Mercedes-Benz R-Klasse W251

V6 diesel with common rail and piezo injectors. Powerful and refined. Known for blocked oil coolers, swirl flap failure and EGR problems. High mileages possible with good maintenance.

• !! Oil Cooler Gaskets Leaking — Oil and Coolant Loss from 120,000 km

  The best-known problem of the OM642: the oil cooler gaskets in the V-section of the engine fail. Oil leaks into the coolant or externally. Particularly prone on vehicles built up to 2009.

  Symptoms: Oil loss warning, oil spots under the left side of the vehicle, oil streaks visible on the belt tensioner
  800–2,500 $
  Ad oil cooler gasket OM642oil cooler OM642 3.0 CDIA6421800615 oil cooler
• !! Swirl Flap Linkage Breaks — Plastic Joint Fails from 100,000 km

  The plastic linkages of the swirl flaps in the OM642 intake manifold break. Aluminium repair kits are available and more durable. In the event of complete failure there is a risk of debris entering the engine.

  Symptoms: Engine warning light, fault codes for intake flap/swirl flap, power loss particularly at low rpm
  200–800 $
  Ad Drallklappe repairsatz OM642swirl flaps-repairsatz OM642 3.0 CDI6420907737 OM642 Ansaug
• !! Injector Seal Leaking — Soot Deposits and Diesel in Oil from 120,000 km

  Piezo injectors seal poorly from heat, diesel enters the engine oil. Oil loses lubrication properties, tar-like residue around injectors is the typical telltale sign.

  Symptoms: Rough idle, increased fuel consumption, knocking noises, oil level rising due to diesel contamination, engine oil smells of diesel
  300–900 $
  Ad fuel injector O-ring seal OM642 A6420170360Kupferdichtscheibe fuel injector CDIfuel injectorgasket OM642

+ 4 more engine weaknesses + vehicle weaknesses

V6 diesel with common rail and piezo injectors. Powerful and refined. Known for blocked oil coolers, swirl flap failure and EGR problems. High mileages possible with good maintenance.

• !! Oil Cooler Gaskets Leaking — Oil and Coolant Loss from 120,000 km

  The best-known problem of the OM642: the oil cooler gaskets in the V-section of the engine fail. Oil leaks into the coolant or externally. Particularly prone on vehicles built up to 2009.

  Symptoms: Oil loss warning, oil spots under the left side of the vehicle, oil streaks visible on the belt tensioner
  800–2,500 $
  Ad oil cooler gasket OM642oil cooler OM642 3.0 CDIA6421800615 oil cooler
• !! Swirl Flap Linkage Breaks — Plastic Joint Fails from 100,000 km

  The plastic linkages of the swirl flaps in the OM642 intake manifold break. Aluminium repair kits are available and more durable. In the event of complete failure there is a risk of debris entering the engine.

  Symptoms: Engine warning light, fault codes for intake flap/swirl flap, power loss particularly at low rpm
  200–800 $
  Ad Drallklappe repairsatz OM642swirl flaps-repairsatz OM642 3.0 CDI6420907737 OM642 Ansaug
• !! Injector Seal Leaking — Soot Deposits and Diesel in Oil from 120,000 km

  Piezo injectors seal poorly from heat, diesel enters the engine oil. Oil loses lubrication properties, tar-like residue around injectors is the typical telltale sign.

  Symptoms: Rough idle, increased fuel consumption, knocking noises, oil level rising due to diesel contamination, engine oil smells of diesel
  300–900 $
  Ad fuel injector O-ring seal OM642 A6420170360Kupferdichtscheibe fuel injector CDIfuel injectorgasket OM642

+ 4 more engine weaknesses + vehicle weaknesses

V6 diesel with common rail and piezo injectors. Powerful and refined. Known for blocked oil coolers, swirl flap failure and EGR problems. High mileages possible with good maintenance.

• !! Oil Cooler Gaskets Leaking — Oil and Coolant Loss from 120,000 km

  The best-known problem of the OM642: the oil cooler gaskets in the V-section of the engine fail. Oil leaks into the coolant or externally. Particularly prone on vehicles built up to 2009.

  Symptoms: Oil loss warning, oil spots under the left side of the vehicle, oil streaks visible on the belt tensioner
  800–2,500 $
  Ad oil cooler gasket OM642oil cooler OM642 3.0 CDIA6421800615 oil cooler
• !! Swirl Flap Linkage Breaks — Plastic Joint Fails from 100,000 km

  The plastic linkages of the swirl flaps in the OM642 intake manifold break. Aluminium repair kits are available and more durable. In the event of complete failure there is a risk of debris entering the engine.

  Symptoms: Engine warning light, fault codes for intake flap/swirl flap, power loss particularly at low rpm
  200–800 $
  Ad Drallklappe repairsatz OM642swirl flaps-repairsatz OM642 3.0 CDI6420907737 OM642 Ansaug
• !! Injector Seal Leaking — Soot Deposits and Diesel in Oil from 120,000 km

  Piezo injectors seal poorly from heat, diesel enters the engine oil. Oil loses lubrication properties, tar-like residue around injectors is the typical telltale sign.

  Symptoms: Rough idle, increased fuel consumption, knocking noises, oil level rising due to diesel contamination, engine oil smells of diesel
  300–900 $
  Ad fuel injector O-ring seal OM642 A6420170360Kupferdichtscheibe fuel injector CDIfuel injectorgasket OM642

+ 4 more engine weaknesses + vehicle weaknesses

Four-valve V6 with variable valve timing. Smooth and powerful. Known for balance shaft wear (revision sprocket) on early build years. Balance shaft gear problem addressed by Mercedes with a recall.

• !! Balance Shaft Sprocket Wears Prematurely from 140,000 km

  The plastic balance shaft sprocket on the M272 wears prematurely from the timing chain. The right cylinder bank goes out of timing. Affected up to end of October 2006.

  Symptoms: Fault codes P0016/P0017 (camshaft right bank), chain rattling, rough running at idle, in extreme cases engine damage
  2,500–5,500 $
  Ad Balance shaft M272Balance shaft sprocket M272 V6A2720500747 balance shaft sprocket
• !! Timing Chain Stretches and Damages Timing from 120,000 km

  Alongside the balance shaft issue the M272 shows premature timing chain stretch. Chain wear on softer sprockets leads to timing deviations.

  Symptoms: Chain noise especially on cold start, camshaft fault codes, rough running, engine irregularities
  2,000–4,000 $
  Ad Timing chain M272 V6Timing chain kit M272Chain guide M272
• !! Camshaft Adjuster Leaks — Wiring Harness Contaminated from 90,000 km

  Four camshaft adjuster solenoids (two per cylinder bank) develop leaks; oil creeps into the wiring harness as far as the ECU. Repair costs up to €1,700 documented.

  Symptoms: CEL, oil traces at camshaft connectors, lambda sensor failure due to oil contamination, non-specific faults that recur
  100–1,700 $
  Ad solenoid valve camshaftnversteller M272Zentralventil M272 A2720500578Zentralventil A2720500478

+ 3 more engine weaknesses + vehicle weaknesses

Four-valve V6 with variable valve timing. Smooth and powerful. Known for balance shaft wear (revision sprocket) on early build years. Balance shaft gear problem addressed by Mercedes with a recall.

• !! Balance Shaft Sprocket Wears Prematurely from 140,000 km

  The plastic balance shaft sprocket on the M272 wears prematurely from the timing chain. The right cylinder bank goes out of timing. Affected up to end of October 2006.

  Symptoms: Fault codes P0016/P0017 (camshaft right bank), chain rattling, rough running at idle, in extreme cases engine damage
  2,500–5,500 $
  Ad Balance shaft M272Balance shaft sprocket M272 V6A2720500747 balance shaft sprocket
• !! Timing Chain Stretches and Damages Timing from 120,000 km

  Alongside the balance shaft issue the M272 shows premature timing chain stretch. Chain wear on softer sprockets leads to timing deviations.

  Symptoms: Chain noise especially on cold start, camshaft fault codes, rough running, engine irregularities
  2,000–4,000 $
  Ad Timing chain M272 V6Timing chain kit M272Chain guide M272
• !! Camshaft Adjuster Leaks — Wiring Harness Contaminated from 90,000 km

  Four camshaft adjuster solenoids (two per cylinder bank) develop leaks; oil creeps into the wiring harness as far as the ECU. Repair costs up to €1,700 documented.

  Symptoms: CEL, oil traces at camshaft connectors, lambda sensor failure due to oil contamination, non-specific faults that recur
  100–1,700 $
  Ad solenoid valve camshaftnversteller M272Zentralventil M272 A2720500578Zentralventil A2720500478

+ 3 more engine weaknesses + vehicle weaknesses

Four-valve V6 in two basic variants: 3.5L naturally aspirated (DES 35, 185–225 kW) with direct injection and stratified charge — refined, linear, no classic naturally aspirated feel. And the 3.0L BiTurbo family (DEH 30 LA, 245–295 kW) with Nanoslide coating, twin turbochargers and water-air charge air cooler. The BiTurbo sings sonorously from 2,500 rpm and screams at the top — exceptional for a V6. Weak points: timing chain wears early when oil change intervals are pushed. Oil cooler in the V-block doesn't seal forever. Camshaft solenoids leak. On the 3.5L stratified-charge variant add NOx sensor failures from short-trip use. Early DES 35 (2011–2013) had piston slap issues. The BiTurbo unit with Nanoslide is considered more robust. Maintained with short oil change intervals (10,000–12,000 km) the engine easily lasts beyond 200,000 km.

• !! Oil cooler leaking in the V-block from 120,000 km

  Known weak point: the oil cooler gasket in the V-block of the M276 ages and starts leaking. Repair is extremely labour-intensive because the engine must be partially disassembled — cost €1,700–3,800.

  Symptoms: Oil spots under the vehicle, rainbow sheen on wet road, dropping oil level, oil smell
  1,700–3,800 $
  Ad oil cooler M276A2761880080oil cooler gasket M276 V6
• !! Camshaft adjuster leaking — wiring harness contaminated from 100,000 km

  Known damage pattern on all newer Mercedes engines: solenoid valves leak, oil migrates into the wiring harness. First the lambda sensors die, then the ECU. Repair documented up to €1,970.

  Symptoms: Check engine light with camshaft fault code, lambda sensor failure, oil film on connectors, in extreme cases ECU failure
  300–1,970 $
  Ad solenoid valve camshaftnversteller M276Zentralventil M276 V6Öl-Stopp-Kabel GLE W166
• !! Piston slap (early 3.5L variants) from 150,000 km

  On early M276 DES 35 (build years 2011–2013) pistons can loosen due to cylinder bore wear. Knock noise on cold start, worst case engine damage.

  Symptoms: Clacking and knocking on cold start from the engine area, getting louder over time, rough idle when cold
  3,000–8,000 $
  Ad piston M276pisto kit M276 3.5LZylinderlaufbuchse M276 DES 35

+ 7 more engine weaknesses + vehicle weaknesses

Large-displacement naturally aspirated V8, second generation with variable valve timing on all four camshafts. Technically related to the M272 V6 — shares its known balance shaft sprocket problem on build years before 2007 (engine number below 088611). From that number the sprocket was made of hardened steel. Check early engines for fault codes P0016/P0017 and cold-start chain noise. The 5.5-litre with 285 kW is the typical variant; the 5.0-litre with 225–250 kW is somewhat more forgiving in heavier vehicles. The M278 BiTurbo successor is more efficient and more powerful, but sounds considerably less characterful — the M273 is the last true naturally aspirated V8 from Stuttgart. Pre-purchase: always check for camshaft adjuster leaks (oil in wiring harness) and thermostat fault P0128. Oil changes every 10,000 km with MB 229.5.

• !! Balance shaft sprocket — same weakness as M272 from 140,000 km

  The M273 V8 shares the balance shaft sprocket issue with the M272 V6. The soft composite material of the sprocket wears down, causing timing chain rattle.

  Symptoms: Fault codes P0016/P0017, chain rattling, rough idle, power loss
  2,500–5,500 $
  Ad Ausgleichswelle M273 V8Ausgleichswellen-Umlenkrad M273Ausgleichswelle M273
• !! Camshaft Adjuster Leaking — Oil in Wiring Harness from 110,000 km

  As with the similarly-designed M272, the solenoid valves leak. The V8 has 4 per bank — consequential costs are particularly high due to more solenoids. Oil migrates by capillary action all the way to the ECU.

  Symptoms: Check engine light with camshaft fault codes P0014/P0015/P0021/P0025, lambda sensor failures, in extreme cases ECU failure
  200–2,000 $
  Ad Solenoid camshaft adjuster M273Centre valve M273 V8Oil stop cable V8 Mercedes
• !! Valve cover gaskets leaking on both sides from 130,000 km

  Like the M113, the valve cover gaskets on the M273 V8 become porous over time. With a V8 having two cylinder heads, the workload doubles.

  Symptoms: Oil spots under the vehicle, oil smell after hard driving, oily engine bay
  400–1,000 $
  Ad valve cover gasket M273 V8valve cover gasket M273Elring M273

+ 6 more engine weaknesses + vehicle weaknesses

6.2-litre naturally aspirated V8, hand-built in Affalterbach to the 'One Man, One Engine' principle. 457–525 hp without any forced induction, rev limit at 7,200 rpm. The sound defines an era: a deep, bass-heavy rumble at 3,000 rpm, a scream above 5,000 that recalls American V8 racing engines. Throttle response is immediate — no turbo lag, no hesitation, every throttle command is executed instantly. Critical issue number one: camshafts wear out through inadequate cold-start lubrication with the factory 0W-40 oil. Switch to Ravenol 5W-40 RCS, let the engine idle for 10 seconds after a cold start, stay below 3,000 rpm until fully warm. Head bolt corrosion on engine numbers below 060658 (up to approx. 2010) — check without fail. Star Technical Bulletin 05.20/20b documents the tappet problem officially. Parts availability is becoming increasingly difficult — hydraulic tappets A1560500225 sometimes not available for months. Maintain the M156 properly and you have one of the most emotional V8s of the last 30 years.

• !! Head Bolt Corrosion — Coolant in Combustion Chamber from 50,000 km

  All M156 up to engine number 060658 affected: head bolts corrode through coolant contact at the bolt head, in worst case break off. Coolant enters the combustion chamber — engine damage. Mercedes fitted improved bolts from approx. 2010 (20x A1560160769 + 4x N000000005754, approx. 108 EUR net). Preventive replacement strongly recommended.

  Symptoms: Coolant loss without visible leak, white smoke from exhaust, oil-coolant mixture, check engine light, misfires on individual cylinders.
  1,500–8,000 $
  Ad Head bolt M156A1560160769 head boltN000000005754
• !! Camshaft Wear — Cold-Start Lubrication Deficit from 60,000 km

  Main problem of the M156: the factory 0W-40 oil drains from the camshafts at standstill; at cold start the lobes run briefly dry. Wear deteriorates exponentially. Camshaft regrinding (TechnoCam and others) as a cheaper alternative to new parts — 50% saving. Always replace the 32 hydraulic tappets (INA A1560500225) at the same time.

  Symptoms: Metallic ticking on cold start that disappears after 30–60 seconds, rough idle, power loss in the upper rev range. Remove valve cover and check lobe tips for wear.
  2,000–6,000 $
  Ad Camshaft M156A1560504301 inlet camshaftA1560500225 hydraulic tappet INACamshaft regrind M156
• !! Camshaft Adjuster — Locking Plate Wears Out from 70,000 km

  The fit of the locking plate in the camshaft adjuster widens over time — the adjuster slips, timing shifts. Clattering on cold start is the first symptom. Reinforced locking plates (300–1,000 EUR for 4 pieces) as a permanent solution. Bearing cap torque 10 Nm, adjuster 45 Nm + 90°.

  Symptoms: Rattling/clattering on cold start for 2–10 seconds, disappears after oil pressure build-up. With advanced wear: also audible when warm, rough idle, shifted timing, power loss.
  800–3,000 $
  Ad Camshaft adjuster M156Locking plate M156 reinforcedCamshaft adjuster repair kit AMG

+ 7 more engine weaknesses + vehicle weaknesses