I originally wrote this article to help my local customers better understand their BlueTec diesels. After 50 years with Mercedes-Benz I retired in 2017. This article is strictly my opinion. I no longer have any affiliation with Mercedes-Benz or their authorized dealers.
More than likely you’re reading this because you want to know what’s with all the problems everyone keeps talking about? Or you’re sick of all the problems and want to know how to actually fix them. There’s a lot to cover so I’ll stick to the bigger issues. What you read here is like the Preface in a novel. The BlueTec diesel engine and emission system is extremely complicated. Every time I think I understand a particular aspect I later realize there’s more to the story. Unlike a well edited novel, Mercedes has published their technical information in a haphazard and disjointed manner. It’s like trying to read a novel with all the pages mixed up. Mercedes has actually addressed all of the nagging problems from the beginning of production in 2007. But the information is so hard to piece together and understand the whole story. Mercedes publishes the information but they expect a busy Mechanic to sort through all of it and put it in the correct order. The customer takes their BlueTec in for a Check Engine light (CEL). The fault code indicates some sensor or valve has failed. The Mechanic remembers there was a Service Bulletin about this fault code. He looks up the bulletin and it tells the Mechanic to do something in addition to just replacing the part indicated by the fault code. But there’s more to the story. There are other bulletins that tell the Mechanic to download updated software into the CDI and SCR control modules before he ever scanned the Diagnostic system for the CEL fault. More bulletins tell the Mechanic this problem was caused by a problem with another part in the emission system. Still more bulletins tell the Mechanic about routine maintenance items that will cause the same symptoms as the CEL, but not leave a fault code. These maintenance items should be addressed before the Mechanic tries to repair the fault which appears to have turned on the CEL. At some point an experienced BlueTec Mechanic realizes there are no simple solutions to BlueTec problems. There always seems to be more and more aspects to the problem. Mercedes may have most of the problems resolved. Understanding and explaining the complete story to owners who don’t want to hear about complicated expensive solutions is no easy task. Owners don’t think they should pay for these “mistakes”. In August 2020 Mercedes admitted to diesel fraud in America. The fraud appears to revolve around the software in the control modules. News reports say Mercedes was switching off the emission system during normal operation. Frankly I don’t understand how Mercedes is switching off the emission system. Something is missing from this chapter of the story. Which has been the problem for the last 13 years. While this fraud chapter is playing out, at least it’s possible to resolve the rest of the problems.

Judging from the emails I receive readers are under the impression that better engine oil will solve the BlueTec diesels problems. I’m inundated with questions about the best BlueTec oil. Oil is just one piece of the puzzle. The correct oil for specific situations is defiantly important. But no less so than the many updates Mercedes has published. There is no one oil that works the best in every situation. Mercedes has made the oil question needlessly confusing. As these engines get to higher milages the Timing Chains stretch. When I help an owner I try to do it so they get the most benefit out of each dollar they spend for maintenance. A weak Timing Chain must be the first concern for a older engine. Mechanics should be recommending the Timing Chain be checked for its amount of stretch. I’ve seen Timing Chains wear out at 50K and others make it to 200K. Once you know how close your chain is to the wear limit, the Timing Chain becomes more important than the DPF (Diesel Particulate Filter). Certain types of oil do a better job protecting the chain. If you have a low mileage engine you can use oils with higher quality and protect both the DPF and the chain.

I’ve lost count of all the Repair Orders owners have sent me. Owners want to know what was actually done and why it still isn’t fixed? I’m sure you already know how expensive BlueTec repairs are. Most owners are paying for repairs that could have been done for much less or avoided all together. Mercedes has published information about virtually every problem and the correct solution. The most common problem… Mechanic’s don’t follow the instructions.

Mercedes-Benz introduced the OM642 V6 BlueTec diesel in 2007. A few years later they introduced the OM651 4 cylinder BlueTec diesel. BlueTec refers to the type of diesel emission system. The very early versions did not have the AdBlue or what’s also referred to as the DEF (Diesel Exhaust Fluid) system. Other than the DEF, they operate the same way. It’s hard to pick an ideal starting point to explain how the problems start. I’ll start when your engine was new. Every BlueTec Diesel engine starts its life with engine oil that was designed for a gas engine. For details, please read the article about the oil. Mercedes has used Mobil One 5W/30 ESP engine oil since 2007. If you read the new Owner’s Manuals for the BlueTec, you’ll notice Mercedes requires the oil certain API or ACEA oil approvals for a BlueTec diesel. If you go to the Exxon Mobil website and look up the Product Data Sheet for Mobil One 5W/30 ESP, you won’t find the API or ACEA BlueTec diesel approvals. I would also suggest reading the 2019 Sprinter BlueTec Owner’s Manual. You can find a pdf on the Sprinter website. On page 282 Mercedes explains the correct oil approvals. Mobil One 5W/30 ESP is primarily a Gas engine oil. I can send you the Product Data Sheets with notes that explain why the oil doesn’t have the correct approvals. Why would Mercedes and Exxon Mobil use a gasoline engine oil in a diesel engine? They do it to achieve the highest CAFE fuel economy. Gas engine oil has lots of friction modifiers that make the oil extra slippery. Shouldn’t oil be slippery? Like I said, this is complicated. Read the oil article. Diesel fuel has soot. When soot mixes with the friction modifiers it causes “bore polishing”. The cylinder walls become mirror smooth. This also causes Blow-by at the piston rings. Blow-by is oil consumption. Mercedes actually explains this in their Workshop Information System. The oil rating agencies also include bore polishing as a test for the highest rated diesel oils. There are Break-in oils for new engines that don’t have friction modifiers. Mercedes does not use Break-in oil in a new BlueTec diesel. If you read the Owner’s Manual, it says one quart of oil every 600 miles is acceptable oil consumption. I’ll bet you’re thinking your engine doesn’t burn oil. Once again, if you read your Owner’s Manual, you’ll see Mercedes mentions “fuel accretion” can cause the oil level to register high. Your engine is actually burning oil. You don’t notice because during DPF regeneration, diesel fuel is washing past the piston rings (bore polishing) and mixing in the crankcase oil. For owners who want the technical details, this is the type of detailed information I send them.

The Blow-by at the piston rings also causes excess crankcase pressure. Blow-by also causes oil leaks. The higher the crankcase pressure, the more oil leaks you’ll have. Emission control laws mandate that hot crankcase oil vapor cannot vent to the atmosphere. The oil vapor must be reintroduced into the engine via the EGR and air intake system. Over time the oil vapor causes sludge to build-up inside the engine. Oil vapor also collects in the Diesel Particulate Filter (DPF). The DPF is part of the exhaust. The DPF traps the black diesel soot and the oil vapor that’s in your diesel exhaust. Pressure sensors tell the engine computer (ECU) the DPF is full and needs to burn out the soot. To do that, the ECU injects extra diesel fuel into the combustion chamber. This is called “regeneration of the DPF”. That extra fuel causes the exhaust temperature to increase to over 1300F. Not all of that extra fuel is burnt. Some washes past the Piston Rings and goes into the crankcase oil. The extra diesel fuel is also carrying soot. The soot mixes with the friction modifiers in the oil. The system needs time to perform the regeneration. If you live in a cold climate or make short trips, the DPF doesn’t have enough time to complete the regeneration. You shut the engine off before it has time to clean the DPF. Diesel fuel keeps building up in the crankcase oil. There are numerous people on owners forums who don’t believe this happens. They confuse other owners who are trying to understand how this works. Go to the Sprinter website and read the 2019 BlueTec owners manual. Mercedes explains this. Mercedes also says it can ruin your engine. Mercedes calls this “fuel accretion” or “fuel dilution”.

Engineers designed the piston rings so the compression gases go behind the rings and push the rings tight against the cylinder walls. If you idle a lot or drive in slow city traffic, the piston rings aren’t pressed tight against the cylinder walls. This causes even more Blow-by. It can take up to one hour to complete the DPF regeneration cycle in those conditions. When the ECU regenerates DPF it also heats the exhaust gases that turn your Garrett Turbocharger. Those exhaust gases can heat the Turbo to over 1600F. This is another fact that some people on owners forums refuse to believe. 1600F is in the Mercedes SCR test procedures. Garrett says it is normal for your Turbocharger to glow red hot. Garrett also says your Turbo runs at 1600F. Your engine oil also lubricates the Turbo. That means your engine oil is exposed to extremely high temperatures. The oil gets so hot that it vaporizes. The hot vapor goes through the Turbo and into the Intercooler. The Intercoolers job is to cool down the hot air from the Turbo before it goes into the combustion chamber. But the oil vapor is caught by the Intercooler. Now the Intercooler can’t cool the air going into the combustion chamber. The oil at the piston rings is even hotter. This is where the oil viscosity becomes so important. Once again, some people on the owners forums say the 5W/30 factory recommended oil is fine. “They’ve never had a problem.” 5W/30 is a low viscosity oil. If you live where it gets hot, 5W/30 oil film gets very thin when it’s 110F and you’re setting in rush hour traffic with the A/C going full tilt. Soot is very abrasive and clumps together in the oil. 5W/30 oil film gets so thin that the clumps of soot are thicker than the oil film. Mixing soot with the friction modifiers in boiling hot 5W/30 oil and you have the perfect polishing compound doing its thing inside your engine. Higher viscosity oil such as 20W/50 or 10W/60 has a thicker oil film at high temperatures. The clumps of soot are thinner than the oil film. This is absolutely true and I have the Petroleum Engineering research papers that prove it.

On page 283 in the 2019 Sprinter owners manual Mercedes tells owners to use the higher viscosity oil in high ambient temperatures. Unfortunately Mercedes doesn’t explain why the higher viscosity is important. The rubber hoses, seals and plastic parts in the Intercooler and elsewhere were not designed for the extreme heat and oil sludge. Even the Turbo had to be redesigned. The extreme heat shatters the Turbo. Garrett calls it the Sever Duty Turbo. The oil vapor rots the rubber hoses and seals. The heat cracks and melts the plastic. This causes air leaks which cause “low Boost” fault codes and Limp Home power loss. As the hot oil vapor air enters the Swirl Flaps they also become clogged with sludge. The Swirl Flaps have movable flaps that were designed to give the engine more power. The flaps are controlled with plastic linkage which finally fails and causes the engine to go into the Limp Home mode. The plastic parts have been updated to metal. If you address the fundamental problems before they cause all of these other problems it cost much less to maintain. The hot oil vapor also passes through the EGR (Exhaust Gas Recirculation) valve and the EGR Cooler. The EGR finally clogs with the burnt oil sludge and triggers a Check Engine Light (CEL). When the EGR Cooler clogs with oil sludge it causes the exhaust temperature to go to high. That causes even more Blow-by. The extreme heat causes the NOx and other exhaust sensors to fail. It even causes the exhaust pipes to crack and cause exhaust leaks into the cabin. If you have exhaust leaks that nobody can seem to find, send me a email and I’ll send you pictures of where the exhaust leaks.

The AdBlue or DEF system is another known problem. The level sensors and heating elements fail. This can cause the engine to stop starting. In Canada Mercedes has extended the warranty to 8 years or 80,000 miles. In America Mercedes has extended the warranty to 7 years or 70,000 miles. Dealers want between $2000 and $3000 to fix the AdBlue after the warranty. There are much less expensive ways to fix the AdBlue. Always keep the tank filled. The AdBlue sensors fail more often when they are not covered with the AdBlue fluid.

NOx sensors are another common failure. Mercedes has updated them at least 5 times. NOx sensors must be replaced as a pair. High exhaust temperatures often cause the NOx sensor to fail. Mercedes even tells mechanics they should replace or clean the EGR cooler when a NOx sensor fails. The Intercooler should also be cleaned.There is finally a actual tool for cleaning the EGR and intake system. In the past, mechanics had to disassemble the intake system for cleaning. This is very time consuming. The tool can also be used for cleaning the DPF. The tool cost less than most dealers charge to clean the DPF. The tool now has the adapters for the OM642 and the OM651. In the past there was no one that made the adapters for BlueTec diesels.

Timing chains are another chronic problem. Mercedes has admitted that all the 2016 and older timing chains will stretch and fail. The sprockets are also wearing out. That means the front cover has to come off. This gets to be very expensive. 5000 mile oil changes with oil containing 1400 ppm to 2000 ppm of zinc (ZDDP) will prevent the timing chain failure. There is also a aftermarket billet oil filter housing with a bigger spin-on oil filter.

Oil leaks are one more frustration. The oil cooler is the most common leak, but not the only leak. When something leaks oil into the valley of the engine, there are 3 drain holes. One on each side and one at the rear. When oil leaks out the drain holes it blows all over the bottom of the engine. It’s hard to tell where all the oil is coming from. Mercedes has a service bulletin that list all the known oil leaks and the updated part numbers. The Heat Exchanger on the side of the oil filter housing is a common oil leak.

Fuel system failures are very expensive. Even if you think you are covered by the warranty, it will be very hard to get Mercedes to pay for fuel system problems. The most common cause of failures is gas in the fuel system. Mercedes has a bulletin that recommends additional lubrication in the fuel. Low sulfur fuel is hard on the high pressure fuel pump. Engineers use diesel fuel to lubricate several elements in the high pressure pump. Gas has no lubricant. If you accidentally get a small percentage of gas in your fuel, the high pressure pump can fail. It shows up as metal in the fuel system. Moisture is also a problem. Rust in the low pressure pumps is not covered by warranty. There are easy fuel additives that protect the system.

The diagnostic system in the ECU is another problem. The fault codes cannot be trusted. You’ll often find fault codes with no meaning. Customers have wasted a lot of money on worthless repairs caused by misleading fault codes. Mercedes has published 40 to 50 software updates for the ECU. They were trying to fix the fault codes and reduce the amount of fuel required for the regeneration of the DPF. Mercedes never really says what the updates fix. Whenever the mechanics have an intermittent or odd problem, they should first try the latest software. Whenever electrical parts were replaced, the software should be updated. Over time this has proven to not work very well. Many dealers tell owners there is nothing wrong with the software or updates are not needed. Mercedes has finally developed a repair program. The ECU must be sent to Germany for repair. The customer pays for the repair. If you have any hope of making the emission system work properly, the ECU must be repaired by Mercedes. Deleting the emission system was the only way some owners could live with their vehicle. It’s not cheap, but at least owners can install the updated parts and get the ECU repaired. You will finally have a BlueTec that is reliable. The biggest hurtle is getting a proper evaluation and the most economical repair plan. (Update 1/2020: Mercedes has stopped the Germany repair program. They’ve gone back to dealers updating the software in the ECU. This is one of the major problems. New 2019 Sprinters are ok for a while. The older the ECU the more software problems you’ll have. Contact me if you want more details. I will not help you delete the emission system. The emission system can operate properly but it’s not cheap.)

If you have a 2015 or older BlueTec, you can assume that every electrical sensor has been updated. You can also assume that every plastic part has been updated. Turbo resonators are plastic and they crack. The plastic linkage for the Swirl Flaps fail and are replaced with metal linkage. Mercedes says Turbo actuators are not available as a separate part. There are three different versions and new actuators are available from the aftermarket. The oil separator has been updated a dozen times. Mercedes is trying to reduce the hot oil vapor going into the Turbo. A catch tank is the only way you can ever stop the crankcase vapor from going into the EGR and intake system. A catch tank also stops blow-by from going into the DPF. The fewer times the DPF regenerates the better it is for your engine. There is no one making a catch tank for the BlueTec. There is a billet catch tank that’s made for a different diesel that works fine. Plastic undersized catch tanks will not work. They can’t handle the back pressure over time. The hoses must be the same diameter as what comes from the Mercedes PCV system.

New Sprinters are vastly improved. They only need a few things and the emission system will last 30 years without problems. Use Break-in oil and the proper diesel oil for your climate. Ignore the recommended 20,000 mile oil changes. Install a catch tank and use a good fuel additive. Do the normal maintenance and you’ll think you’ve got the best diesel on the road.
Never let the engine idle for long periods. When Sprinters are used as Delivery Vans, Limos or Ambulances they often let their engines idle for long periods. They have the highest Turbo failure rate. When Turbos fail, you must remove the Intercooler and clean out the broken parts.The oil sludge is very difficult to clean. Once the engine is routinely switching on the Check Engine Light the owner is forced to clean up the oil sludge in the EGR and other systems.

Mercedes also makes an Auxiliary battery kit for all the Sprinters. These vehicles have so many computers that one battery can’t supply enough voltage at peek load. The special battery kit Mercedes makes for the Sprinter will give the computers enough voltage when demand is the highest. Low Voltage fault codes are quite common. The computers need 12.5 volts to operate properly. Mercedes also makes a block heater for cold climates. It’s important to keep condensation from freezing in the oil. If you live in a cold climate, you should change the oil before the start of freezing weather. Frozen moisture really makes a mess in the oil.

I get a lot of request for more specific help from owners. I’m happy to advise you on whatever BlueTec questions you have. I can give you the best maintenance plan for your specific vehicle and for where you live. I’m getting 30 to 40 emails per day. People always want to know what’s the best oil. I’ve tried to explain this is more complicated than switching oils. If you only want to know a better oil, use Amazon Basic’s 5W/40 Synthetic Diesel oil. It’s cheap and way better than the Mobil One ESP. It meets all the API and ACEA requirements. For owners who actually want to understand this engine and solve the problems, there are better and more expensive oils. It depends on your unique situation. I send owners a lot of technical information. I explain how it relates to them and how they can cut their future repair cost. I show you where to get things like the Catch Tank and how to install it. If you have specific problems I send you the diagnostic information that explains what needs to be fixed. There’s a lot to read and it’s complicated. I’ll guide you through it and explain it until you really understand the problems. Like I said at the beginning, I retired in 2017. I’m doing this to help my 4 year old Grandson save for his college. I set up a PayPal account that goes to his 529 Savings account. It cost $200. I have all sorts of people contact me. Some have no idea what they’ve bought. They believe everything they’ve read in the Sales Brochure. I feel bad for them but it’s to hard to explain all of this to them. This works best for owners who are mechanically experienced and they want to protect a very expensive engine. If you totally rely on the dealer for everything, this will be a challenge. You can email me at; tom54stephens@gmail.com. (I get so many phone calls that I had to remove my phone number. Once we start I’ll send you my number and you’re welcome to call as often as it takes.)