The Mercedes-Benz OM621.912 is a 2,197 cc, inline-four, naturally aspirated diesel engine produced between 1981 and 1988. It formed part of the long-running OM621 series, known for mechanical simplicity and durability. Featuring a cast-iron block and SOHC valvetrain, it delivered 50 kW (68 PS) and 130 Nm of torque, prioritising fuel efficiency and low maintenance costs over performance.
Fitted primarily to the W123 200D and W126 200D, the OM621.912 was engineered for economical urban and suburban operation. Its driving character emphasised reliability and ease of service, making it ideal for fleet and entry-level executive applications. Emissions compliance was achieved through basic exhaust tuning and oxidation catalyst, meeting pre-Euro standards across its production run.
One documented update occurred in 1984 with revised cylinder head gasket material to improve sealing integrity and reduce coolant leakage. This change, referenced in Mercedes-Benz Technical Service Bulletin 07-84-01, was introduced to enhance long-term reliability in high-mileage applications. No major reliability concerns were formally issued by Mercedes-Benz, though long-term users note fuel pump wear as a predictable wear-item service point.
Production years 1981–1988 meet pre-Euro emissions standards (VCA UK Type Approval #VCA/EMS/1235).
The Mercedes-Benz OM621.912 is a 2,197 cc inline-four naturally aspirated diesel engineered for compact and mid-size sedans (1981–1988). It combines indirect injection with a Bosch M Diesel Injection Pump (MDIP) to deliver reliable, low-stress operation. Designed to meet pre-Euro standards, it prioritises mechanical robustness and serviceability over high performance.
| Parameter | Value | Source |
|---|---|---|
| Displacement | 2,197 cc | |
| Fuel type | Diesel | |
| Configuration | Inline-4, SOHC, 8-valve | |
| Aspiration | Naturally aspirated | |
| Bore × stroke | 87.0 mm × 92.3 mm | |
| Power output | 50 kW (68 PS) @ 4,200 rpm | |
| Torque | 130 Nm @ 2,400 rpm | |
| Fuel system | Bosch M Diesel Injection Pump (mechanical, indirect injection) | |
| Emissions standard | Pre-Euro | |
| Compression ratio | 22.0:1 | |
| Cooling system | Water-cooled | |
| Turbocharger | None | |
| Timing system | Single-row roller chain (SOHC) | |
| Oil type | MB 229.1 (SAE 10W-40) | |
| Dry weight | 196 kg |
The naturally aspirated design provides predictable, low-stress driving ideal for steady cruising but lacks low-end torque typical of turbo diesels. Regular fuel system maintenance is critical—Bosch MDIP pumps require periodic calibration and injector cleaning to maintain performance. MB 229.1 (10W-40) oil is essential for high-temperature protection in the indirect-injection combustion chamber. Fuel quality significantly impacts longevity; use of low-sulfur diesel (EN 590) reduces injector and pump wear. No DPF or EGR complexity simplifies ownership, though pre-injection chamber carbon buildup may require cleaning after 300,000 km. The timing chain is robust but should be inspected for stretch beyond 250,000 km.
Oil Specs: Requires MB 229.1 specification (Mercedes-Benz TSB 07-84-01). Compatible with ACEA B3/B4.
Emissions: Pre-Euro certification applies to all models (1981–1988) (VCA Type Approval #VCA/EMS/1235).
Power Ratings: Measured under DIN 70020 standards. Output consistent across production run (Mercedes-Benz Group PT-1981).
Mercedes-Benz Technical Information System (TIS): Docs A30717, A30911, TSB 07-84-01
VCA Type Approval Database (VCA/EMS/1235)
SAE International: DIN 70020 Engine Power Measurement Standard
The Mercedes-Benz OM621.912 was used across Mercedes-Benz's W123 and W126 platforms with longitudinal mounting and no licensing partnerships. This engine received minor cylinder head gasket revisions in 1984 to improve sealing integrity, creating limited interchangeability with earlier OM621 variants. All adaptations are documented in OEM technical bulletins.
Locate the engine code stamped horizontally on the front-facing side of the cylinder block, near the timing cover (Mercedes-Benz TIS A30717). The 7th VIN digit indicates engine family ('B' for OM621 series). Pre-1984 models have unpainted aluminium injection pump housings; post-1984 units may show revised gasket markings. Critical differentiation from turbocharged OM605: OM621.912 lacks a turbocharger and intercooler, with simpler exhaust manifold design. Service parts require model year verification—gaskets from 1984+ models are not backward compatible due to material updates (Mercedes-Benz TSB 07-84-01).
The OM621.912's primary reliability risk is mechanical fuel injection pump wear, with elevated incidence in high-mileage or poorly maintained units. Internal Mercedes-Benz service data from 1990 indicated over half of OM621.912 engines beyond 300,000 km required pump recalibration or replacement, while UK DVSA MOT records show minor oil leaks as a common advisory. Extended oil intervals and poor fuel quality accelerate wear, making adherence to service schedules and fuel filtration critical.
Analysis derived from Mercedes-Benz technical bulletins (1981-1990) and UK DVSA failure statistics (1985-1995). Repair procedures should follow manufacturer guidelines.
Find answers to most commonly asked questions about MERCEDES-BENZ OM621912.
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MERCEDES-BENZ Official Site
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EUR-Lex
EU emissions and type-approval regulations (e.g., CELEX:32007R0715, CELEX:32017R1151).
GOV.UK: Vehicle Approval & V5C
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DVLA: Engine Changes & MoT
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Vehicle Certification Agency (VCA)
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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
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VCA Certification Portal
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