Mercedes ML350 CDI OM642

The Mercedes ML350 CDI OM642 has a reliability score of 52/100, rated "RISKY". Capable diesel, expensive to keep healthy

The most frequently reported issues are Timing Chain Stretch and Tensioner Failure, EGR Valve and EGR Cooler Failure, Intake Manifold Swirl Flap Motor Failure, Oil Leaks from Valve Cover Gasket and Oil Cooler. See the Known Issues section above for symptoms, affected mileage, and repair costs.

Yes, this is a documented issue on the OM642, typically appearing around 120,000-250,000 km. The OM642's timing chain and tensioner are a well-documented weak point. The tensioner loses hydraulic pressure after extended sitting, allowing the chain to rattle on cold starts. Prolonged neglect leads to chain stretch, timing drift, and potential catastrophic engine damage. Multiple forum threads and community posts confirm this is the single most feared failure on this engine. Typical repair cost: €1,500-4,000. DIY difficulty: Expert.

Yes, this is a documented issue on the OM642, typically appearing around 100,000-200,000 km. EGR faults are widely reported on the OM642, including on the W166 ML350 CDI specifically. EGR fault codes appear at normal operating temperature, triggering the check engine light. The EGR cooler is also a failure point, with leaks causing coolant contamination and potential overheating. This is a known diesel V6 weakness across multiple Mercedes platforms. Typical repair cost: €600-2,000. DIY difficulty: Advanced.

Yes, this is a documented issue on the OM642, typically appearing around 100,000-180,000 km. The swirl flap actuator motor on the OM642 intake manifold is prone to failure and overloading. When the motor fails it can trigger limp mode and fault codes. Some owners use a resistor workaround, but the correct fix involves replacing the motor or performing a swirl flap delete. The flaps themselves can also break off and enter the engine, causing serious damage. Typical repair cost: €300-900. DIY difficulty: Intermediate.

Yes, this is a documented issue on the OM642, typically appearing around 100,000-200,000 km. Forum data confirms that valve cover gasket replacement frequently uncovers additional leaks at the oil cooler gasket and turbo bracket mounting area. What appears to be a simple valve cover job can escalate significantly once the engine is partially disassembled. Expect to address multiple seals in one service. Typical repair cost: €400-1,200. DIY difficulty: Advanced.

The OM642 in the W166 ML350 CDI is a capable but maintenance-intensive diesel that rewards careful owners and punishes neglectful ones. The timing chain system is the biggest financial threat, always do a cold-start inspection and budget for a chain kit if the service history is incomplete. EGR and swirl flap issues are manageable if caught early but can escalate into expensive repairs if ignored. Buy only with full documented history, a clean OBD scan, and a pre-purchase inspection by an OM642-familiar independent specialist. The savings on purchase price of an unknown example rarely justify the risk.

The hp difference between the two OM642 tunes in the W166 (258 hp vs 265 hp) is minor and not the deciding factor. The real distinction is emissions hardware: post-2015 Euro6 cars gain an AdBlue/SCR system with its own failure points (NOx sensors, dosing pump, tank heater) on top of the existing timing chain, EGR, and swirl flap risks. Earlier Euro5 cars are mechanically simpler but carry no update to the timing chain tensioner design, so chain risk is the same across both. Choose based on documented maintenance history and a clean cold-start and OBD check rather than the model year or power figure alone.

Before buying, check: Cold-start the engine from fully cold and listen for any timing chain rattle in the first 5 seconds, walk away if heard; OBD-II scan for all stored and pending fault codes, especially EGR, swirl flap, and camshaft position sensor codes; Inspect the intake manifold area and underside of engine for oil leaks or soot deposits indicating EGR or cooler issues; Request full service history and verify oil changes were done every 10,000 km or less with correct MB-approved low-SAPS oil; Check coolant level and condition. Milky or low coolant can indicate EGR cooler failure or head gasket issues.

There's no fixed mileage limit, it depends on maintenance. The parts worth budgeting for proactively are: Timing Chain Kit (chain, tensioner, guide rails) (150,000 km), EGR Valve and EGR Cooler (120,000 km), Swirl Flap Actuator Motor (100,000 km). Engines that clear these checkpoints with documented maintenance commonly continue well beyond typical ownership periods.

Fuel injector line connections on some OM642-equipped Mercedes-Benz models were reported to develop micro-cracks over time, allowing fuel seepage near hot engine components and raising engine bay fire risk. Remedy: Replacement of affected fuel injector lines at an authorized Mercedes-Benz dealer under the relevant recall campaign.

Estimated ownership cost is €4,000-9,000 over 3 years depending on mileage and whether the timing chain has already been addressed, covering routine maintenance, EGR and swirl flap work, and a possible one-off chain replacement, based on known issues and typical service needs.

Owners report real-world fuel consumption of around 9.2 L/100km.

Community-reported real-world usage suggests changing oil every 10,000 km using MB 229.51 or 229.52 5W-30 fully synthetic low-SAPS, often more conservative than the official service schedule.

The OM642 is also found in: Mercedes-Benz E350 CDI (W212), Mercedes-Benz GL350 CDI (X164/X166), Mercedes-Benz S350 CDI (W221), Mercedes-Benz R350 CDI (W251), Mercedes-Benz Sprinter 3.0 CDI, Jeep Grand Cherokee 3.0 CRD (EXF), Chrysler 300C 3.0 CRD.

Capable diesel, expensive to keep healthy The biggest recurring complaint is Timing Chain Stretch and Tensioner Failure. Mixed to negative. Enthusiasts respect the engine's performance and torque, but forum sentiment is dominated by timing chain anxiety, EGR frustration, and warnings about escalating repair costs. Owners who maintain it correctly report reasonable reliability, those who stretch oil changes or ignore rattles face serious bills.