Mercedes-Benz E500 W212

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 harnesses costs around €500-800. However, if the oil reaches the ECU, the repair bill escalates dramatically as the entire engine wiring harness and ECU may need replacement, costing €5,000-8,000 or more. Mercedes released a protection repair kit (sacrificial harness extensions) to address this. Proactive owners install these preventively. Symptoms of advanced contamination include multiple random fault codes appearing simultaneously, especially for oxygen sensors and ignition coils. This affects all M278 engines from 2011-2020.

Early-build M278 engines have problems with timing chain tensioners losing oil pressure overnight. The check valve weakens over time, allowing hydraulic pressure to bleed off. This produces a distinctive metallic rattle on cold start that lasts from a few seconds up to 30 seconds. The M278 uses a complex chain drive with one primary and two secondary chains, each with its own tensioner (3 total). Replacing tensioners alone costs €2,000-3,000, but specialists recommend replacing chains, guides, and sprockets simultaneously to avoid paying for labor twice, bringing total cost to €4,000-5,500. The rattle typically develops between 80,000-120,000 km. Engines with serial numbers ending above 30 191844 have revised tensioners. Ignoring the rattle risks chain skip and catastrophic valve-piston contact.

The twin turbochargers on the M278 use internal wastegates that can develop worn bushings over time. Symptoms include a rattling noise at specific RPM ranges and eventually a P0299 underboost code. The wastegate rod on the affected side visibly shakes at idle. The complication is that the turbochargers are welded to the exhaust manifolds, so a wastegate failure requires replacing the entire turbo assembly. Furthermore, replacing the turbochargers requires removing the engine from the vehicle due to tight packaging, which adds substantial labor. Wastegate repair kits (new flapper and stainless steel bushing) are available as an alternative for €300-500, but require specialist fitting. Full turbo replacement per side runs €2,000-3,000 for parts plus €2,000-3,000 in labor for engine removal. Most owners address only the affected side.

AIRMATIC was standard on the E500. The rubber air spring bladders become brittle and develop cracks from age and temperature cycling. Rear springs fail most often. Symptoms include one corner sitting lower after overnight parking, or the compressor running constantly. OEM replacement per spring is €600-900 fitted at a dealer. Aftermarket alternatives from Arnott or Bilstein are available for €300-500 per spring. The compressor pump itself can fail from overwork if springs have been leaking slowly, costing €500-900. A full air suspension overhaul (4 springs plus compressor) can reach €3,000-3,500 at a dealer. Some owners convert to conventional coil springs as a permanent solution for around €800-1,200.

The M278 uses plastic valve covers that gradually warp from the heat generated by the twin turbochargers mounted in the V of the engine. Once the cover distorts, the gasket alone cannot seal and oil escapes onto the turbochargers, creating a burning oil smell. Simply replacing the gasket is insufficient as the warped cover must also be replaced. The repair involves significant disassembly (high-pressure fuel pump, injectors, coils) making labor expensive. Independent specialists charge €800-1,200 while dealers typically quote €1,500-2,000 for both sides. This is one of the most common M278 issues and affects most engines eventually, typically becoming noticeable after 80,000-100,000 km.

The 722.9 7G-Tronic automatic uses an internal conductor plate that houses speed sensors and solenoids for gear selection. Over time, the internal components degrade, leading to incorrect rotational speed readings. Symptoms include rough gear changes, delayed shifts, transmission holding gears too long, and eventually limp mode. The conductor plate requires programming to the vehicle due to the integrated anti-theft system. Dealer replacement costs €1,800-2,500, while specialists can repair the existing unit for €1,200-1,600. Regular transmission fluid changes every 60,000 km (despite Mercedes marketing it as lifetime fill) significantly extend the conductor plate's lifespan.

Like all direct injection engines, the M278 does not wash fuel over the intake valves, allowing oil vapors from the PCV system to bake onto the valve surfaces. This creates hard carbon deposits that restrict airflow, leading to rough idling, hesitation under acceleration, and eventually misfires. The buildup becomes noticeable around 60,000-100,000 km depending on driving patterns. Short trips and frequent cold starts accelerate the problem. Walnut blasting is the standard remedy, costing €800-1,500 at a specialist. Some owners perform this preventively every 60,000 km. A class-action lawsuit was filed in the US in 2023 over this issue, though it does not apply in Europe.

The W212 has complex electronics with numerous control modules that should enter sleep mode when the car is turned off. If any module fails to sleep properly, parasitic draw exceeds the normal 50-80 mA range and can drain the battery within 2-5 days. Common culprits include the COMAND infotainment system, the keyless entry receiver, stuck relays, and aftermarket accessories. Diagnosis involves measuring current draw with an ammeter and pulling fuses systematically. A new OEM battery costs €200-350. If a faulty control module is the cause, replacement runs €400-800 depending on the module. Keeping the car on a trickle charger when parked for extended periods is recommended.