep marking, this is the route the escaping combustion gasses have taken to ‘carbon-up’ your engine with ‘Black-Death’. Indeed any gas-cut passages in the sealing surfaces will have most certainly also cut into the aluminium injector seat and in severe cases damaged the steel injector face that mates with the copper seal.

I would recommend always at least cleaning the recessed seat of any removed injector with a special tool made for the job. This special tool is a flat face end-mill or reamer with the correct diameter sleeve to cut or reface the base surface with some precision. These tools are available from Laser for under £60 from most motor factors and are essential to DIY replacement and the successful re-sealing of injectors – experience has found in every case, if you don’t at the very least ‘lightly face’ and prepare the injector seat before reinstallation, you are not giving yourself the best chance of success, therefore I would get the proper tool for the job, period!

Mercedes Diesel Injector Removal and Fitting 3

Escaping combustion gasses often cut the aluminium seat and this needs to be re-faced so that a perfectly gas tight seal can be maintained

One or two rotations with the seating tool will be sufficient to enable you see if the seat is damaged in any way, using your inspection mirror and light. If there are any black marks on the seat (rather like worms!) as the drawings below show, then continue to use the seating tool, checking and wiping aluminium shavings as you progress to remove material. Eventually the tool will re-face the damaged seat to show a clean, uninterrupted ring of aluminium. It is possible if you do not fully rotate the tool when using it, mistakenly rotating the tool left to right rotationally, may cause the finished surface to have ‘chatter’ marks on it. This is surface unevenness that must be removed and is only achieved by turning the tool lightly and continually in one direction, stopping movement only as you simultainiously relieve pressure from the cutter. Inspect and clean often, to obtain the best surface finish possible.

Mercedes Diesel Injector Removal and Fitting 5

Improper use of the seating tool sometimes causes chattering on the surface of the seat. This needs to be smooth and not disrupted with tool marking

Now turn your attention to the face of the injector body that compresses against the copper seal. This must be free of any carbon build-up and ‘shiny clean’. You can take some fine emery paper and lightly work this surface until it is bright and continuously clean around its circumference. Now you are almost ready to reassemble!

The injector hold down bolts are a one-time-use, stretch to yield fastening, that must be new and unused on each replacement. Cut a short groove with a saw file to make a thread cleaner out of the old bolt and use this to freshen up the tapped hole for the clamp bolt. Use a thin screwdriver to clear the oily debris from the bottom of the hole as it often builds up here and can if not removed, be forcefully compressed into a plug, causing it to burst through into the cylinder head water jacket – take special care to clean this hole out!  If the thread is damaged in this deep tapped hole you have a number of repair options, these are covered briefly in this post.

Mercedes Diesel Injector Removal and Fitting 6

Now you can use ceramic grease to coat the outside of the injector body (not the tip or any mating/sealing surfaces) place a new preferred Honda Accord Diesel Seal onto the injector tip and carefully lower it into the clean and prepared hole. Do be sure to have one final check with torch and mirror to make doubly sure there is no debris left on the sealing surface before final assembly. Once the injector is fully inserted, with the fuel connection pointing to the fuel rail, add the clamp and loose-fit the clamp bolt to hold things in place. Now refit the fuel union to the injector, removing the tissue plug that has prevented dirt getting inside, then refit the steel fuel line from the injector union to the fuel rail, finger tightening the union nuts. Now torque the clamp hold down bolt to 7Nm plus 90 degrees (MB Specs say 90 plus 90 degrees after 7Nm, but actually the yield has occurred after the first 90 degrees and clamping force reached, so I see little point in risking further stressing an otherwise ‘spindly’ and fragile fixing). You may now refit the fuel bleed off pipework, electrical connector and tighten off the steel fuel line unions. Rebuilding your engine is now a reversal of dismantling. (remember to remove the manifold intake plug of rags or paper before fitting the upper section!)

There are a few tips here that hopefully will help you achieve this sometimes daunting job, but with care, patience, the right tools and a little effort this can easily be done by a DIY mechanic.

32 thoughts on “Mercedes Diesel Injector Seal Replacement – Notes

  1. Just wanted to say thanks very much for this write up. Helped me get through this job with almost no issues…. the tapped hole in the head was wrecked so had to use a helicoil as it was all I had and still had some leakage after the first replacement which was fixed by swapping out for a Honda seal…. great tip!

  2. Hi, just changed an injector in my 04 plate c220 cdi estate. Programmed the new injector to the ECU replaced seal . now the car starts and after 2 or 3 seconds the car cuts out? I’m stumped as to what it can be , any ideas ?

    1. Hi Mark,

      Any codes showing? It could be electrical, any injector solenoid connector that is slightly intermittent/not plugged on correctly will stop the engine. If you have put a new filter on or disturbed the LP or HP feed pipes to and from the low pressure pump this could be a probable cause as they may not be drawing correctly and be pulling air into the lines.

      I am pretty sure you have an in-tank lift pump in that model, so make sure this is running and pulling in the fuel to the pump block. Check lift pump relay/fuse (you will have to search for these as I can’t remember the location numbers off hand but I remember they are on the fuse board in the boot – I think!) Try banging hard under the round protruding section on the tank base to see if you can get things running. Do you hear it prime in a pre start condition? I suppose if it has had a period of standing, while you have carried out the repairs, it may have decided to stick – they have a habit of doing that!. You can access the pump and tank sender unit from under the rear passenger seat base (at least on the saloon model you can).

      This worth a read – underlines the pipework/air leak scenario.-

      Just an outside thought – you did reconnect the MAF (Intake air flow sensor) when you put things back together? If so, try disconnecting it and see if it maintains idle.

      All the best,
      Do let me know what you find.


  3. I wonder how many times the injector seat can be recut before it becomes too deep?

    I have had my fair share of this problem with my car, a 2003 C220 and it is a problem that recurs every so often. My mechanic now tells me that one of the injector seats has been cut for the last time and it is now time to replace the car or face replacing the head next time this happens. It is a pity as my car is pretty good in every respect apart from that.

    I wish I had known about the Honda washers before!

    1. Hi Michael,

      It is not widely known but Mercedes sell thicker washers for instances where the face/seat has been heavily recut and would protrude the injector tip too far into the cylinder. I don’t know the exact amount that can be removed but armed with this information you should be able to find out from a friendly dealer.

      hope this helps

      1. Thanks Steve. I was aware of two different thicknesses of washer and sad to say I have been making the best of the thicker ones on the last couple of times the problem arose. It is common practice among the mechanics I know to clean the seat and then to apply Loctite on the bottom surface of the washer. They then leave the car overnight before starting it up the next day and allowing the engine to warm gently to full temperature. That is what has been done on mine more than once and it seems to last for quite some time though will obviously be down to individual conditions. It’s hardly a cure but may be of some use to someone.

        It’s such a poor design and makes buying a used Mercedes a bit of a gamble as to whether or not there is any seat left to be cut next time they start leaking. With that in mind, how long in your experience would you expect the Honda washers to last? I am probably going to buy another Mercedes and it would be good to know that if caught in time the problem could be dealt with long term.

        Thanks again,


      2. Hi Michael,
        I have been fitting the Honda alternative for good number of years on a courier fleet of vans and as yet once done I have not had to return to the work – so works for me! The loctite would not be a method I would try as the cylinder pressures are high and a good clean metal to metal surface torqued to the correct amount should really be the best fitment. The copper compresses to the face of the base of the seat and should seal perfectly, any imperfections in either your finishing or cutting will cause problems. A shiny bright perfect ring is what you are looking for and it doesn’t take too long to achieve with the proper tools. The Honda washer does appear softer material and I even had one chap who suggested that he lightly annealed the MB seal with heat and this made it softer and more compliant in the compression fit – can’t say if this worked but on paper there is some logic in it.

        All the best

      3. If you pretty happy with your car try to to find a good bare head as it is the only problem you faced.

  4. Hi Steve, my garage has just changed all the copper washers on the injectors of my ML270, he also fitted a new turbo and decoked the engine, replaced the oil and filter and added a cleaning agent in the fuel.
    Now it smokes heavy all the time even on tick over, it runs ok only the smoke is a problem, he also cleaned the egr valve and replaced the air mass.
    Previously the car ran fine smoke free, only had the problem of leaky injectors.
    Now he is saying it needs new injectors, hope you can help? I would appreciate any advice.
    Thanks John

    1. Hi John,
      There is a chance one or more of the injectors or its nozzle has been damaged during the extraction process especially if one was hard to remove. (often the very small ball bearing inside is lost if the internals were stripped to attach a puller) This could now be over-fuelling and causing the issues you have. Conduct a leak off test with clear plastic tube on the fuel bleed off lines and see if anything looks unusual.

      All the best

  5. I am not a DIY, but I had a question I hoped you could answer. What are the average labor hours necessary to deal with the “black death” issue? My mechanic is telling me 15+ hours?

    1. Hi James,
      He is probably erring on the side of caution as the job can often sometimes ‘grow’ if things are stuck (injectors) or hold down bolts shear or strip. What I would do is ask him if this price is based on a worse case scenario, if not get him to quote on this basis then you know exactly what you could be into if things do go south, before he starts!

      All the best

  6. Hi Steve,
    I have attempted to post this on your other write up and I’m not sure if it went on so this may be a duplicate sorry in advance.

    I have just fixed a black death Leak on a 59 plate 330 cdi 651957 2.1 engine and after doing the job and attempting to start the van I found the engine jamming , when barring over it hits hard at tdc on cylinder 4 where I have done the work. I am assuming something has fallen into the cylinder and is jamming! (nightmare) any ideas ? I thought at first maybe debris from the coked up mess but now I’m not so sure as it seems hard when hitting. I did not need to split the Injector as I managed to wiggle it out. I also haven’t lost any Nuts / Bolts but saying that the hole is tiny and not a lot would go through! Do you think it possible for debris to have gone in and got stuck behind a valve causing it to not fully close ? Maybe a leak down tester to prove valves closing ? I’m also thinking it maybe good to remove the Rocker cover to see if valves are closed by looking at stem heights ? Can the Rocker covers be removed without removing injectors ? Any ideas would be appreciated as I really don’t fancy taking the Head off!

    Thanks Andy

    1. Hi Andy,
      As you say it seems some debris or item has fallen into the cylinder. Use a small magnetic retriever down the hole and see if it just happens to be metallic/magnetic. If its just some coke jamming a valve then take the cylinder down to BTDC and beyond (up to the same point on the other side of the stroke) and use some compressed air in the hole, hopefully it should shift the muck either into the exhaust manifold or inlet ports. I suppose this is may cause a problem in itself that it may just lie in wait only to jam again. Or probably a better bet would be to bring the engine close to TDC then you could spray some Mr Muscle oven cleaner down into the cylinder through the injector hole, this should dissolve or soften the carbon lump quite easily (if that is what it is) to the point that it no longer causes a problem and turns into tar that will just burn off once the engine gets the all clear and can be started.
      Rocker cover removal will entail the removal of all injectors sadly.
      All the best

      1. Excellent Steve, thanks for your reply, I had already tried the compressed air idea to send it up and out the valves but i think having already tried to start the engine i may have squashed the carbon together tightly. Also I spoke to a couple of friends before getting your reply and they too suggested the dissolving idea as apparently it is not unusual for this to happen, one said use diesel by pouring into the cylinder and leaving to soak and after a few hours bar it backwards and forwards, squashing the carbon until it passes out. The other said the same but suggested using Caustic Soda to dissolve it, which is fairly similar to your idea as I believe Mr muscle contains similar chemicals. I had already used the Caustic soda before being told any other and left it for a couple of hours to dissolve and then barred back and forward, each time hitting the carbon until it suddenly gave and barred round ok, then to rinse thoroughly with water to remove Caustic soda, good blow down with compressed air and set about rebuilding. I did remove all Injectors to remove the Rocker cover for valve stem inspection and to be honest I’m glad I did as the seals on all 3 were ready to give at any time so now 4 new Seals, 4 new Bolts and rebuilt. Ran a bit lumpy for the first 5 minutes but then cleared up, took it for a good half hour run on the motorway and all is now good. Thank you for your help

  7. 1.What is standard injector bore height of a 2000 UK mercedes benz 313cdi.
    2.How to fix an over cut injector seat

    1. Hi there GABS,

      As it would impossible to tell if the individual seat you are dealing has been recut before without factory data (that I don’t have available, but you can purchase here along with other engine builder data if you required it: – ), the best you can hope to achieve is to get it at least the same as the others or that of a known good engine if all seats have been overcut. What I would do is measure a known good injector pocket, or compare all on the engine you have if just the one seat is an issue. Use a vernier depth gauge to measure from the flat surface of the alloy seat down the hole to the piston crown at TDC. (or to an engineers plate if the head is on the bench)

      From this you will ascertain the required ‘extra thickness’ of sealing washer required. This is very scientific and unless you suspect/know a huge amount has been cut I would feel you were pretty safe and sound using the Honda washer. This is a little thicker (appx half a mil) and when compressed would compensate for some light refacing. Mercedes do sell oversize washers (you would have to ask for them specifically at the dealer, as I do not have part numbers, I know they exist but doubt they wont go into ‘multi-millimetre’ thicknesses) If you can get hold of some accurate data this would be the way to go, but to be honest in practical terms the Honda washer has always done the job with normal recutting sub 0.75 mm.

      If the head has been largely undercut by a few millimetres then I guess the head would be rendered unusable.

      Hope that helps
      All the best

  8. Hi Steve
    I followed your instructions to the letter and have replaced 5 seals in my ml270 all good.
    I read that poor starting can be a consequence of excessive leak off in the injector.
    Mine starts great but cuts out under full throttle @3000rpm.
    I read that could be low pressure in the rail ( injector leak off ? ) or too high pressure in the rail ( rail pressure regulator buggered ? )
    Also noted is that I can loosen an injector pipe almost immediately after shut down with very little or no pressure release at the pipe connector.
    If the rail can lose pressure so quickly there should be an external leak? Or internal?
    I’m looking at the rail pressure reg next for a buggered o-ring ( leaking would be internal or external ?.. I haven’t actually visually looked yet )
    If o-ring is good then reg is faulty?
    Your thoughts?
    Cheers from Australia again

    1. Hi Peter,
      Your bang on track, though I would look to check out / replace the fuel filter first, make sure you replace it with a OEM MB one as some do not allow the respective flow rate.
      Let me know how you get on,
      All the best

  9. Hi Steve,
    My vehicle a Mercedes ML270 had a blowing injector, it was running perfectly but fumes could be smelt in the car when stationary. I took it to a garage which changed the seals on a couple of injectors. On picking up the car from the garage it was blowing out black smoke on tick-over and when being driven and lacked power through all the gears, only able to attain 60 mph on the flat and less uphill. A couple of days later one of the injectors blew out of the head completely. I would be grateful for your input as to the probable cause of the problems.

    1. Hi Dave,
      Oops.. Sounds like one of the threads on the injector hold down bolts has stripped. Probably at the time of removal/refitting. There is an outside chance the injectors could have been replaced into the wrong cylinders this would have caused a slight mismatch to the individual ‘coding’ that is applied to each injector, but to be honest I would not have expected large amounts of black smoke – this can be re-coded using the Mercedes Star system diagnostics. This action adjusts the fuel trim on each injector for best economy and balance. However there is a greater chance that the smoke was caused by unburnt fuel from a poorly sealed injector, fingers would point to the one that eventually gave way, as it was quite possibly not clamping down to seal the injector correctly – causing low compression (more-so at idle) on that cylinder and allowing incomplete combustion – hence smoky exhaust output.

      Hope that helps and you get it fixed soon, the hold down bolt thread will have to be repaired using a Time-sert, a kind of upmarket helicoil. The kits are available from Wurth with the long series drills and taps needed to accomplish the work. There is a little more reading here that may be of interest.

      Best regards

  10. This is a great write up. I have the ml270 engine in my jeep and am having an ongoing issue with number 3 seal. I have bought the correct seat cutting tool but now my mechanic says just to pull the head and get it replaced. I have been using the merc copper washers and was going to try the honda ones that seem to be working a treat for you, just not sure the specs of them. Are you able to let me know so I can buy some and recut the seat?

    1. Hi Adrian,
      You are best to obtain one and have a look for yourself, they are slightly thicker than the MB ones and softer. Thats the best way to see if they suit your requirement, they should be perfect especially if you have recut the seat.
      All the best

  11. Reading the above (very informative by the way for us novice mechanics! – I have just replaced an injector seal on one of my injectors (ML270CDI) The issue I currently have is that I followed everything to the letter and it all went well, but now the car won’t fire up. It’s not even trying to, it just turns over. There is fuel at the fuel pipe connector that connects to the injector I am being told that I need to reprogramme the injectors /brain – but I only took one out, cleaned it up, new seal and put it back in – is this correct? Do I need to reprogramme the lot?

    1. Eric,
      It should not prevent the engine running, I would be looking for an ill fitted injector electrical connector that if not pushed fully home will prevent the vehicle from starting. Your problem is most probably electrical and is something connected to the work you have been doing. Cam shaft sensor plug or wiring being top of my list!

  12. Fantastic write up! I’ve had so many Merc’s & I’m sick of the problems! My problem is the same as above…..

    1. I might suspect the cam shaft position sensor – I have had to replace them on all 7 german cars I have owned over last 2 decades…..

  13. This is the best write up I have ever read and I am extremely grateful for all the great advice. Thanks so much, how do I join the club…

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