Mercedes-Benz GLS X166

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

5.5-litre V8 BiTurbo — AMG's first forced-induction V8 and the successor to the naturally aspirated M156. 525–585 hp depending on model, up to 900 Nm in the S version. The character is fundamentally different from the M156: instead of a high-revving scream, a brutal torque wave from 1,750 rpm. The sound is deeper, more bass-heavy, with a characteristic turbo hiss on lift-off. Less emotional than the naturally aspirated engine, but in a different league on the motorway — above 200 km/h the M157 still accelerates like others do at 100. Timing chain is the known Achilles heel: chain tensioner failure at 80,000–120,000 km, Mercedes has improved the design (non-return valve in cylinder head oil gallery). Piezo injectors are sensitive to fuel quality, service life 100,000–150,000 km. Turbos last 200,000+ km with proper warm-up and cool-down — shut off immediately after a motorway run and they'll fail significantly earlier. Warm up the engine, allow cool-down after hard use, oil changes every 10,000 km with 0W-40 — then 300,000 km is realistic.

• !! Timing Chain Stretch — M157 BiTurbo V8 from 100,000 km

  The timing chains on the M157 5.5L BiTurbo V8 stretch, especially on vehicles with frequent cold starts and short trips. Engine damage possible if chain jumps. Extensive repair on the V8.

  Symptoms: Metallic rattling on cold start, camshaft position fault codes, rough running, in extreme cases engine damage
  3,000–8,000 $
  Ad Timing chain M157Chain tensioner AMG 5.5 BiTurbo
• !! Oil Consumption from Piston Ring Wear from 120,000 km

  Under heavy use the piston rings on the M157 wear prematurely. Oil consumption above 1 L/1,000 km is possible. Engine overhaul required with advanced wear.

  Symptoms: Rising oil consumption, blue smoke on acceleration, oil mist from exhaust, oil level warnings
  4,000–10,000 $
  Ad Piston rings M157Pistons AMG 5.5 BiTurbo
• !! High-Pressure Fuel Pump Fails — No Fuel Pressure from 100,000 km

  The high-pressure fuel pump on the M157 can wear internally at high mileage. Pressure drop in the rail causes difficult starting and power loss.

  Symptoms: Difficult starting, power loss under load, hesitation at high rpm, fuel pressure fault codes
  1,200–3,000 $
  Ad High-pressure pump M157HPFP AMG 5.5 BiTurbo

+ 8 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

4.7-litre BiTurbo V8 with direct injection (CGI), 90-degree bank angle and hot-V layout — turbos sit between the cylinder banks. Timing chain and tensioners are the first thing to check on used examples: cold-start rattle is a warning sign. Alusil bores react badly to wrong oil or extended intervals; piston slap develops gradually from oil film loss, often triggered by fuel-washing injectors. Ignition coils rarely last beyond 120,000 km. The oil separator should be checked after 100,000 km — when faulty it forces oil vapour into the intake and accelerates intake valve carbon build-up. Sound character: subtle BiTurbo rumble, no naturally aspirated roar. Shorten maintenance intervals to 10,000 km, oil MB 229.5 with low Noack value. Engine long-lived with correct maintenance — problem cars are almost always those with missed oil changes.

• !! Timing chain tensioner failure (early production) from 60,000 km

  All M278 built before Feb. 2013 are affected by prematurely wearing chain tensioners. Mercedes responded with a goodwill campaign: new tensioners plus a check valve in the cylinder head.

  Symptoms: Metallic rattle on cold start (2–5 seconds), later also rattling when warm and on hot restart
  2,500–4,500 $
  Ad M278 chain tensionerM278 timing chain repair
• !! Cylinder bore scoring (Silitec coating) from 120,000 km

  No M278 with over 120,000 km known without cylinder bore damage. Silitec coating is sensitive to knock. Full rebuild approximately €14,000–18,000, replacement engine approximately €20,500.

  Symptoms: Increased oil consumption (1–1.6 L/1,000 km), engine knock, rough idle, brass swarf in oil
  5,000–20,000 $
  Ad M278 ZylinderlaufbahnM278 piston ringsSilitec cylinder
• !! Camshaft adjuster solenoid — oil destroys ECU from 80,000 km

  The M278 is the best-known example of Mercedes' oil migration problem. Oil seeps from the camshaft adjuster solenoid capillary-style through the entire engine wiring harness to the ECU. Costs up to €11,000 if ECU and harness both need replacing.

  Symptoms: Cold-start rattle, fault codes P0010/P0011 camshaft adjuster, oil traces on connectors, sporadic engine misfires, complete ECU failure
  400–11,000 $
  Ad camshaftnversteller M278 Ölstoppringwiring loom M278 Öl A2781500433Motorcontrol unit M278 A2789001600Ölstoppring M278 A2769050143

+ 6 more engine weaknesses + vehicle weaknesses