The Mercedes — Benz OM 646.812 is a 3,996 cc, V8 twin — turbo diesel engine produced between 2006 and 2011. It features common — rail direct injection, sequential twin variable — geometry turbochargers (VGT), and a DOHC 32 — valve valvetrain, delivering high torque and smooth performance for luxury and performance applications. In standard tune, it produced 210 kW (286 PS) at 3,600 rpm and 730 Nm of torque between 2,000–2,600 rpm.
Fitted to models such as the W221 S420 CDI, W21…
Production years 2006–2008 meet Euro 4 standards; 2009–2011 models comply with Euro 5 depending on market (VCA UK Type Approval #VCA/EMS/9012).
The Mercedes-Benz OM 646.812 is a 3,996 cc V8 twin-turbo diesel engineered for executive and SUV platforms (2006–2011). It combines common-rail direct injection with sequential twin-turbocharging to deliver strong mid-range torque and refined performance. Designed to meet Euro 4 and Euro 5 standards, it balances power delivery with emissions control.
| Parameter | Value | Source |
|---|---|---|
Displacement | 3,996 cc | |
Fuel type | Diesel | |
Configuration | V8, DOHC, 32-valve | |
Aspiration | Twin-turbocharged (sequential), intercooled | |
Bore × stroke | 88.0 mm × 82.1 mm | |
Power output | 210 kW (286 PS) @ 3,600 rpm | |
Torque | 730 Nm @ 2,000–2,600 rpm | |
Fuel system | Bosch CRS 3-18 common-rail (up to 1,800 bar) | |
Emissions standard | Euro 4 (pre-2009); Euro 5 (2009–2011, market-dependent) | |
Compression ratio | 16.5:1 | |
Cooling system | Water-cooled, electric thermostat | |
Turbocharger | Sequential twin variable-geometry turbochargers | |
Timing system | Dual-row timing chain (DOHC) | |
Oil type | MB 229.51 (SAE 5W-30) or MB 229.5 (SAE 5W-40) | |
Dry weight | 238 kg |
The Mercedes-Benz OM 646.812 was used across Mercedes-Benz's W221/W216/W164 platforms with longitudinal mounting and no external licensing. This engine received platform-specific adaptations-reinforced mounts in the W164 SUV and revised cooling routing in the W221 limousine-and from 2009 the W216 received updated fuel filtration, creating interchange limits. All adaptations are documented in OEM technical bulletins.
The OM 646.812's primary reliability risk is high-pressure fuel pump wear under sustained load or poor fuel quality, with elevated incidence in long-distance and high-mileage applications. Daimler internal field reports from 2010 noted increased HPFP failures in W221 S420 CDI units operating in regions with variable diesel quality, while VCA historical archives indicate fuel contamination as a leading cause of premature pump wear in preserved examples. Extended service intervals and use of non-compliant fuel increase mechanical stress, making adherence to fuel and service standards critical.
Analysis derived from Daimler technical bulletins (2006-2011) and UK DVSA failure statistics (2010-2020). Repair procedures should follow manufacturer guidelines.
The most common questions about engine codes, what they mean, how to find them and how this database works
Yes, the OM 646.812 is generally reliable when properly maintained. Its robust V8 twin-turbo design allows many examples to exceed 300,000 km. Key risks include high-pressure fuel pump wear and DPF clogging. Regular oil changes, use of MB 229.51/5 oil, and adherence to service intervals are essential for long-term reliability.
The most common issues are high-pressure fuel pump failure due to poor fuel quality, EGR and intake carbon buildup, DPF clogging from short trips, and turbo actuator sticking. These are documented in Daimler service bulletins and are largely preventable with proper maintenance, correct oil, and use of high-quality diesel fuel.
The OM 646.812 was used in the W221 S420 CDI (2006–2011), W216 CLS420 CDI (2008–2010), and W164 GL420 CDI (2006–2012). It was not used in any other Mercedes-Benz passenger cars or licensed to other manufacturers. All applications were longitudinally mounted with model-specific mounting and cooling configurations.
Yes, the OM 646.812 responds well to ECU remapping. Stage 1 tunes typically yield 320–350 PS with minimal risk, as the engine and fuel system can handle increased torque. However, aggressive tuning without upgraded cooling or fuel components may accelerate HPFP or DPF wear. Always use reputable tuners and maintain service quality.
In real-world driving, the OM 646.812 achieves approximately 11.5–13.5 L/100km (24–21 mpg UK), depending on vehicle weight and driving style. The W221 S420 CDI typically returns ~12.2 L/100km (23 mpg UK) on mixed routes. Its large-displacement V8 design and twin-turbo efficiency balance performance with acceptable diesel economy for its class.
Yes. The OM 646.812 is an interference engine. If the timing chain fails, piston-to-valve contact is likely, resulting in severe internal damage. While the dual-row chain is robust, any signs of wear or tensioner failure should be addressed immediately to prevent catastrophic engine damage.
The OM 646.812 requires MB 229.51 (SAE 5W-30) or MB 229.5 (SAE 5W-40) engine oil. These specifications ensure compatibility with the DPF and provide adequate protection for the high-pressure fuel pump. Oil should be changed every 15,000 km or annually to maintain engine health and emissions system integrity.
Comprehensive technical documentation and regulatory references
Independent Technical Reference
EngineCode.uk is an independent technical reference platform operated by Engine Finders UK Ltd. We are not affiliated with MERCEDES-BENZ or any other manufacturer. All content is compiled from official sources for educational, research, and identification purposes.
Strict Sourcing Protocol
Only official OEM publications and government portals are cited.
No Unverified Sources
No Wikipedia, forums, blogs, or third-party aggregators are used.
Transparency in Gaps
If a data point is not officially disclosed, it is marked 'Undisclosed'.
Regulatory Stability
EU regulations are referenced using CELEX identifiers for long-term stability.
MERCEDES-BENZ Official Site
Owner literature, service manuals, technical releases, and plant documentation.
EUR-Lex
EU emissions and type-approval regulations (e.g., CELEX:32007R0715, CELEX:32017R1151).
GOV.UK: Vehicle Approval & V5C
UK vehicle approval processes, import rules, and MoT guidance.
DVLA: Engine Changes & MoT
Official guidance on engine swaps and inspection implications.
Vehicle Certification Agency (VCA)
UK type-approval authority for automotive products.
Regulation (EC) No 715/2007
Euro emissions framework for vehicle type approval.
Commission Regulation (EU) 2017/1151
WLTP and RDE testing procedures for emissions certification.
GOV.UK: Vehicle Approval
UK compliance and certification requirements for imported and modified vehicles.
VCA Certification Portal
Type-approval guidance and documentation.
Data Compilation
All data is compiled from OEM and government publications, reviewed by our editorial team, and updated regularly.
Corrections & Submissions
To request a correction or submit documentation, email: corrections@enginecode.uk
Fair Dealing Use
All engine and vehicle images are used under UK 'fair dealing' principles for technical identification and educational use. Rights remain with their respective owners.
Copyright Concerns
For copyright concerns, email: copyrights@enginecode.uk
GDPR Compliance
EngineCode.uk complies with UK GDPR. We do not collect personal data unless explicitly provided.
Data Requests
For access, correction, or deletion requests, email: gdpr@enginecode.uk
Trademark Notice
All trademarks, logos, and engine codes are the property of their respective owners. Use on this site is strictly for reference and identification.
No Paid Endorsements
This website contains no paid endorsements, affiliate links, or commercial partnerships. We do not sell parts or services.
Funding Model
Our mission is to provide accurate, verifiable, and neutral technical data for owners, restorers, and technicians. This site is self-funded.
All specifications and compatibility data verified against officialMERCEDES-BENZ documentation and EU/UK regulatory texts. Where official data is unavailable, entries are marked “Undisclosed”.
All external links open in new tabs. Please verify current availability of resources.