The OM651 uses a simplex (single-row) timing chain which is known to stretch prematurely. The tensioner can fail to maintain adequate pressure, especially on cold starts when oil pressure is low. This is a widely reported issue across European markets, with chain slack leading to rattling, misfires, and potential catastrophic engine damage if left unaddressed. Multiple forum and Reddit threads confirm this is the engine's defining weakness.
Plastic timing chain guide rails are prone to cracking and breaking. Broken fragments can fall into the oil sump and cause secondary engine damage. Forum reports confirm that even during routine valve cover gasket replacement, technicians have found broken guide rails. Engine must be stripped to fully assess damage.
The EGR cooler on the OM651 is known to develop leaks over time. Forum discussions specifically reference EGR cooler failure as a notable issue, with some reports of white smoke from exhaust potentially linked to cooler leakage. Access to the EGR cooler is awkward due to the plastic intake manifold, increasing labour costs.
White smoke from exhaust
Coolant loss without visible external leak
Rough idle
Loss of power
Coolant smell from engine bay
DIY: Hard
Cold Start Shudder and Transmission Jerk€100-600 · 50,000-150,000 kmMODERATE
Owners report the car shuddering or jerking when selecting Drive on cold starts. This can be related to engine hesitation or may require a transmission control unit reflash. Not always a mechanical failure but is a documented and recurring complaint on the W212 OM651.
Community discussion indicates DPF issues are common enough that owners consider DPF delete as a solution. Short-distance or urban driving prevents proper DPF regeneration cycles, accelerating clogging. DPF delete is noted as a modification but carries legal implications in most markets.
DPF warning light
Loss of power
Increased fuel consumption
Frequent failed regeneration cycles
DIY: Moderate
Oil Pressure and Flow Issues Affecting Tensioner€200-800 · 60,000-180,000 kmMODERATE
Forum members note that oil flow issues may contribute to inadequate timing chain tensioner operation, particularly on cold starts. Using the correct high-spec oil and maintaining short oil change intervals is considered essential to mitigate this risk. Some owners have switched to higher anti-wear oil grades as a precaution.
Chain rattle on cold start
Low oil pressure warning
Lifter tick at startup
DIY: Easy
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Buying Checklist
Cold start the engine yourself and listen for timing chain rattle — walk away if any metallic rattling is present
Request full documented service history with proof of timing chain replacement or inspection, especially on vehicles over 100,000 km
Run a full OBD diagnostic scan checking for P0016, P0017, DPF, EGR, and any pending timing-related fault codes
Pressure test the cooling system to detect EGR cooler or coolant leaks before purchase
Check oil condition and colour — dark, sludgy oil suggests neglected intervals which accelerates chain wear
Budget €1,500-4,500 for a preventive timing chain kit replacement if history is unknown — factor this into your offer price
Service Reality
Community oil interval10,000 km
Oil specificationMB 229.51 or MB 229.52, 5W-30 low SAPS — community strongly recommends 10,000 km maximum intervals, not manufacturer's extended drain intervals
Est. ownership cost (2yr)€1,200-5,500 over 3 years depending on timing chain status at purchase — budget for chain kit as a near-certainty on any example over 120,000 km without documented replacement
The OM651 in the W212 E-Class is a capable and refined diesel engine that can cover enormous mileages, but its timing chain and guide rail weakness is a genuine critical risk that must be addressed before or immediately after purchase. Never buy one without a cold-start inspection and a full diagnostic scan — the chain rattle on startup is your clearest warning sign. If the timing chain history is undocumented on a high-mileage example, treat replacement as mandatory and negotiate the cost off the asking price. Maintained with short oil change intervals using the correct MB-spec oil, these engines can be rewarding long-term ownership propositions, but neglected examples can become money pits very quickly.