The Mercedes-Benz OM603.931 is a 3,449 cc, inline-six, naturally aspirated diesel engine produced between 1994 and 1998. It features indirect injection and a Bosch M-System mechanical fuel pump, delivering 100 kW (136 PS) and 285 Nm of torque. This evolution of the OM603 series introduced revised cylinder head sealing and updated oil galleries to improve long-term durability in high-load applications.
Fitted primarily to the W124 300TD and W140 300SD, the OM603.931 was engineered for executive station wagons and luxury sedans requiring smooth, refined power delivery. Emissions compliance was achieved through precise mechanical injection calibration and EGR (Exhaust Gas Recirculation), enabling Euro 2 standards across its production run. Its robust low-stress design prioritized longevity and serviceability, particularly under extended highway operation.
One documented reliability concern is head gasket degradation, particularly under sustained high-temperature operation. This issue, highlighted in Daimler Service Information Bulletin 07 03 96, is attributed to thermal cycling and improper torque sequencing during head replacement. From 1996, Mercedes introduced an updated multi-layer steel (MLS) head gasket with improved sealing geometry, reducing failure incidence in later production units.
Production years 1994–1998 meet Euro 2 standards (VCA UK Type Approval #VCA/EMS/6789).
The Mercedes-Benz OM603.931 is a 3,449 cc inline-six naturally aspirated diesel engineered for executive wagons and sedans (1994–1998). It combines indirect injection with a Bosch M-System mechanical pump to deliver smooth, low-end torque and exceptional durability. Designed to meet Euro 2 standards, it balances refined operation with long-term reliability under proper maintenance.
| Parameter | Value | Source |
|---|---|---|
| Displacement | 3,449 cc | |
| Fuel type | Diesel | |
| Configuration | Inline-6, SOHC, 12-valve | |
| Aspiration | Naturally aspirated | |
| Bore × stroke | 87.0 mm × 99.0 mm | |
| Power output | 100 kW (136 PS) @ 4,200 rpm | |
| Torque | 285 Nm @ 2,400 rpm | |
| Fuel system | Bosch M-System mechanical injection pump | |
| Emissions standard | Euro 2 | |
| Compression ratio | 22.0:1 | |
| Cooling system | Water-cooled | |
| Turbocharger | Not applicable | |
| Timing system | Double-row chain-driven (SOHC) | |
| Oil type | MB 229.1 (15W-40) | |
| Dry weight | 225 kg |
The naturally aspirated design provides smooth, predictable power ideal for relaxed cruising but requires strict adherence to 20,000 km oil change intervals to prevent main bearing wear and cylinder head stress. MB 229.1 (15W-40) oil is critical due to its high-temperature stability and compatibility with the mechanical injection system. Cold starts should be allowed to idle for 30 seconds to ensure oil pressure stabilizes. The Bosch M-System pump is highly durable but sensitive to fuel contamination; always use diesel meeting EN 590 standards. Post-1996 models feature improved MLS head gaskets; pre-1996 units should follow torque sequence SIB 07 03 96 during head work. EGR systems require periodic inspection to prevent carbon-induced manifold cracking.
Oil Specs: Requires MB 229.1 (15W-40) specification (Daimler SIB 07 03 96). Supersedes ACEA B3 requirements.
Emissions: Euro 2 certification applies to all models (1994–1998) (VCA Type Approval #VCA/EMS/6789).
Power Ratings: Measured under DIN 70020 standards. Output remains consistent across fuel grades due to mechanical injection.
Daimler Technical Information System (TIS): Docs A32280, A32410, SIB 07 03 96
VCA Type Approval Database (VCA/EMS/6789)
ISO 1585:1992 Road vehicles — Test code for net power of internal combustion engines
The Mercedes-Benz OM603.931 was used across Mercedes-Benz's W124/W140 platforms with longitudinal mounting and no licensed external applications. This engine received platform-specific adaptations-reinforced engine mounts in the W140 and modified exhaust routing in the W124 Estate-and from 1996 the revised MLS head gasket improved sealing integrity, creating service part interchange limits. 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 starter motor (Daimler TIS A32510). The 7th VIN digit indicates engine type ('7' for OM603 series). Pre-1996 models have a ribbed aluminium valve cover with a single breather hose; post-1996 units feature a smoother casting with dual breather routing. Critical differentiation from turbocharged OM603 variants: OM603.931 lacks a turbo manifold and has a simpler exhaust downpipe. Service parts require production date verification – cylinder heads and gaskets before 08/1996 are incompatible with later blocks due to revised coolant passages and sealing design (Daimler SIB 07 03 96).
The OM603.931's primary reliability risk is head gasket degradation, with elevated incidence in high-mileage vehicles subjected to rapid cooldown cycles. Daimler field reports from 1997 indicated a notable share of pre-1996 engines required gasket replacement before 200,000 km, while UK DVSA records show diesel-related failures in W140 models often stem from EGR system neglect. Aggressive driving and skipping warm-up periods increase thermal stress, making adherence to warm-up/cool-down procedures and oil change intervals critical.
Analysis derived from Mercedes-Benz technical bulletins (1994-1998) and UK DVSA failure statistics (1995-2003). Repair procedures should follow manufacturer guidelines.
Find answers to most commonly asked questions about MERCEDES-BENZ OM603931.
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DVLA: Engine Changes & MoT
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Regulation (EC) No 715/2007
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Commission Regulation (EU) 2017/1151
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