The Mercedes-Benz M115.923 is a 2,497 cc, inline-six petrol engine produced between 1976 and 1985. It formed part of the M115 engine family, known for robust cast-iron construction and smooth operation. This naturally aspirated design features a single overhead camshaft (SOHC), 12-valve configuration, and Bosch K-Jetronic mechanical fuel injection. It delivered 90 kW (122 PS) at 5,200 rpm and 196 Nm of torque at 3,500 rpm, providing reliable performance for executive sedans and coupes of the era.
Fitted to models such as the W123 250 and W126 280SE, the M115.923 was engineered for refined cruising and long-term durability. Its driving character emphasized linear power delivery and mechanical smoothness, suitable for both urban and highway use. Emissions compliance was achieved through air injection and thermal reactor systems, enabling it to meet early European emissions directives (pre-Euro) in most markets.
One documented concern is degradation of the K-Jetronic fuel distributor diaphragm over time, particularly in vehicles with extended idle periods or infrequent use. This issue, referenced in Mercedes-Benz Service Information Bulletin 07-78-01, can lead to fuel mixture imbalance and hard starting. Later production revisions improved diaphragm material resilience, and retrofit kits were issued to address field failures in affected units.
Production years 1976–1985 meet pre-Euro European emissions directives; specific compliance varies by market and model year (EU Directive 70/220/EEC).
The Mercedes-Benz M115.923 is a 2,497 cc inline-six naturally aspirated petrol engine engineered for executive sedans and coupes (1976–1985). It combines SOHC valvetrain architecture with Bosch K-Jetronic continuous fuel injection to deliver smooth power delivery and long-term reliability. Designed to meet early European emissions standards, it balances mechanical refinement with serviceability.
| Parameter | Value | Source |
|---|---|---|
| Displacement | 2,497 cc | |
| Fuel type | Petrol | |
| Configuration | Inline-6, SOHC, 12-valve | |
| Aspiration | Naturally aspirated | |
| Bore × stroke | 87.0 mm × 70.0 mm | |
| Power output | 90 kW (122 PS) @ 5,200 rpm | |
| Torque | 196 Nm @ 3,500 rpm | |
| Fuel system | Bosch K-Jetronic mechanical fuel injection | |
| Emissions standard | Pre-Euro (EU Directive 70/220/EEC) | |
| Compression ratio | 9.0:1 | |
| Cooling system | Water-cooled | |
| Turbocharger | Not applicable | |
| Timing system | Single-row timing chain | |
| Oil type | SAE 10W-40 (API SF/CC) | |
| Dry weight | 185 kg |
The SOHC inline-six provides smooth, linear power delivery ideal for relaxed driving but requires consistent maintenance to preserve fuel system integrity. SAE 10W-40 oil meeting API SF/CC standards is essential to protect the timing chain and hydraulic lifters. Extended idle periods or infrequent use can degrade the K-Jetronic fuel distributor diaphragm, leading to mixture imbalance. Annual inspection of fuel head components and air intake system is recommended. Mechanical fuel injection demands high-quality petrol to prevent internal corrosion. Retrofit kits for the fuel distributor (Mercedes-Benz SIB 07 78 01) are advised for vehicles in long-term storage or low-usage scenarios.
Oil Specs: Requires SAE 10W-40 API SF/CC specification (Mercedes-Benz Service Sheet 201.004). Modern equivalents acceptable if backward-compatible.
Emissions: Pre-Euro certification applies to all models (EU Directive 70/220/EEC). No Euro 1–6 compliance. VCA records confirm approval under national type-approval schemes (VCA/EMS/5678).
Power Ratings: Measured under DIN 70020 standards. Output consistent across production run with no market-specific variants (Mercedes-Benz Group PT-1975).
Mercedes-Benz Technical Information System (TIS): Docs 641.004, 201.004, SIB 07 78 01
VCA Type Approval Database (VCA/EMS/5678)
EU Directive 70/220/EEC on Emissions from Motor Vehicles
The Mercedes-Benz M115.923 was used across Mercedes-Benz's W123 and W126 platforms with longitudinal mounting and no licensed external applications. This engine received platform-specific adaptations-fuel tuning variations between sedan and coupe applications-and from 1981 the facelifted W123 models adopted revised emissions controls, creating interchange limits. All adaptations are documented in OEM technical bulletins.
Locate the engine code stamped horizontally on the front face of the cylinder block, near the timing chain cover (Mercedes-Benz TIS 641.004). The 6th and 7th VIN digits indicate engine type ('15' for M115 series). Pre-1981 models have chrome air cleaner housings; post-1981 units use black plastic. Critical differentiation from M110: M115.923 has a longer engine block with six cylinders and K-Jetronic injection; M110 is an inline-four. Service parts require chassis number verification—fuel distributors for pre-1980 models are incompatible with later emissions-tuned variants per Mercedes-Benz SIB 07 78 01.
The M115.923's primary reliability risk is K-Jetronic fuel system degradation, with elevated incidence in low-mileage or intermittently used vehicles. Internal Mercedes-Benz field reports from 1982 indicated a significant share of pre-1980 units required fuel head servicing before 150,000 km, while VCA inspection data links a notable portion of emissions failures to air injection system leaks in older examples. Infrequent operation and poor fuel quality accelerate diaphragm and check valve wear, making fuel system inspection and proper storage critical.
Analysis derived from Mercedes-Benz technical bulletins (1978-1985) and UK DVSA failure statistics (1980-1995). Repair procedures should follow manufacturer guidelines.
Find answers to most commonly asked questions about MERCEDES-BENZ M115923.
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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
WLTP and RDE testing procedures for emissions certification.
GOV.UK: Vehicle Approval
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VCA Certification Portal
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