Once upon a time, Mercedes-Benz diesel engines were synonymous with longevity, dependability, and robust engineering. Models like the 240D, 300D, and 300SD were legendary for their durability, prioritizing rugged simplicity over outright speed. However, the landscape has shifted, and some of the later diesel engines from Mercedes-Benz, particularly the Mercedes-Benz OM642, have unfortunately earned a reputation as potential money pits due to a series of recurring and significant issues. Let’s delve into the common problems plaguing the OM642 diesel engine, an engine that has become an Achilles heel for certain Mercedes-Benz models.
It might seem harsh to single out an engine, but the Mercedes-Benz OM642 V6 diesel is one that warrants caution. While some engine flaws can be anticipated and managed, the OM642 presents a range of problems that can lead to substantial repair bills. Even for those accustomed to maintaining older European vehicles, the issues with this engine can be particularly daunting.
So, what exactly is the OM642 engine, and in which Mercedes-Benz vehicles can it be found? Developed in the mid-2000s, Mercedes-Benz designed the OM642 as a replacement for their aging inline-six and inline-five diesel engines. The result was a turbocharged, 72-degree, all-aluminum V6 engine with an undersquare design and variable geometry turbocharger. This engine found its way into a wide array 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, 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 numerous Sprinter vans.
Given its extensive use, vehicles equipped with the OM642 diesel engine are readily available in the used car market. However, they are also frequently encountered in repair shops dealing with a variety of mechanical failures. Let’s explore some of the most prevalent issues associated with this engine.
Turbo Actuator Failures
One common problem with the OM642 turbodiesel engine is turbo failure, often manifesting as a lack of boost. This issue frequently stems from the variable geometry flaps within the turbocharger becoming stuck, typically in a position that restricts boost. Often, the culprit is a malfunctioning externally-mounted vane actuator. Mercedes-Benz typically recommends replacing the entire turbocharger assembly, a repair that can cost several thousand dollars.
While aftermarket turbo actuators are available, they have often proven to be unreliable, suffering from poor build quality, fitment problems, and limited lifespan. However, if the turbocharger itself is in good condition, a complete replacement might not be necessary.
Fortunately, a cottage industry has emerged specializing in refurbishing these turbo actuators. These rebuilt units can offer a more affordable solution, potentially getting an OM642 engine back on the road for a few hundred dollars. While these rebuilt actuators can be successful, it’s important to note that turbo actuator failure is just one of several potential issues that can plague the OM642.
Swirl Flap Problems
Like many modern diesel engines, the Mercedes-Benz OM642 utilizes swirl flaps in the intake manifold. These flaps are designed to create air turbulence at lower engine speeds, which theoretically improves the mixing of fuel and air for more efficient combustion. As MS Motorservice explains, swirl flaps enhance fuel-air mixture at low speeds, reducing fuel consumption and emissions. However, these flaps should open at higher engine speeds to maximize airflow.
Unfortunately, the swirl flaps in the OM642 engine are prone to becoming clogged with carbon deposits from the exhaust gas recirculation (EGR) system. This buildup can cause the flaps to seize, often damaging the delicate swirl flap mechanism components, such as the connecting rods. Because these components are not individually serviceable, intake manifold replacement becomes necessary. In many cases, replacing both intake manifolds (one for each cylinder bank) is recommended.
Replacing just one intake manifold can cost around $700 for the part alone, not including labor. With approximately 6 hours of labor per manifold, this repair can quickly escalate into a significant expense, easily reaching four figures if you rely on a professional mechanic.
Oil Cooler Leaks
Another common problem area in the OM642 engine, and often located near the problematic swirl flaps, is oil leaks originating from the oil cooler gaskets. These gaskets are known to be of inferior quality and are situated deep within the engine valley. While oil cooler leaks are not unique to the OM642, the engine’s design makes accessing the oil cooler particularly challenging. The intake manifolds and turbocharger, among other components, obstruct access to the valley where the oil cooler resides.
This issue is so prevalent that Mercedes-Benz issued a technical service bulletin for it back in 2016. While the replacement gaskets themselves are inexpensive, the labor required for replacement is substantial. Replacing the oil cooler seals can take upwards of 12 hours of book time, making it a multi-day DIY project or a very costly repair at a shop.
For those considering a DIY approach, a detailed thread on the Peach Parts forum documents the intricate process of accessing the oil cooler in an OM642 engine. The complexity of this repair highlights why many owners opt to entrust this job to professionals, despite the high labor costs.
NOx Sensor Failure
Moving to the emissions system, the NOx sensors in the OM642 engine are notorious for frequent failures. NOx sensor replacements are a common repair for this engine. The cost for each NOx sensor can range from $450 to $500, and prompt replacement is crucial. A malfunctioning NOx sensor can trigger Mercedes-Benz’s “10-start countdown,” an emissions safeguard that limits the number of engine starts before immobilization.
Once the countdown reaches zero, the vehicle will be immobilized and require towing to a dealership to rectify the issue. Beyond the inconvenience of immobilization, NOx sensors are critical emissions components that monitor nitrogen oxide levels, an important aspect of diesel emissions control.
Main Bearing Wear
Similar to some high-performance BMW engines, early versions of the Mercedes-Benz OM642 engine can suffer from premature main bearing wear. In 2013, Mercedes-Benz issued a technical service bulletin addressing a ticking noise at idle and low engine speeds.
The recommended solution from 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 significant engine damage and premature engine failure.
Used replacement OM642 engines can range from $6,500 to $8,500, excluding installation costs. Considering the depreciation of many vehicles equipped with the OM642, major engine failure can render them economically unviable to repair.
If you are considering a V6 diesel Mercedes-Benz, opting for a naturally-aspirated gasoline V6 or V8 model from 2009 or newer might be a more prudent choice, potentially saving you from significant headaches and expenses. The potential fuel economy benefits of the diesel can easily be negated by engine repair costs, possibly exceeding the fuel costs of a gasoline V8. While the OM642 engine offers smoothness, torque, and efficiency when functioning correctly, its reliability issues make it a risky proposition. For those set on a diesel SUV, alternatives like the BMW X5 35d or even the Porsche Cayenne Diesel, Audi Q7 TDI, and Volkswagen Touareg V6 TDI, despite their own potential issues, have historically presented fewer unscheduled repairs compared to diesel ML-Class models.
(Photo credits: Mercedes-Benz, Jeep, Mhobsonjr – Own work, CC BY-SA 4.0, eBay, Amazon, Peach Parts, Sprinter Forum, Benzworld, Mercedes Benz Owners)
