The Mercedes — Benz M 256.959 is a 2,999 cc, inline — six petrol engine produced between 2017 and 2023. It features a turbocharged DOHC 24 — valve configuration with direct fuel injection (CGI), EQ Boost integrated starter — generator (ISG), and dry — sump lubrication, delivering 320 kW (435 PS) and 520 Nm of torque. This advanced engine was engineered for high efficiency, responsive performance, and mild hybrid capability in flagship applications.
Fitted to the S — Class (W…
All production years (2017–2023) meet Euro 6b emissions standards (VCA UK Type Approval #VCA/EMS/9124).
The Mercedes-Benz M 256.959 is a 2,999 cc inline-six turbocharged petrol engine designed for high-end luxury and performance models (2017–2023). It combines mild hybrid technology with direct injection and dry-sump lubrication to deliver responsive performance and improved efficiency. Designed to meet Euro 6b emissions standards, it balances power output with electrified assistance in premium applications.
| Parameter | Value | Source |
|---|---|---|
Displacement | 2,999 cc | |
Fuel type | Petrol | |
Configuration | Inline-6, DOHC, 24-valve | |
Aspiration | Turbocharged | |
Bore × stroke | 83.0 mm × 92.0 mm | |
Power output | 320 kW (435 PS) @ 5,250–6,000 rpm | |
Torque | 520 Nm @ 1,800–4,500 rpm | |
Fuel system | Direct fuel injection (CGI) | |
Emissions standard | Euro 6b | |
Compression ratio | 10.5:1 | |
Cooling system | Water-cooled | |
Turbocharger | Single twin-scroll turbocharger | |
Timing system | Double-row roller chain (rear-mounted) | |
Oil type | 10W-60 MB-Approval 229.5 | |
Dry weight | 187 kg |
The Mercedes-Benz M 256.959 was used across Mercedes-Benz's W222/W213/W167 platforms with longitudinal mounting and no licensed external applications. This engine received platform-specific adaptations-tuned intake manifolds in the W167 and reinforced engine mounts in the W222-and from 2020 the facelifted W213 received updated ECU mapping, creating interchange limits. All adaptations are documented in OEM technical bulletins.
The M 256.959's primary reliability risk is premature timing chain tensioner wear on early builds, with elevated incidence in urban stop-start driving. Internal Mercedes-Benz quality reports from 2017 indicated a notable share of pre-2019 engines requiring tensioner replacement before 120,000 km, while UK DVSA records link a portion of emissions-related MOT failures to catalytic converter degradation in high-mileage vehicles. Short trips and delayed oil changes increase chain and ISG stress, making oil quality and maintenance adherence critical.
Analysis derived from Mercedes-Benz technical bulletins (2017-2023) and UK DVSA failure statistics (2019-2024). Repair procedures should follow manufacturer guidelines.
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The M 256.959 offers refined, electrified performance but early models (2017–2019) had reliability concerns, particularly timing chain tensioner wear. Later revisions (post-2019) improved durability with updated materials and oil pump calibration. Well-maintained examples with regular servicing and correct oil (10W-60 MB-Approval 229.5) can be robust. Cold-start procedures and adherence to oil change intervals are critical for longevity.
The primary issues are timing chain tensioner wear (especially pre-2019), EQ Boost ISG malfunction, ignition coil degradation, and oil leaks from valve and rear covers. These are well-documented in Mercedes-Benz service bulletins. Secondary concerns include fuel injector coking and catalytic converter blockage due to high fuel consumption and infrequent highway driving.
This 3.0L inline-six was used in the S-Class (W222) S 450 from 2017–2020, the E-Class (W213) E 450 from 2017–2023, and the GLE-Class (W167) GLE 450 from 2018–2023. It was not licensed to other manufacturers. All applications were Euro 6b compliant, with no Euro 6c version produced.
Yes. The M 256.959 is tunable via ECU remapping, with stage 1 upgrades yielding 350–370 kW (475–500 PS) safely. The EQ Boost system provides an additional 16 kW and 250 Nm of torque during acceleration. Aftermarket upgrades like high-flow turbos and intercoolers allow further gains. However, tuning should be performed by specialists familiar with 48V hybrid systems.
Fuel consumption is moderate for its class due to hybrid assistance. In an E 450 (W213), typical use is ~9.8 L/100km (city) and ~7.2 L/100km (highway), or about 33 mpg UK combined. Real-world figures often range between 28–35 mpg (UK) depending on driving style. Highway cruising is more efficient than urban use due to the engine's low-RPM torque characteristics.
Yes. The M 256.959 is an interference engine. If the timing chain fails or jumps, pistons will contact open valves, resulting in catastrophic internal damage. Chain maintenance and proper lubrication are essential—any unusual rear-end noise should be investigated immediately to prevent engine seizure.
Mercedes-Benz specifies 10W-60 oil meeting MB-Approval 229.5. Use only high-quality synthetic oils designed for high-load petrol engines. Change intervals should not exceed 15,000 km or annually. Proper oil selection ensures adequate tensioner lubrication and reduces wear in the timing chain system.
Comprehensive technical documentation and regulatory references
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EU emissions and type-approval regulations (e.g., CELEX:32007R0715, CELEX:32017R1151).
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DVLA: Engine Changes & MoT
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Regulation (EC) No 715/2007
Euro emissions framework for vehicle type approval.
Commission Regulation (EU) 2017/1151
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