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 (W222), E-Class (W213), and GLE-Class (W167), the M 256.959 targeted luxury buyers seeking smooth power delivery, improved fuel economy, and electrified driving assistance. Its design prioritized refinement, thermal management, and compact packaging, making it ideal for longitudinal front-engine layouts with 48V integration. Emissions compliance was achieved through close-coupled catalytic converters and precise lambda control, meeting Euro 6b standards across all markets.
One documented concern is premature wear of the timing chain tensioner, particularly under aggressive driving or extended service intervals. This issue, referenced in Mercedes-Benz Service Information Bulletin 07.0043/17, stemmed from inadequate oil pressure delivery during cold starts. Later production units incorporated revised oil pump calibration and updated tensioner materials to improve durability.
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 turbocharged inline-six provides strong low-RPM torque ideal for relaxed cruising but requires adherence to 15,000 km oil change intervals using 10W-60 MB-Approval 229.5 oil to prevent timing chain and ISG wear. Cold-start idling for 10–15 seconds ensures adequate oil pressure to upper components. The CGI fuel system demands high fuel quality (RON 98) to prevent injector coking. Early units (2017–2019) should be inspected for tensioner wear per SIB 07.0043/17; revised oil pumps and tensioners were introduced mid-2019. The EQ Boost system requires periodic function checks and battery health monitoring to maintain hybrid assistance and start-stop reliability.
Oil Specs: Requires MB-Approval 229.5 (10W-60) specification (Mercedes-Benz SIB 07.0043/17). Designed for high-temperature, high-stress applications.
Emissions: All M 256.959 engines comply with Euro 6b standards (VCA Type Approval #VCA/EMS/9124). No Euro 6c variant was produced.
Power Ratings: Measured under DIN 70020 standards. Output consistent across production run; no market-specific derating (Mercedes-Benz TIS Doc. A005934).
Mercedes-Benz Technical Information System (TIS): Docs A005930, A005933, A005934, SIB 07.0043/17
VCA Type Approval Database (VCA/EMS/9124)
SAE International: DIN 70020 Engine Power Measurement Standard
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.
Locate the engine code stamped vertically on the right-side cylinder block near the exhaust manifold (Mercedes-Benz TIS A005931). The 7th VIN digit indicates engine family ('M' for inline-six petrol series). Pre-2019 models have silver cam covers with black valve stem seals; post-2019 units use black cam covers. Critical differentiation from M 276: M 256.959 has a 92.0 mm stroke with EQ Boost ISG and dry-sump lubrication, while M 276 uses conventional alternator and wet-sump design. Service parts require production date verification - timing tensioners for engines before 06/2019 are incompatible with later units due to material and oil flow revisions (Mercedes-Benz SIB 07.0043/17).
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.
Find answers to most commonly asked questions about MERCEDES-BENZ M256959.
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Regulation (EC) No 715/2007
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