There was a time when diesel Mercedes-Benz vehicles were synonymous with longevity, dependability, and unwavering reliability. Models like the 240D, 300D, and 300SD, though not known for speed, were celebrated for their robust design and ability to reach any destination. However, the landscape has shifted, and some of the more recent diesel offerings from Mercedes, particularly those equipped with the OM642 diesel engine, have become notorious for their problematic nature and potential for costly repairs. Let’s delve into the issues plaguing the OM642 engine and understand why it’s become an “Achilles heel” for Mercedes-Benz.
While engine critiques can sometimes feel like hyperbole, the concerns surrounding the Mercedes-Benz OM642 diesel V6 are grounded in widespread reports and repair experiences. Many seasoned mechanics and owners express a level of caution rarely seen with other powertrains. While some might be more forgiving of typical wear items or predictable maintenance needs, the OM642 presents a range of issues that can quickly escalate into significant financial burdens. Even for individuals accustomed to maintaining older European vehicles, the OM642 can be a source of considerable frustration.
So, what exactly is the OM642, and which Mercedes-Benz models house this engine? Introduced in the mid-2000s, Mercedes-Benz aimed to consolidate its aging inline-six and inline-five diesel engines into a single, modern turbocharged diesel V6 unit. The result was the OM642, an undersquare (long stroke) 72-degree all-aluminum engine featuring a variable geometry turbocharger. This engine found its way into a wide array of Mercedes-Benz vehicles, including the 2007 and newer E320 CDI and E350 BlueTec sedans, the 2007 and newer R320 CDI, R350 BluTec, ML320 CDI, ML350 BluTec, and 2008 and newer GL320 CDI and GL350 BlueTec SUVs. It also appeared in the 2012-2013 S350 BlueTec sedans, the 2007-2008 Jeep Grand Cherokee diesel models, the less common GLE350d and GLS350d SUVs, and a significant number of Sprinter vans.
Given its broad application across various models, Mercedes-Benz vehicles equipped with the OM642 engine are readily available in the used car market. However, their prevalence in repair shops is equally noteworthy. Let’s examine some of the common problems associated with this engine that contribute to its less-than-stellar reputation.
Turbo Actuator Failures
A frequent complaint with the OM642 turbodiesel is a sudden loss of turbo boost. This often stems from a malfunctioning variable geometry turbocharger (VGT) actuator. The vanes inside the turbocharger, controlled by this actuator, become stuck in a position that prevents proper boost generation. When this externally mounted vane actuator fails, Mercedes-Benz typically recommends a complete turbocharger replacement, an expensive proposition costing several thousand dollars due to the assembly being sold as a single unit.
While aftermarket turbo actuators are available as a supposedly simpler solution, experiences suggest they often suffer from poor manufacturing quality, fitment problems, and limited durability. However, if the turbocharger itself is in good condition, a complete replacement might be an unnecessary expense.
Fortunately, a specialized industry has emerged focusing on refurbishing these turbo actuators. Opting for a rebuilt actuator can offer a more economical repair, potentially getting an OM642 back on the road for a few hundred dollars. While rebuilt actuators can be effective, it’s important to recognize that turbo actuator issues are just one of several potential problems that can plague the OM642 engine.
Swirl Flap Mechanism Problems
Like many modern diesel engines designed to meet stringent emissions regulations, the OM642 incorporates swirl flaps within its intake manifold runners. These flaps are designed to enhance air turbulence at lower engine speeds, theoretically promoting better mixing of the injected fuel and air for more efficient combustion. As explained by parts manufacturers like MS Motorservice, swirl flaps create a swirling motion of air in the cylinder, improving fuel-air mixture, reducing fuel consumption, and lowering emissions, particularly at low engine speeds. At higher speeds, the flaps open to maximize airflow into the cylinders.
However, the exhaust gas recirculation (EGR) system, while effective in reducing emissions, introduces carbon deposits into the intake system. These deposits accumulate on the swirl flaps, causing them to become sticky and eventually seize in place. This carbon buildup can also damage the delicate swirl flap mechanism components, such as the connecting rods. Unfortunately, swirl flaps are not designed to be individually serviceable, necessitating the replacement of the entire intake manifold, or manifolds. Each cylinder bank on the OM642 has its own intake manifold, and it’s not uncommon for both to require replacement due to swirl flap issues.
Replacing just one intake manifold can cost around $700 for the part alone, plus approximately 6 hours of labor. If you’re not equipped to tackle this repair yourself, professional intake manifold replacement can easily result in a four-figure repair bill.
Oil Cooler Leaks in the Engine Valley
Adding to the complexity and cost of OM642 ownership is the issue of oil cooler leaks. These leaks are often caused by the failure of inferior oil cooler gaskets. And where are these gaskets located? Deep within the “valley” of the engine, nestled beneath the intake manifolds and turbocharger. While an oil leak might seem manageable, the location of the OM642 oil cooler makes this repair exceptionally labor-intensive. Mercedes-Benz has packed a significant number of components into the engine valley, creating a challenging access point for repairs.
The oil cooler leak issue is so prevalent that Mercedes-Benz issued a technical service bulletin (TSB) in 2016 addressing it. While the replacement gaskets themselves are relatively inexpensive, the labor involved is substantial. Replacing the oil cooler seals requires approximately 12.3 hours of book time, making it a multi-day DIY project, and a very expensive repair at typical European auto repair shop labor rates.
For a visual understanding of the intricate process involved in accessing the OM642 engine valley, online forums like Peach Parts host detailed DIY guides, illustrating why even experienced mechanics might prefer to outsource this repair.
NOx Sensor Failures
Moving beneath the vehicle, the OM642’s NOx sensors are another frequent source of trouble and repair expenses. These sensors, crucial for monitoring nitrogen oxide emissions, are prone to failure. Replacing a NOx sensor can cost between $450 and $500 per sensor. Prompt replacement is essential because a failed NOx sensor can trigger Mercedes-Benz’s “10-start countdown” – an emissions safeguard that limits the number of engine starts to ten before immobilizing the vehicle.
“Immobilized” in this context is literal. Once the countdown reaches zero, the vehicle will require a tow truck to reach a dealership for reprogramming and repair. Beyond the inconvenience of immobilization, NOx sensors are vital components of the emissions control system, ensuring the engine operates within acceptable environmental standards. Diesel engines already face scrutiny regarding emissions, making functional NOx sensors crucial.
Main Bearing Wear Issues
Similar to some high-performance BMW engines, early versions of the Mercedes-Benz OM642 engine can experience main bearing wear. Mercedes-Benz issued a technical service bulletin in 2013 addressing a ticking noise present at idle and low engine speeds.
The recommended solution from Mercedes-Benz involves “Replacement of bearing shells of 1st crankshaft main bearing.” While this repair can be performed with the engine still in the vehicle, neglecting this ticking noise can lead to significant engine damage and premature engine failure.
Searching online parts marketplaces reveals that used replacement OM642 engines range in price from $6,500 to $8,500, excluding installation costs. Given that many vehicles equipped with the OM642 have depreciated to similar price points, major engine failure can render them economically unrepairable.
If you’re considering a V6 diesel Mercedes-Benz, opting for a naturally-aspirated gasoline V6 or V8 model from 2009 or newer might be a wiser choice, potentially saving you from significant headaches and repair bills. The potential fuel economy gains offered by the diesel OM642 can easily be overshadowed by the cost of addressing its common engine problems. In some cases, the repair expenses could have been better allocated to fuel costs for a gasoline V8 model. While the OM642 engine offers smoothness, torque, and efficiency when functioning correctly, its inherent reliability issues make it a risky proposition unless you’re prepared for potentially substantial maintenance expenses. For those committed to diesel power, alternatives like the BMW X5 35d (E70) may present a more reliable option compared to an ML350 BlueTec. Even though models like the Porsche Cayenne Diesel, Audi Q7 TDI, and Volkswagen Touareg V6 TDI have their own sets of quirks, repair shops generally report seeing fewer unscheduled repairs for these models combined than for diesel ML-Class vehicles alone.
(Photo credits: Mercedes-Benz, Jeep, Mhobsonjr – Own work, CC BY-SA 4.0, eBay, Amazon, Peach Parts, Sprinter Forum, Benzworld, Mercedes Benz Owners)
