For decades, the words “Mercedes-Benz diesel” conjured images of unwavering reliability, engines that seemingly ran forever. Models like the 240D, 300D, and 300SD were legendary for their robust simplicity and over-engineered design, prioritizing longevity over outright speed. However, the landscape has shifted, and the later diesel iterations from Mercedes-Benz, particularly the OM642 V6 engine, have unfortunately tarnished this reputation, becoming notorious for their potential to be costly headaches. Let’s delve into the issues plaguing the OM642 diesel engine, an unwelcome chapter in the otherwise storied history of Mercedes-Benz diesels.
While engine criticism should not be taken lightly, in this instance, it’s warranted. My tolerance for automotive imperfections is generally higher than most. Wear items like timing chain components are expected, predictable failures can be planned for, and even significant flaws manageable when integrated into routine maintenance schedules. As someone who daily drives an older BMW and Porsche and whose profession revolves around automotive content, I’m certainly not risk-averse. Yet, the Mercedes-Benz OM642 diesel V6 stands out as an engine I would actively avoid.
So, what exactly is the OM642, and which Mercedes-Benz vehicles 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. The result was the OM642, an undersquare (long stroke) 72-degree all-aluminum block engine equipped with a variable geometry turbocharger. This engine found its way into a wide range of Mercedes-Benz models, including the 2007 and newer E320 CDI and E350 BlueTec sedans, the 2007 and newer R320 CDI, R350 BluTec, ML320 CDI, ML350 BluTec, the 2008 and newer GL320 CDI and GL350 BlueTec SUVs, and even the 2012 to 2013 S350 BlueTec sedans. Beyond Mercedes-Benz, it also appeared in the 2007 to 2008 Jeep Grand Cherokee diesel models, the rarer GLE350d and GLS350d SUVs, and extensively in Sprinter vans.
This widespread application means that vehicles fitted with the Mercedes-Benz OM642 diesel engine are commonly available in the used car market. However, they are also frequently seen occupying service bays at repair shops, often due to a recurring set of problems. Let’s explore some of the common issues associated with this engine.
Turbo Actuator Failures: Silence of the Boost
A frequent complaint with the OM642 engine is a sudden loss of turbocharger function. The variable geometry flaps within the turbo, responsible for optimizing boost across the RPM range, can become stuck in a “no boost” position. Often, this is due to a failure of the externally mounted vane actuator. Mercedes-Benz’s official solution is often a complete turbocharger replacement, a costly repair running into thousands of dollars, as the components are sold as a single unit.
While aftermarket turbo actuators are available, marketed as a straightforward fix, experience suggests they often suffer from poor manufacturing quality, fitment problems, and questionable long-term reliability. However, if the turbocharger itself is in good condition, a complete unit replacement isn’t the only viable option.
Fortunately, a niche industry has emerged, specializing in refurbishing these turbo actuators. Services like Turbo Actuator Repair offer rebuilt actuators that can potentially restore functionality for a fraction of the cost of a new turbocharger. While these rebuilt actuators can offer a temporary reprieve, other potential OM642 issues may still arise.
Swirl Flap Issues: Intake Manifold Obstruction
Like many modern diesel engines aiming for efficiency and lower emissions, the Mercedes OM642 incorporates swirl flaps in the intake manifold runners. These flaps are designed to create air turbulence at lower engine speeds, theoretically promoting better mixing of the direct-injected fuel and air, leading to more efficient combustion. MS Motorservice, a wholesaler for OE engine parts from brands like Pierburg and TRW, explains the function:
Swirl flaps produce a swirl alongside the cylinder axle. They are used in diesel vehicles to improve the mixing of the fuel-air mixture at low engine speeds. For this purpose, the air is fed to each cylinder through two separate channels in the intake manifold. One of the two channels can be closed with a swirl flap. This creates a swirling of the fresh air. Better mixing reduces fuel consumption and pollutant emission. At higher engine speeds and torques, the swirl flap is opened to achieve a better filling level. The swirl flaps are also opened at starting of the engine and in overrun condition.
However, the exhaust gas recirculation (EGR) system, while effective in reducing low-speed emissions, contributes to carbon buildup within the intake manifold. This carbon accumulation can cause the swirl flaps to become sticky and eventually seize in place. This often leads to damage to the delicate swirl flap mechanism components, such as the connecting rods. Unfortunately, these swirl flaps are not designed to be serviced individually, necessitating the replacement of at least one entire intake manifold. Given the engine’s V6 configuration, with separate manifolds for each cylinder bank, replacing both manifolds is not uncommon.
Expect to pay around $700 for just one intake manifold unit, as seen at retailers like FCP Euro, plus approximately 6 hours of labor for a single bank replacement. For those not equipped for DIY repair, intake manifold replacement can easily result in a four-figure repair bill.
Oil Cooler Leaks: Valley of Tears
Compounding the intake manifold woes, the OM642 engine is also prone to oil leaks originating from the oil cooler gaskets, located inconveniently in the engine’s valley. While an oil leak might seem minor, the valley location on the OM642 is particularly problematic. Mercedes-Benz’s engine design, while powerful, tightly packs components in the engine bay, placing the oil cooler beneath layers of obstructions, including the problematic intake manifolds and the turbocharger itself.
This issue is so prevalent that Mercedes-Benz issued a technical service bulletin (TSB) MC-10135712-9999.pdf in 2016 addressing the oil cooler gasket leaks. While the replacement gaskets themselves are inexpensive, the labor involved is substantial. Replacing the oil cooler seals requires around 12.3 hours of book time, making it a labor-intensive, multi-day DIY project or a costly repair at a European car specialist shop.
For a detailed firsthand account of this repair, the Peach Parts forum hosts a comprehensive DIY thread, illustrating why even experienced mechanics might opt to outsource this repair.
NOx Sensor Failure: Emissions System Alert
Moving to the underside of the vehicle, the Mercedes OM642 diesel engine is also known for frequent NOx sensor failures. These sensors are a common replacement item in the engine’s service history. Each NOx sensor can cost between $450 to $500 for parts alone. Prompt replacement is crucial, as a failed NOx sensor can trigger Mercedes-Benz’s “10-start countdown,” an emissions safeguard that severely limits vehicle use by allowing only ten more engine starts before immobilization.
“Immobilized” in this context means the vehicle will require a tow truck to reach a service center. Beyond the inconvenience, NOx sensors are vital components of the emissions system, monitoring harmful nitrogen oxide levels. Given the environmental concerns surrounding diesel engines, maintaining a functioning NOx sensor system is essential.
Main Bearing Wear: Ticking Time Bomb
Similar to certain high-performance BMW engines, early versions of the Mercedes-Benz OM642 engine can suffer from premature main bearing wear. In 2013, Mercedes-Benz issued another technical service bulletin MC-10135950-9999.pdf addressing a ticking noise originating from the engine at idle and low speeds.
The recommended fix involves “Replacement of bearing shells of 1st crankshaft main bearing.” While this repair can be performed with the engine in situ, neglecting this ticking noise can lead to catastrophic engine failure.
Searching car-part.com, a marketplace for used auto parts, reveals that a used replacement OM642 engine ranges from $6,500 to $8,500, excluding installation. Considering the significant depreciation of many vehicles equipped with the OM642, major engine damage can render them economically unrepairable.
If you are considering a V6 diesel Mercedes-Benz, opting for a naturally aspirated gasoline V6 or V8 model from 2009 onwards might be a more prudent choice, potentially saving you from significant future expenses. The fuel economy benefits of the diesel can easily be negated by the cost of engine repairs, potentially exceeding the fuel costs of a V8. While the OM642 offers smoothness, torque, and efficiency when functioning correctly, its potential for costly repairs makes it a less desirable option. If you are determined to own a diesel-powered vehicle, alternatives like the E70 BMW X5 35d offer greater reliability 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 issues, repair frequencies for these models appear lower than for diesel MLs.
(Photo credits: Mercedes-Benz, Jeep, Mhobsonjr – Own work, CC BY-SA 4.0, eBay, Amazon, Peach Parts, Sprinter Forum, Benzworld, Mercedes Benz Owners)
