Mercedes-Benz S500 W222

The M278 has a well-documented design flaw where engine oil wicks through the camshaft adjuster solenoid seals and migrates via capillary action along the wiring harness. If caught early, replacing the solenoid seals and installing sacrificial pigtail harness extensions (approximately €15 each) costs €500-800 total at a specialist. However, if the oil reaches the ECU, the full repair including engine wiring harness and ECU replacement can reach €5,000-8,000. Mercedes released a protection repair kit to address this. Proactive owners install sacrificial pigtail harnesses preventively. Symptoms of advanced contamination include multiple simultaneous fault codes for oxygen sensors and ignition coils. This affects all M278 engines from 2011-2017 and is the single most important item to check before purchase.

The AIRMATIC suspension is standard on the W222 S-Class and the rubber bladders degrade with age and temperature cycling. Rear springs fail most frequently. Symptoms include one corner sitting lower after overnight parking or the compressor running constantly. OEM strut replacement at a dealer runs €800-1,200 per corner including labor. Aftermarket alternatives from Arnott or Bilstein cost €400-600 per strut. If the compressor pump fails from overwork due to leaking springs, add €500-900 for the pump. A full suspension refresh (4 struts plus compressor) can reach €3,500-4,000 at a dealer. Failures typically begin between 80,000-120,000 km. Cars equipped with Magic Body Control have even more complex and expensive suspension with hydraulic ABC pump failures potentially costing €2,000-5,000.

Early M278 engines (2013-2014 production) are more susceptible to timing chain tensioner issues. The check valve weakens over time, allowing hydraulic pressure to bleed off overnight. This produces a distinctive rattle on cold start lasting 1-30 seconds. The M278 uses three chains (one primary, two secondary) with three tensioners. Replacing tensioners alone costs €2,000-3,000 at a specialist. However, replacing chains, guides, and sprockets simultaneously is strongly recommended, bringing total cost to €4,000-5,500. The rattle typically develops between 80,000-120,000 km. Mercedes improved the tensioner design from mid-2015 onwards. Ignoring the rattle risks chain skip and catastrophic valve-piston contact.

The M278 uses plastic valve covers that gradually warp from the heat generated by the turbochargers mounted in the valley of the engine. Once the cover distorts, the gasket cannot seal and oil drips onto the turbos, producing a burning oil smell. Simply replacing the gasket is not sufficient as the warped cover must be replaced as well. The repair involves significant disassembly including high-pressure fuel pump, injectors, and coils. Independent specialists typically charge €800-1,200, while Mercedes dealers quote €1,500-2,000 for both sides. This is among the most common M278 issues and affects most engines eventually, typically becoming noticeable between 80,000-120,000 km.

The twin turbochargers use internal wastegates with bushings that wear over time. The valve disk vibrates in its seat during partial load operation, causing a rhythmic rattle. Mercedes issued a TSB acknowledging this as a characteristic of the design and offers a software recalibration via Xentry (adding AO code 31O) that keeps the wastegate closed longer in partial load. If the rattling becomes severe or causes boost control issues, full turbo replacement is required. The turbochargers are welded to the exhaust manifolds and cannot be replaced without removing the engine, making labor costs substantial. Aftermarket wastegate repair kits with stainless steel bushings are available for €200-400, but turbo replacement per side runs €2,000-3,000 for parts plus €2,000+ for engine removal labor.

The W222 has extensive electronics with over 100 electric motors and numerous control modules. When any module fails to enter sleep mode properly, parasitic draw exceeds the normal 40-80 mA range and drains the battery. The COMAND infotainment system, keyless entry receiver, and various driver assistance modules are common culprits. A new OEM battery costs €200-350, and diagnosing the parasitic draw (pulling fuses systematically with an ammeter) can take several hours at a specialist. If a faulty control module is identified, replacement runs €300-800. A quality trickle charger (CTEK MXS 5.0 recommended) is essential if the car sits for more than a week.

Many 2014-2017 W222 owners report rough or jerky shifting. On the 7G-Tronic (pre-facelift S500), this is often caused by degraded transmission fluid or the conductor plate losing calibration. Mercedes issued software updates that recalibrate shift points. A transmission adaptation reset combined with fresh fluid and filter costs €400-600 at a specialist. If the conductor plate (valve body electronics) has degraded beyond software correction, replacement costs €1,200-2,500. Regular fluid changes every 60,000 km are strongly recommended despite Mercedes marketing the fluid as a lifetime fill.

The M278 engine mounts are hydraulic and wear out over time, typically after 80,000-120,000 km. Failed mounts cause noticeable vibration through the cabin at idle and at low engine speeds (1,000-2,000 RPM), which is particularly jarring in a car designed for supreme comfort. Replacement requires careful work around the turbochargers and dense engine bay. OEM mounts cost approximately €200-350 each, and the job requires 6-8 hours of labor. Total cost including both sides runs €800-1,200 at an independent specialist and €1,200-1,500 at a Mercedes dealer.