"These engines are amazing, they will go wrong but in my experience it's always the same things — no surprises. Chain stretch, oil leaks, injector issues."
The OM651 timing chain is a well-documented weak point across all applications of this engine. Chain stretch causes rattling on cold starts, and a failing tensioner can allow slack that leads to timing loss and catastrophic engine damage if ignored. Forum data confirms this affects both low and high power variants.
Community reports consistently identify the rocker cover gasket and the base of the oil filter housing as the two primary leak points on the OM651. Leaks tend to develop gradually and can lead to low oil levels if not monitored.
Burning oil smell
Visible oil residue on top of engine or below oil filter housing
Forum data highlights that the oil pressure smart valve blanking plug (A0019905317) can cause significant oil leaks from the oil temperature sensor area shortly after installation or as the seal deteriorates. This is a known failure point specific to the OM651.
Large oil leak within minutes of startup
Oil leak near oil temperature sensor
Low oil pressure warning
DIY: Moderate
Thermostat & Coolant System Failure€250-700 · 80,000-150,000 kmMODERATE
Multiple forum threads report OM651 thermostats failing in the open position, preventing the engine from reaching operating temperature even after extended driving. The thermostat is a separate item from the water pump on this engine.
Forum data references persistent intermittent starting problems on the OM651, with owners having replaced multiple components without resolving the root cause. Glow plugs, sensors and fuel system components are common culprits.
Hard or no-start especially when cold
Extended cranking before firing
Intermittent fault codes
DIY: Moderate
Transmission Mount Wear€150-400 · 80,000-160,000 kmLOW
Community data notes transmission mount degradation on OM651-powered 4MATIC models, often discovered alongside engine mount replacement. Worn mounts cause increased drivetrain vibration and clunking.
Clunking on acceleration or gear changes
Increased vibration through cabin
Visible mount deterioration on inspection
DIY: Easy
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Buying Checklist
Cold-start the engine and listen for timing chain rattle in the first 10-30 seconds — walk away if present
Check full oil service history; verify intervals did not exceed 10,000 km or 12 months
Inspect rocker cover top and oil filter housing base for fresh oil leaks or heavy residue
Confirm engine reaches normal operating temperature within 10-15 minutes of driving — suspect thermostat if it does not
Scan for fault codes including timing, fuel pressure, glow plug and coolant temperature faults before purchase
Check transmission and engine mounts for deterioration, especially on 4MATIC variants with higher mileage
Service Reality
Community oil interval10,000 km
Oil specificationMB 229.51 or MB 229.52 5W-30 fully synthetic
Est. ownership cost (2yr)€1,500–4,500 over 3 years depending on mileage and chain condition at purchase
The OM651 in the GLC 220d is a capable diesel that rewards disciplined ownership but punishes neglect, especially around oil change intervals. The timing chain is the single biggest financial risk — a well-documented failure that becomes expensive if missed. Prioritise examples with full stamped service history, short oil change intervals, and a cold-start that is completely silent. Budget €1,500-3,000 in contingency for chain work on any higher-mileage example, and have an independent Mercedes specialist inspect the car before committing.