Most people are finding this website after they’ve had a problem with their Mercedes-Benz BlueTec diesel.   Other people find it as they research a new Sprinter.  Still others have found the link on a owners forum.   The article was originally written as a way to explain the BlueTec diesel to my local customers.  As time pasted, I began to hear from BlueTec owners all over the world. I’ve learned a lot from the thousands of stories.  If I tried to cover them all, this article would be impossible to read.  I’m constantly searching for the most concise way to convey what I’ve learned about this engine and its emission system.  I worked for Mercedes-Benz as a Mechanic, Shop Foreman, Instructor, and a Service Manager for 50 years.  I retired last year.  A BlueTec diesel is the most complicated engine Mercedes-Benz ever built. What follows is my opinion about what I’ve learned.

In 2008, Mercedes put the first OM642 V6 diesel in their passenger cars and their Sprinters.  After that, they added the OM651 4 cylinder BlueTec diesel.   I’m just going to deal with the hard stuff. If you want the production information, you can look it up on Wikipedia.  Mercedes has dropped the diesel from their passenger cars and only puts the OM642 engine in the Sprinter.  I still hear from a few passenger car owners, but most have given up.  The cost of repairs is to much for most people.  However, Sprinters are one of Mercedes most popular models.  For the most part, owners love their Sprinters.  People have invested a lot of money in custom features.  Sprinters hold their resale value.  Most owners want to do the best preventive maintenance and are willing to spend the money.  Magazines & Websites can’t get enough of all the Sprinter’s cool features.  But when the reviews get around to the engine, they don’t have much to say.

The BlueTec diesel has been out for over ten years.  There should now be plenty of technical information about them, so where is it?   You can find a little on some owner’s forums.  YouTube has some repair advice.  Most of these are the common repairs.  They don’t get into why it happened in the first place.  I’ll be the first to admit, I can never seem to find the best way of explaining this engine & its emission system.  It’s a lot like peeling an onion.  Every time I think I’ve got it covered, there is another level… and then another… and another.  I suppose that’s why you don’t find anyone else trying to explain this thing.

Diesel engines are inherently the dirtiest engines on the planet.  Practically every problem with a BlueTec stems from the emissions system.  It’s hard to pick the best starting spot, so I’ll start with soot.  Soot is the black smoke you see in a diesel exhaust.  Engineers installed a Diesel Particulate Filter (DPF) to capture the soot and not let it into the air.  At some point the DPF can’t hold any more soot.  To get rid of it, the engine’s computer (ECU) needs to heat the exhaust to 1600F.   The extreme heat incinerates the soot in the DPF.  It’s like a self cleaning oven. This self cleaning process is know as “regenerating the DPF”.  It’s the process of regenerating the DPF that causes so many other problems.   When pressure sensors tell the (ECU) the DPF is full of soot, the ECU injects extra fuel into the combustion chamber.   The extra fuel heats the exhaust and the soot is incinerated.  It sounds simple, but of course it’s not. During regeneration, some of the extra fuel washes past the piston rings and into the crankcase oil.  Mercedes calls this “fuel accretion”.  This is obviously not very good for the engine oil.  If you’re using B20 biodiesel, it makes the problem much worse.  How and where you drive determines how much diesel fuel will get into the crankcase oil.  If you make short trips in cold weather, the engine never gets hot enough to regenerate the DPF.   The oil also never gets hot enough to burn off condensation that’s naturally in the oil.  Freezing temperatures cause the condensation to freeze.  Until it thaws, the frozen water acts like sand in the oil.  Soot keeps building up in the DPF.  The extra fuel keeps washing past the piston rings.  Mercedes-Benz explains it this way.  “The normal regenerating intervals of the BlueTec DPF (the driving distance between two regeneration periods) is 130 miles of City driving, & 320 miles of Highway driving.”   The problem is, very few people only drive in the City or Highway.    “The duration of a regeneration period generally lies between 15 & 25 minutes.”   (The regeneration of the DPF takes 15 minutes if you’re on the Highway, & 25 minutes in City traffic.)    It takes the engine about 30 minutes to warm-up enough to begin the regeneration.  If you only make short trips in stop & go traffic, it can take up to an hour of driving before you’ll complete a regeneration of the DPF.  Thirty minutes to warm up the engine, & another 25 minutes to complete the regeneration.  The colder the weather, the longer it will take.   If you shut the engine off before the reiteration is complete, all that extra fuel was a complete waste.  You got a load of diesel fuel into your crankcase, and nothing to show for it.   If you keep making short trips, you’ll eventually have a crankcase full of diesel fuel and a clogged up DPF.  On the other hand, Freeway driving makes the regeneration go more quickly and with less frequency.  If you kick it into passing gear while going up a hill, the DPF is in heaven.  Vehicles primarily driven on the Highway will have far fewer problems.  This is only a tiny part of what’s actually happening, but this should help explain why some owner’s have more trouble than others.  But even with this simplified explanation, there are plenty of mitigating factors.  How old is the ECU software?  What type of oil is used?  When was it changed? Has the Oil Separator been replaced?  How dirty is the air filter?  It goes on & on.

Now you see how your driving style & the climate will effect the regeneration of the DPF.  So what happens to all that diesel fuel in the engine oil? How can Mercedes-Benz tell all the BlueTec diesel owners they can go 20,000 miles on oil changes?  Some people live where it’s freezing & others live in 100F heat.  How can they tell everyone to use the same Mobil One 5W/30 ESP oil? Obviously some engines will have more fuel dilution than others.  Well, Mercedes actually did address this in a 2012 Service Bulletin S.I.00.20-D-0029A.  The bulletin explains “fuel accretion” and the damage it can do to the engine.  The factory tells dealers to advise owners who make short trips to “dramatically” reduce the service intervals.   Owner’s driving short trips should change the oil every 3,000 miles.  Owners in warm climates that drive both city & freeway, should change the oil every 5,000 miles. Owners in warm climates who drive exclusively at freeway speeds, may go 20,000 miles on oil changes.  The factory has published many service bulletins about this.  But Mercedes Marketing Department and Mercedes dealers continue to tell owners they can go 20,000 miles on oil changes.  Then Mercedes Warranty Department has its say.  The Warranty Department tells dealers the factory will not pay for damage caused by 20,000 mile oil changes.  I get calls every day from owners who can’t believe their engine locked up at 40,000 or 50,000 miles.  Dealers want $30,000 to $40,000 for a new engine.  People think it won’t happen to them.  They’ve heard some owners say they’ve gone 300,000 miles and never had a problem.  I’m sure their have been owners that never have problems.  For every 300,000 mile owner, I can show you ten where the engine locked up at 60,000 miles.  How lucky do you feel?  You can buy a whole lot of maintenance for $30,000 or $40,000.  Dealers tell owners they haven’t seen any problems with the BlueTec diesel.  They don’t know how this rumors get started.  They’ve never seen a problem with 20,000 mile oil changes.  The Mercedes sales brochures tell owners they can go 20,000 miles.   But hidden away in the back of the Owners Manual, Mercedes-Benz says: “The low temperature characteristics of engine oils can noticeably deteriorate during operation, e.g. from aging, soot and fuel accretion. For this reason, regular oil changes using an approved engine oil from the suitable SAE classification are urgently recommended.”  “But my Salesman told me I could go 20,000 miles.”

When the DPF requires regeneration, additional diesel fuel mixes with your engine oil.  The fuel dilutes the oil & makes it thinner.    That’s the fuel accretion Mercedes talks about in their bulletins.  If you start with a low viscosity 5W/30 oil, it becomes very thin when it’s mixed with diesel fuel.  Then the Turbocharger heats the oil to 1600F during the regeneration.  Fuel dilution is nothing new.  The traditional method for dealing with fuel dilution is with a heavier viscosity oil.  20W/50 or 10W/60 oil is a much better choice for fuel dilution.  Mercedes has actually put this recommendation in their 2018 Owner’s Booklet.  However, they say it in such a convoluted legalistic manner, that it’s hard the average owner to understand what Mercedes is saying.  They say different oil viscosity is “urgently recommended” & “severe engine damage can result”.   But they never tell owners exactly what oil they should use.   You’re supposed to do your own research and figure out all of these technical terms.  The owners manual reminds owners that engine damage caused by the wrong oil is not covered by the warranty.  Mercedes own branded oil, is 5W/30 and is Mobil One ESP.  If this is not the appropriate oil, why can’t Mercedes tell owners what is the appropriate oil?  Why this disjointed convoluted explanation?

I once had a long section about the properties of diesel oil.    It got overly complicated, so I added a new page on the website that will explain the problems with diesel oil.  How & where you drive, makes a big difference in the type of oil your engine needs.    There are different oils for all types of driving.  If you want to know more about how the oil works, check the section about oil.
From time to time I’ve mentioned oil Catch Tanks.  Most owners don’t understand them, and they are scared to try one.  I’ve never found one that was made specifically for a BlueTec diesel.  If you want one, you have to modify one that was made for a different vehicle.  This makes a Catch Tank something that most owners won’t mess with.  The crankcase oil vapor is what causes all of the oil sludge in the EGR and air intake system.  Better oil and more frequent oil changes reduce the oil vapor.  If you could completely stop the oil vapor, your engine would pollute less and run better.  A Catch Tank would stop all the oil vapor.  It’s not for everyone.  A Catch Tank is for owners who are mechanically inclined and want to keep their engine as clean as possible.  Contact me if you want to know more.

Not only does the recommended 5W/30 oil easily vaporize, it also lacks enough zinc to protect the timing chain.   The timing chain stretches & then you get to replace a $4,000 timing chain.   If you had used a proper oil, the chain would last the life of the engine.  The EPA oil has been ruining timing chains for 25 years.  None of this is new.  Mercedes admits to the timing chain failures in Service Bulletin LI05.10-P-049909.
Once you’re out of warranty, the oil vapor finally becomes visible all over the engine.   Remove the top cover & you’ll see what looks like oil leaking from everywhere.   A little later, & the oil cooler seals give up.   Then it’s the heat exchanger on the oil filter housing that warps from the heat & it leaks oil.   The rubber seals in the air intake get so hot they leak & the engine goes into limp home with low boost pressure fault codes.   NOx sensors fail because the EGR cooler is clogged with burnt oil. Oil sludge in the Swirl Flaps causes the linkage to bind & break.   All of these problems are solved with better preventive maintenance.  If you think preventive maintenance is expensive, wait until you get the bill for cleaning up this mess.
Rust in the fuel system causes injectors to clog and drip fuel.  That causes the glow plugs to fail.  When a glow plug fails the DPF regeneration stops.  When the regeneration stops, the DPF clogs & the check engine light comes on.  It’s $17,000 to replace the fuel system because of the rust.  Then the glow plugs will need to be replaced.  As the glow plugs are removed, they can break off in the cylinder head.  In order to remove the broken glow plug, the cylinder head has to be removed and sent to a Machine Shop.  You could have replaced the engine for $30,000, but even that doesn’t include the DPF & fuel system.  Rust in the fuel system is easily preventable.

Mercedes-Benz is constantly updating the software for their computers.   The updates solve all sorts of intermittent problems.   Many updates are related to the fuel accretion & DPF regeneration.   Mercedes dealers don’t like doing the updates, so they tell owners they aren’t needed, or have already been performed.   Owners waste money on expensive repairs that are destine to fail again, & again, & again, simply because the software is obsolete.  Mechanics and owners ignore the root causes, because nobody wants to pay for it.    Mercedes admits that all 2009 to 2014 SCR / AdBlue systems will fail.    It’s a known problem with the level sensors & the heating elements.    The list goes on & on.

As of 2019, B20 biodiesel will be sold in every state.    You better start paying close attention to the pump.   At one time, Mercedes did a fair job of explaining the damage caused by B20 biodiesel.   However, Mercedes must have gotten a call from the EPA.  Mercedes really toned down their admonitions about B20 biodiesel in their latest sales brochure.  Just so we’re clear on this. Mercedes does not approve any biodiesel above B5.  There are large parts of America where you can only buy B20 biodiesel.  Mercedes will not warranty the damage caused by anything above B5 biodiesel.  So what do you do, when you have no choice?  There are additives you can put in the oil & fuel which will mitigate the damage.  But Mercedes says you can’t use additives in the oil or fuel.  Now what?

I’ve shortened this article because it was getting long & technical.  It’s still to long and technical.  I’ve left a lot out.  Every problem has a solution and a way to prevent the problem.  I get a lot of calls and emails from owners who want more information about how to take care of their BlueTec.   Most Sprinter owners have invested a lot of money in their vehicles.  When SUV owners have problems with theirs, they abandon the car.  They can’t justify the repair cost.  Sprinter owners can’t walk away from their investment.

If you’re buying a new Sprinter, you have a once in a lifetime opportunity to properly break in the engine’s piston rings.    The engines come with standard Mobil One ESP synthetic motor oil.  Because of fuel dilution, BlueTec diesels need to have a perfect seal with their piston rings.  The EPA wanted to reduce the waste oil going into the environment.  So the EPA made everyone stop using the break-in oil.  If the piston rings have a perfect seal against the cylinder walls, you’ve got a way better chance of keeping fuel accretion to a minimum.   It’s much harder for fuel to get past the piston rings and into the crankcase oil.  You only get one chance on a new engine to get it right.   Plus, it’s simple to do.   But don’t bother asking a dealer to do it.   You’ll just get a big horse laugh & “you don’t need to do that”.   “Who told you to do that?”   “That’s old school.  Nobody does that any more.”  If you hear those comments, ask them to explain fuel accretion and blow-by in the OM642 diesel?  Ask them to explain Mercedes-Benz service bulletin S-B-09.20/29 and what’s causing the oil to go into the Turbocharger?  Ask them where the oil goes after it enters the Turbo?  The oil is caused by Blow-by on the piston rings.  There’s Blow-by at the rings because they were never broken-in properly.

Mercedes-Benz no longer offers factory rebuilt engines for the early versions of the OM648, OM642, & OM651 BlueTec Diesel.  The EPA wants these old engines off the road.  The emissions are way to dirty.   There are currently a number of class action lawsuits over the BlueTec Diesel.   The emission systems are stressed to the limit of technology.  Owners hold out some false hope that these lawsuits will somehow help them.  I hear from so many distressed owners who have been denied warranty on their ruined engine.  They want to know if there is some way to force Mercedes to honor their warranty.  Honestly, there isn’t much hope.  There is only one source for a decent third party rebuilt engine.  Even then, there is more to it than just replacing the engine.  I hear from owners who are so financially upside down, that they can’t walk away from their car.  If you’re really stuck, there is only one way to untangle the mess.  It will still cost about $15000, but the repair will last.  I’ve never understood why people would rather fight with Mercedes than change their maintenance routine.  Some people think I’m just trying to drum up business for my shop.  I’ve retired.  It doesn’t matter to me if you service your vehicle at the dealer or do it yourself.  I hear from owners all over the world.  I doubt the S-class BlueTec owner in Tasmania will drop by very soon.  However, I would like to fund my 2 year old Grandson’s college savings plan.  I have a small window of opportunity to use my experience to help owners and my Grandson.  I’ll explain the preventive maintenance in a way that you will understand.  Once people understand what’s going on, they find life with their BlueTec diesel much easier.

If you want help, you can email or call me.  It cost $200 to my Grandson’s college savings plan.  I’ll send you a link to PayPal.  I’ll also send you detailed instructions about the problem areas and how to avoid them.  It helps if you are somewhat mechanically inclined.  But even if you aren’t, you’ll get the general idea.  You’ll know how to recognize a local repair shop that will help you follow the maintenance.  It doesn’t matter if you have a new Sprinter under warranty or a older model that’s burning a quart of oil every 200 miles.  Nothing I tell you will effect the warranty.  I’ve heard of dealers telling owners not to change their oil before 20,000 miles or it will void the warranty.  There is no way Mercedes can void your warranty because you change your oil every 5,000 miles.  My goal is for you to never need the warranty.  Zero warranty claims is a success.  I want your emission system to work just like new when you have 200,000 miles.   I’m not trying to sell you any products.  I’ll tell you what works, & you can buy them where ever you want.  If you’re thinking of a new Sprinter, I can explain the obscure options that are really helpful.  For example; Sprinter offers a third auxiliary battery.  You should order that battery, because it gives the electrical system the extra voltage it often needs at peak load.  When mechanics scan your diagnostic computers, they often find low voltage fault codes.  Even though there is often nothing wrong, the low voltage code is still telling you the system voltage is intermittently to low.  It’s better not to have low voltage fault codes.  So, just order your Sprinter with the extra battery.  That’s why Mercedes offers it.  I will tell you what’s a waste of money.  Never buy the Prepaid maintenance plan.  $600 for a oil change is a complete rip off.  I used to think the extended warranty was a smart idea.  But I hear from to many owners who can’t get Mercedes to pay the claims.
You can reach me at, or 916.715.0665.