For decades, diesel Mercedes-Benz vehicles were synonymous with longevity and robust engineering. Models like the 240D and 300D were celebrated for their durability and reliability, even if they weren’t the fastest. However, the landscape has shifted, and the OM642 diesel engine, found in many modern Mercedes-Benz models, has become a source of significant concern for owners due to various reliability issues. This article will delve into the common problems associated with the Mercedes-Benz OM642 engine, providing essential information for current owners and those considering purchasing a vehicle equipped with this motor.
While some may consider engine issues as just part of vehicle ownership, the OM642 presents a pattern of failures that are hard to ignore. Even for car enthusiasts who are comfortable with maintenance and repairs, the OM642 engine can be particularly challenging. Issues are not just limited to typical wear items but often involve critical components with complex and costly repair procedures. If you are considering a Mercedes-Benz diesel from the mid-2000s onwards, understanding the potential pitfalls of the OM642 engine is crucial.
Alt text: Mercedes-Benz OM642 engine overview video thumbnail, highlighting common diesel engine concerns.
The Mercedes-Benz OM642 engine was introduced in the mid-2000s as a replacement for the older inline-six and inline-five diesel engines. This turbocharged diesel V6 engine, with an undersquare design and aluminum construction, found its way into a wide range of Mercedes-Benz vehicles. You can find the OM642 in models like the E320 CDI and E350 BlueTec sedans (2007 onwards), R-Class, ML-Class, GL-Class SUVs (2007 onwards), the S350 BlueTec sedan (2012-2013), and even some Jeep Grand Cherokee diesels (2007-2008) and Sprinter vans. The widespread use means that while vehicles equipped with the OM642 are readily available on the used car market, they also frequently appear in repair shops due to a number of recurring problems. Let’s explore some of the most prevalent issues encountered with the OM642 diesel engine.
Turbo Actuator Failures
One common complaint with the OM642 engine is turbocharger malfunction, often manifesting as a lack of boost. This issue frequently stems from the variable geometry turbocharger (VGT) actuator failing. The actuator controls the vanes within the turbo, and when it malfunctions, the vanes can get stuck in a position that prevents proper boost pressure. Mercedes-Benz typically recommends a complete turbocharger replacement, a repair that can cost thousands due to the integrated nature of the components.
While Mercedes-Benz may push for full turbo replacement, the good news is that aftermarket solutions exist. Although some aftermarket turbo actuators are available, their quality and longevity can be questionable, often leading to fitment problems and premature failure. However, a more reliable alternative to a complete turbo replacement is emerging in the form of specialized refurbishment services.
Several companies specialize in rebuilding OM642 turbo actuators. These services offer a cost-effective repair by focusing specifically on the faulty actuator, assuming the turbocharger itself is in good condition. While these rebuilt actuators can successfully restore turbo function and get your OM642 back on the road, it’s important to recognize that turbo actuator failure is often a symptom of broader issues within the engine’s system, and other problems may still arise.
Swirl Flap Problems
Like many modern diesel engines designed for improved emissions and efficiency, the OM642 engine incorporates swirl flaps in the intake manifold. These flaps are designed to create turbulence in the air entering the cylinders at lower engine speeds, enhancing the mixing of fuel and air for more efficient combustion. As explained by engine component specialists like MS Motorservice, swirl flaps optimize fuel-air mixture, reducing fuel consumption and emissions, especially at lower engine speeds.
However, the exhaust gas recirculation (EGR) system, while effective in reducing emissions, contributes to carbon buildup within the intake manifold. This carbon accumulation can cause the swirl flaps to become sticky and eventually seize. The delicate mechanisms controlling the swirl flaps, including connecting rods, are also prone to failure when the flaps become stuck. Unfortunately, swirl flaps are not typically serviceable as individual components, often requiring replacement of the entire intake manifold assembly. Since the OM642 is a V6 engine, it has two intake manifolds, one for each cylinder bank, and replacing both may be necessary, significantly increasing repair costs.
The cost of a single intake manifold for the OM642 engine can be around $700, excluding labor. With labor times for a single manifold replacement estimated at over 6 hours, and potentially needing both manifolds replaced, a swirl flap repair can easily result in a substantial four-figure bill, especially if you rely on a professional mechanic.
Oil Cooler Leaks
Adding to the complexity and potential expense, the OM642 engine is also known for oil cooler leaks. These leaks are often caused by the degradation of the oil cooler gaskets, which are located in the engine’s valley, a particularly inaccessible area nestled beneath the intake manifolds and turbocharger. While oil leaks are not uncommon in engines, the location of the OM642 oil cooler gaskets makes this issue particularly problematic due to the extensive labor required for access.
The prevalence of this issue led Mercedes-Benz to issue a technical service bulletin (TSB) in 2016 addressing the oil cooler gasket leaks. While the replacement gaskets themselves are relatively inexpensive, the repair is incredibly labor-intensive. Book time for replacing the oil cooler seals can be over 12 hours, making it a multi-day DIY project or a very costly repair at a mechanic. The complexity of accessing the oil cooler, as detailed in online forums by DIY mechanics, highlights why even experienced individuals might prefer to outsource this repair due to the sheer time and effort involved.
NOx Sensor Failure
Moving to the exhaust system, NOx sensor failures are another frequent issue with the OM642 engine. These sensors, crucial for monitoring nitrogen oxide emissions, are known to fail relatively often and are a common repair item for OM642-equipped vehicles. The cost of replacing a NOx sensor can range from $450 to $500 per sensor. Prompt replacement is essential because a faulty NOx sensor can trigger Mercedes-Benz’s “10-start countdown” system.
The 10-start countdown is an emissions safeguard that limits the number of engine starts after a NOx sensor failure is detected. Once the countdown reaches zero, the vehicle becomes immobilized and will require towing to a service center to rectify the issue. Beyond the immobilization risk, functioning NOx sensors are vital for effective emissions control, a particularly important aspect for diesel vehicles given environmental concerns.
Main Bearing Wear
Perhaps the most concerning issue affecting the OM642 engine is main bearing wear, particularly in earlier production years. Similar to some high-performance BMW engines, the OM642 can experience premature wear of the main bearings. Mercedes-Benz issued a TSB in 2013 regarding a ticking noise at idle and low engine speeds, directly related to main bearing issues.
The recommended fix, according to Mercedes-Benz, is the “replacement of bearing shells of 1st crankshaft main bearing.” While this repair can be performed with the engine in the vehicle, neglecting this ticking noise can lead to more severe engine damage and potentially engine failure.
The financial implications of main bearing failure can be significant. Used OM642 replacement engines can cost between $6,500 and $8,500, excluding installation. Considering the depreciation of many vehicles equipped with this engine, major engine damage can render them economically unviable to repair.
Conclusion
If you are in the market for a V6 Mercedes-Benz, particularly a diesel model, it’s wise to carefully consider the potential long-term costs associated with the OM642 engine. Opting for a naturally-aspirated gasoline V6 or V8 model from 2009 or newer might be a more reliable choice and could save you from significant repair bills down the line. While diesel engines offer fuel efficiency advantages, the potential repair expenses with the OM642 can easily outweigh these savings, potentially costing more than running a gasoline V8.
While the OM642 engine offers smooth performance, ample torque, and good fuel economy when functioning correctly, its history of reliability issues should give potential buyers pause. Unless you are prepared for potentially substantial maintenance and repair costs, exploring alternative diesel vehicles or gasoline-powered Mercedes-Benz models might be a more prudent decision. Alternatives like the BMW X5 35d (E70) or even the Porsche Cayenne Diesel, Audi Q7 TDI, and Volkswagen Touareg V6 TDI, while not without their own potential issues, have generally shown fewer unscheduled repairs compared to OM642-equipped Mercedes-Benz models. Thorough research and careful consideration are essential when evaluating a Mercedes-Benz with the OM642 diesel engine.
