Mercedes/OM642

Mercedes · Single turbo

OM642

ML350 CDI·W166·2011–2019·2,987 cc

"The timing chain tensioner is tired of these no-start mornings — and so is your wallet."

RISKY

52/ 100

"Capable diesel, expensive to keep healthy"

261

Max HP

620

Torque Nm

10,000

Oil interval km

Recalls

Known issues

Known Issues

Timing Chain Stretch and Tensioner Failure€1,500-4,000 · 120,000-250,000 kmCRITICAL

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.

Rattling or ticking noise on cold start lasting 2-5 seconds

Rattling after engine sits for more than 2 days

Rough running or misfires at idle

Check engine light with camshaft position sensor codes

DIY: Expert

EGR Valve and EGR Cooler Failure€600-2,000 · 100,000-200,000 kmHIGH

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.

Check engine light at normal coolant temperature (~80°C)

EGR-related fault codes in the ECU

Rough idle or hesitation

Coolant loss without visible external leak (EGR cooler)

DIY: Advanced

Intake Manifold Swirl Flap Motor Failure€300-900 · 100,000-180,000 kmHIGH

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.

Limp mode activation

Intake manifold-related fault codes

Rough idle or poor low-RPM response

Rattling from intake manifold area

DIY: Intermediate

Oil Leaks from Valve Cover Gasket and Oil Cooler€400-1,200 · 100,000-200,000 kmMODERATE

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.

Visible oil seeping from valve cover area

Oil smell after driving

Oil residue on engine bay surfaces

Gradual oil consumption between services

DIY: Advanced

Buying Checklist

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

Ask specifically if timing chain, EGR valve, or swirl flap work has ever been carried out and request receipts

Service Reality

Community oil interval10,000 km

Oil specificationMB 229.51 or 229.52 5W-30 fully synthetic low-SAPS

Est. ownership cost (2yr)€1,200–3,500/year depending on mileage, with a potential one-off timing chain hit of €1,500–4,000 if not already replaced

Real fuel consumptionNo data

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 CDIJeep Grand Cherokee 3.0 CRD (EXF)Chrysler 300C 3.0 CRD

Buying Advice

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.

Parts People Buy

OM642 Timing Chain Kit (chain, tensioner, guides)Link unavailable

EGR Valve OM642Link unavailable

EGR Cooler OM642Link unavailable

Swirl Flap Delete Kit OM642Link unavailable

Valve Cover Gasket Set OM642Link unavailable

MB 229.51 5W-30 Engine OilLink unavailable

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