There was a time when diesel Mercedes-Benz vehicles were synonymous with longevity, dependability, and unwavering reliability. Models like the 240D, 300D, and 300SD, while not known for their speed, were celebrated for their robust simplicity and over-engineered design, reliably transporting owners wherever they needed to go. However, those golden days are unfortunately behind us. Some of the last diesel Mercedes-Benz cars sold in the American market, particularly those equipped with the OM642 diesel engine, have become notorious for being expensive to maintain due to a series of inherent issues. Let’s delve into the problems plaguing this engine in this installment of “Achilles Heels.”
It may seem like a cliché to criticize an engine, but it’s not a stance I take lightly. I tend to be more understanding than some when it comes to vehicle reliability. After all, timing chain components are technically wear items, and issues with predictable failure timelines can be planned for. Even significant flaws that can be addressed as part of routine maintenance are manageable. I personally drive an 18-year-old BMW and a 24-year-old Porsche and write about cars for a living, so I’m not exactly risk-averse. Yet, the Mercedes-Benz OM642 diesel V6 is one engine I would strongly advise against owning.
Alt text: A close up shot of a mechanic’s hand pointing to the engine bay of a Mercedes-Benz, highlighting potential issues with older models.
So, what exactly is the OM642, and which vehicles are fitted with it? In the mid-2000s, Mercedes-Benz sought to replace their aging inline-six and inline-five diesel engines with a single, modern turbocharged diesel V6. The result was the OM642, an undersquare (long stroke) 72-degree all-aluminum unit featuring a variable geometry turbocharger. This engine found its way into a wide range of models, including the 2007 and newer E320 CDI and E350 BlueTec sedans, the 2007 and newer R320 CDI, R350 BluTec, ML320 CDI, ML350 BluTec, and the 2008 and newer GL320 CDI and GL350 BlueTec SUVs. It also appeared in the 2012 to 2013 S350 BlueTec sedans, the 2007 to 2008 Jeep Grand Cherokee diesels, the less common GLE350d and GLS350d SUVs, and a significant number of Sprinter vans.
Given its extensive application, vehicles equipped with the OM642 diesel engine are frequently encountered on used car marketplaces. Unfortunately, they are also commonly seen occupying service bays in local repair shops. Let’s examine some of the recurring problems associated with this Motor Diesel Mercedes Benz.
Turbo Actuator Failures
A frequent issue with this turbodiesel engine is the turbocharger becoming inactive. This occurs when the variable geometry flaps inside the turbocharger cease to adjust properly, becoming stuck in a “no boost” position. Often, this lack of boost is due to a malfunctioning externally-mounted vane actuator. Mercedes-Benz’s official solution is a complete turbocharger replacement, costing several thousand dollars, as they sell the components as a single unit.
While aftermarket turbo actuators are available, marketed as simple fixes, my experience suggests they are often poorly manufactured, have fitment problems, and lack durability. However, if the turbocharger itself is in good condition, a full unit replacement isn’t the only viable option.
Fortunately, a small but dedicated industry has emerged, specializing in refurbishing these actuators. This service can get an OM642 engine back on the road for a few hundred dollars. However, this might only be a temporary fix. While I’ve seen successful outcomes with rebuilt actuators, numerous other potential issues can still sideline an OM642 motor diesel mercedes benz.
Swirl Flap Problems
Like many modern diesel engines, the OM642 incorporates swirl flaps within some intake manifold runners. These flaps are designed to generate air turbulence at lower engine speeds. The theory is that this turbulence enhances the mixing of the direct injection fuel charge with the incoming air in each cylinder. MS Motorservice, a wholesaler of OE engine parts for brands like Pierburg and TRW, explains the function of swirl flaps:
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.
Regrettably, these swirl flaps are prone to accumulating carbon deposits from the exhaust gas recirculation system. This buildup causes them to seize, often damaging fragile components of the swirl flap mechanism, such as the flap connecting rods. Since these flaps are not individually serviceable, replacing at least one complete intake manifold is a common necessity. Replacing both intake manifolds (as each cylinder bank has its own manifold) is also a frequent occurrence.
Expect to pay around $700 for just one intake manifold, plus approximately 6.1 hours of labor for a single bank replacement. If you’re not comfortable working on cars yourself and have limited free time, intake manifold replacement can easily result in a four-figure repair bill for this motor diesel mercedes benz.
Oil Cooler Leaks in the Engine Valley
Adding to the issues in the same area as the swirl flaps, pressurized oil leaks are common due to subpar oil cooler gaskets. The location of these gaskets? Right in the engine’s valley. While this might not be a major issue on some engines, Mercedes-Benz packed the valley of the OM642 with numerous components, including the problematic intake manifolds and the turbocharger itself.
This problem is so prevalent that Mercedes-Benz issued a technical service bulletin back in 2016 addressing it. While the replacement parts themselves are inexpensive, replacing the oil cooler seals requires around 12.3 hours of labor according to service manuals. This is a time-consuming, multi-day DIY job, and the high labor rates at European car specialty shops make this repair very expensive to outsource for your motor diesel mercedes benz.
For a detailed look at the process of accessing the OM642 engine valley, a comprehensive thread on the Peach Parts forum provides a DIY guide. It clearly demonstrates why even experienced mechanics might prefer to pay someone else to handle this repair.
NOx Sensor Failure
Moving to the underside of the vehicle, the OM642’s NOx sensors frequently fail and were among the most common repairs I quoted in my previous role. Expect to pay roughly $450 to $500 per NOx sensor for parts. These sensors require prompt replacement upon failure because a faulty NOx sensor can trigger Mercedes-Benz’s dreaded 10-start countdown. This emissions safeguard limits the driver to only ten more engine starts before the vehicle becomes immobilized.
And when I say immobilized, I mean completely immobilized. Once the countdown reaches zero, the only way to get the vehicle to a dealership is by tow truck. Furthermore, NOx sensors are critical emissions system components that detect harmful nitrogen oxide levels. Diesel engines already face enough environmental scrutiny, making functional NOx sensors essential for responsible operation of your motor diesel mercedes benz.
Main Bearing Wear
Similar to certain high-performance BMW engines, early versions of the Mercedes-Benz OM642 engine can also suffer from engine bearing wear. In 2013, Mercedes-Benz released a technical service bulletin addressing a ticking noise audible at idle and low engine speeds.
The recommended solution? According to Mercedes-Benz, it’s “Replacement of bearing shells of 1st crankshaft main bearing.” While this repair can be performed with the engine still in the vehicle, ignoring this ticking noise can lead to premature engine failure for your motor diesel mercedes benz.
Browsing car-part.com, a popular online marketplace for used auto parts used by many independent repair shops, a good used replacement OM642 engine ranges from $6,500 to $8,500, excluding installation costs. Considering that many vehicles equipped with the OM642 motor diesel mercedes benz have depreciated to around this value, catastrophic engine damage can make repairs economically unviable.
If you are considering a V6 diesel Mercedes-Benz, opting for a naturally-aspirated gasoline-powered V6 or V8 model from the 2009 model year or newer might save you considerable trouble. The potential fuel savings offered by the diesel engine can easily be negated by expensive engine repairs, potentially exceeding the cost of fuel for a V8. While the OM642 is undeniably smooth, torquey, and efficient when functioning correctly, unless you are intent on financially supporting your mechanic’s family, exploring alternative diesel-powered vehicles is advisable if you are set on owning a diesel. The E70 BMW X5 35d represents a superior choice compared to an ML350 BlueTec. Although the Porsche Cayenne Diesel, Audi Q7 TDI, and Volkswagen Touareg V6 TDI also have their weaknesses, I encountered significantly fewer unscheduled repairs for these three models combined than for diesel MLs alone.
(Photo credits: Mercedes-Benz, Jeep, Mhobsonjr – Own work, CC BY-SA 4.0, eBay, Amazon, Peach Parts, Sprinter Forum, Benzworld, Mercedes Benz Owners)
Support our mission of championing car culture by becoming an Official Autopian Member.
Got a hot tip? Send it to us here. Or check out the stories on our homepage.
