The OM651's timing chain system is its most notorious and well-documented failure point. The primary and secondary chains stretch prematurely, tensioners lose pressure, and plastic guide rails crack and shed debris into the oil system. Cold-start rattles are the classic warning sign. Multiple Reddit threads and forum posts confirm this is widespread across W204 applications. Owners who disable stop/start and shorten oil change intervals report significantly better outcomes. A full chain kit replacement including all guides and tensioners is the only proper fix.
Loud rattling or clattering on cold start
Rattle that persists beyond 10-15 seconds after start
Loss of power and limp mode (slackened chain affecting timing)
Engine unable to exceed 2,000 rpm
P0340 camshaft position sensor fault codes
DIY: Expert only — requires engine front disassembly, timing lock tools, and 1.5–2 days labour
The EGR cooler on the OM651 is prone to internal cracking, allowing coolant to leak into the intake tract. This coolant is then ingested by the engine at startup, causing white smoke, rough running when cold, and in severe cases, hydrolocking damage. Forum data confirms this is a known failure requiring EGR cooler replacement. Coolant pressurisation of the system is a related symptom requiring investigation of the EGR cooler, water pump, and oil cooler as a set.
White smoke from exhaust especially on cold start
Coolant level dropping without visible external leak
Rough or stumbling idle when cold
Coolant system pressurising abnormally
Sweet smell from exhaust
DIY: Moderate — accessible with mechanical experience but requires coolant system bleeding
The OM651 intake swirl flaps are known to cause issues, with owners on forums actively seeking ECU maps to disable them. Swirl flap actuator failure or flap breakage can result in debris entering the engine, rough idle, and power loss. A popular solution in the tuning and reliability community is to blank or software-disable the swirl flaps to prevent catastrophic ingestion of broken flap material.
Rough idle or hesitation at low rpm
Power loss under load
Rattling from inlet manifold area
Intake-related fault codes
DIY: Moderate — inlet manifold removal required; blanking plates straightforward to fit
NHTSA complaints highlight premature corrosion of rear brake backing plates and brake lines on W204 models, including garage-kept examples. Critical brake pipe failure due to corrosion has been reported even on well-maintained cars. This is a structural safety concern, particularly on vehicles in salt-belt regions.
Grinding or scraping noise from rear wheels
Visible rust on brake backing plates
Spongy or soft brake pedal
Visible corrosion on underbody brake lines
DIY: Moderate — brake line replacement requires flaring tools and bleeding; backing plates are straightforward
NHTSA complaints document electronic steering wheel lock module failures that prevent the vehicle from starting. This is confirmed as a known issue across multiple Mercedes-Benz owners online. The car becomes immobilised, and replacement of the ELV (Electronic Steering Lock) module is required.
Car refuses to start with no visible reason
Steering lock warning on dashboard
Key turns but engine does not crank
EZS or ELV fault codes present
DIY: Difficult — requires steering column disassembly and module coding
Fuel Tank Leak / Fuel Smell€200-800 · 80,000-150,000 kmLOW
NHTSA complaints note fuel smell after filling the tank, with fuel found pooling on top of the tank under the rear seat. Mechanics have confirmed the issue and advised against filling the tank completely. While isolated, a fuel leak represents a fire risk and should be investigated immediately.
Strong fuel smell after filling tank
Fuel residue visible on top of tank under rear seat
Smell intensifies when tank is full
DIY: Moderate — requires rear seat removal and fuel system access
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Buying Checklist
Cold-start the engine from fully cold and listen for any rattling or clattering in the first 15 seconds — walk away if present
Demand a full documented service history showing oil changes every 10,000 km or less with Mercedes 229.51/229.52 spec oil
Connect an OBD scanner and check for timing, camshaft position (P0340), EGR, and ELV-related fault codes before purchase
Inspect the coolant reservoir for brown sludge or oil contamination and check for white smoke on startup indicating EGR cooler failure
Crawl under the car and inspect all underbody brake lines for corrosion, especially at clips and brackets near the rear axle
Ask for proof the stop/start system is disabled or was never used — community consensus is this accelerates timing chain tensioner wear significantly
Service Reality
Community oil interval10,000 km
Oil specificationMercedes-Benz 229.51 or 229.52 compliant 5W-30 fully synthetic; community strongly recommends higher anti-wear spec oils (229.52) and 8,000–10,000 km intervals to protect timing chain tensioners
Est. ownership cost (2yr)€1,500–5,000 over 3 years depending on whether timing chain work is needed; budget €2,000–4,500 for a full chain kit service if history is unknown
The OM651 in the W204 facelift is a capable and punchy diesel engine that has been fundamentally undermined by a well-documented timing chain problem — budget for a full chain kit replacement if the car has not had one and has more than 100,000 km. Only buy with a full, verifiable service history showing short oil change intervals; a cheap purchase price means nothing if you are walking into a €2,000–4,000 chain job within months. The EGR cooler and swirl flap issues are secondary but real concerns that add to the ownership risk profile. If you find a well-documented example with a recent chain service, it can be a rewarding ownership experience — but treat any unknown-history car as a financial gamble.