"Oil cooler seals are a major weakness and a royal pain to replace, but the seals seem to... [fail repeatedly]. Summarizing what I've learned so far about the OM642."
The OM642 timing chain and tensioner are a well-documented failure point. Chain stretch is common, particularly on older dual-row chain variants. A worn tensioner can allow chain slack, leading to rattles on cold start and, if neglected, catastrophic engine damage. All reported cases of excessive chain wear appear linked to poor or infrequent oil changes.
Rattling noise on cold start lasting 2-5 seconds
Persistent ticking from the front of the engine
Reduced power or rough running in advanced cases
DIY: Hard
Oil Cooler Seal Failure€800-2,500 · 100,000-200,000 kmHIGH
The oil cooler seals are widely regarded as the OM642's most notorious recurring weakness. Failure causes oil and coolant cross-contamination or external leaks. Replacement is labour-intensive due to the engine's V6 layout and the cooler's buried position, making labour costs the dominant factor.
Oil in coolant reservoir or coolant in oil
Visible oily residue around the oil cooler area
Rising coolant temperature or oil temperature irregularities
EGR valve clogging and swirl flap failure are common across OM642-equipped vehicles. A stuck or seized swirl flap actuator can result in power loss, especially above 2,000 RPM. EGR issues cause carbon buildup in the intake manifold, reducing efficiency and throttle response.
Community sources note the OM642 turbo actuators are more fragile compared to older Mercedes diesel engines. Actuator failure leads to limp mode and power loss. Full turbo failure, while less common than actuator issues, is expensive to rectify given the V6 configuration.
Valve cover gasket leaks are reported on the OM642, often discovered during other maintenance. The V6 layout means both banks can develop leaks. The turbo bracket mounting area and surrounding seals are also noted leak points, sometimes discovered when investigating the oil cooler.
Oil smell from engine bay
Visible oil seepage around valve covers
Oil residue on engine block or underside
DIY: Moderate
Advertisement
Buying Checklist
Perform a cold-start and listen for any timing chain rattle lasting more than 2 seconds before buying
Check the coolant reservoir for oily contamination and the oil filler cap for white emulsion indicating oil cooler seal failure
Request full documented service history — verify oil was changed every 10,000 km or less with correct MB 229.51/229.52 spec oil
Connect an OBD scanner and check for fault codes related to EGR, swirl flaps, turbo boost, and cam timing deviation
Inspect the engine bay for oil staining around valve covers, oil cooler area, and turbo bracket mounting points
Test drive under full load above 2,000 RPM to confirm no power choke or limp mode triggered by turbo or swirl flap faults
Est. ownership cost (2yr)€1,500–5,000 per year depending on maintenance history and mileage; higher if oil cooler seals or timing chain require attention
The OM642 in the W164 ML350 CDI is a genuinely capable diesel V6, but it comes with a well-documented list of expensive failure points that make buying history critical, not optional. A well-maintained example with oil changes every 10,000 km or less can be a rewarding ownership experience, but a neglected one will quickly expose you to a chain replacement, oil cooler job, or turbo actuator bill that can exceed the vehicle's value. Budget €2,000-4,000 for preventive work if service history is incomplete or unknown. Only buy with a full pre-purchase inspection from an independent Mercedes specialist who knows the OM642 specifically.