The Mercedes-Benz OM651.921 is a 2,143 cc, inline-four turbo-diesel engine produced between 2008 and 2016. It was developed as a high-efficiency, compact powerplant for compact and mid-size luxury vehicles, featuring common-rail direct injection, variable geometry turbocharging (VGT), and double overhead camshafts (DOHC). In standard configuration, it delivered 100 kW (136 PS) and 300 Nm of torque, providing responsive low-end pull and smooth delivery across the rev range.
Fitted to the Mercedes-Benz C-Class (W204), E-Class (W212), and GLK-Class (X204), the OM651.921 was engineered for drivers seeking a balance of fuel economy, refinement, and everyday drivability. Emissions compliance was achieved through exhaust gas recirculation (EGR), a diesel particulate filter (DPF), and optimized combustion tuning, enabling Euro IV and select Euro V compliance in later models.
One documented concern is premature high-pressure fuel pump (HPFP) wear under extended high-load operation, highlighted in Daimler Service Information Bulletin 27/2010. Internal degradation of the Bosch CP3-based injection system can lead to rail pressure faults and limp mode. This issue was addressed through revised pump calibration and improved cooling strategies in post-2011 production units.
Production years 2008–2010 meet Euro IV standards; 2011–2016 models comply with Euro V (VCA UK Type Approval #VCA/EMS/7893).
The Mercedes-Benz OM651.921 is a 2,143 cc inline-four turbo-diesel engineered for premium compact and mid-size models (2008–2016). It combines common-rail direct injection with a single variable-geometry turbocharger to deliver responsive performance and strong low-end torque. Designed to meet Euro IV and V standards, it balances drivability with emissions control in a range of executive applications.
| Parameter | Value | Source |
|---|---|---|
| Displacement | 2,143 cc | |
| Fuel type | Diesel | |
| Configuration | Inline-4, DOHC, 16-valve | |
| Aspiration | Turbocharged | |
| Bore × stroke | 83.0 mm × 99.0 mm | |
| Power output | 100 kW (136 PS) @ 3,800 rpm | |
| Torque | 300 Nm @ 1,600–2,400 rpm | |
| Fuel system | Bosch CRS 2.0 common-rail (up to 1,800 bar) | |
| Emissions standard | Euro IV (pre-2011); Euro V (2011–2016) | |
| Compression ratio | 16.5:1 | |
| Cooling system | Water-cooled | |
| Turbocharger | Single variable-geometry turbo (Garrett GT1549V) | |
| Timing system | Dual chain (front-mounted, long-life design) | |
| Oil type | MB 229.5 (SAE 5W-30) | |
| Dry weight | 168 kg |
The inline-four configuration delivers efficient, linear power ideal for daily driving but demands strict adherence to 15,000 km oil change intervals using MB 229.5 specification oil to maintain fuel pump longevity. The Bosch CRS 2.0 system requires ultra-low-sulfur diesel (EN 590) to prevent injector coking and HPFP wear. Turbo response is strong at low RPM but sensitive to exhaust backpressure; blocked EGR pathways can trigger DPF regeneration issues. Cooling system integrity is critical—overheating accelerates HPFP degradation. Post-2011 units benefit from revised pump calibration and improved airflow management per Daimler SIB 27/2010.
Oil Specs: Requires MB 229.5 (5W-30) specification (Mercedes-Benz SIB 27/2010). ACEA B5 compliance insufficient.
Emissions: Euro IV applies to 2008–2010 models; Euro V certification for 2011–2016 units (VCA Type Approval #VCA/EMS/7893).
Power Ratings: Measured under DIN 70020 standards. Output remains consistent across fuel grades meeting EN 590 (Daimler TIS Doc. A38540).
Mercedes-Benz Technical Information System (TIS): Docs A38102, A38540, SIB 27/2010
VCA Type Approval Database (VCA/EMS/7893)
SAE International: J1349 Engine Power Test Code
The Mercedes-Benz OM651.921 was used across Mercedes-Benz's W204, W212, and X204 platforms with longitudinal mounting and designated for compact and mid-size executive applications. This engine received platform-specific adaptations-higher-flow oil cooler in the GLK 220 CDI and revised EGR calibration for stop-start duty-and from 2011 updated emissions control software to meet Euro V, creating minor service part and calibration differences. All adaptations are documented in OEM technical bulletins.
Locate the engine code stamped horizontally on the front-left cylinder block near the timing cover (Mercedes-Benz TIS A38102). The 8th VIN digit indicates engine type ('M' for OM651 series). Pre-2011 models have silver valve covers with black intake manifolds; post-2011 units feature revised emissions control modules and updated ECU firmware. Critical differentiation from OM651.910: OM651.921 has Bosch CRS 2.0 injection system with CP3 pump and Garrett GT1549V turbo; OM651.910 uses CP1 and smaller turbo. Service parts require build-date verification—fuel pumps before 06/2011 are incompatible with later calibrated ECUs (Daimler SIB 27/2010).
The OM651.921's primary reliability risk is high-pressure fuel pump degradation under sustained load, with elevated incidence in urban and commercial applications. Internal Daimler reports from 2012 noted increased HPFP failure rates in pre-2011 units exceeding 150,000 km, while VCA MOT data links turbocharger performance faults to EGR clogging in city-driven C-Class models. Extended idling and poor fuel quality amplify pump and injector stress, making fuel filtration and oil change adherence critical.
Analysis derived from Mercedes-Benz technical bulletins (2008–2017) and UK DVSA failure statistics (2010–2019). Repair procedures should follow manufacturer guidelines.
Find answers to most commonly asked questions about MERCEDES-BENZ OM651921.
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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