The Mercedes-Benz M 102.991 is a 2,799 cc, inline-six petrol engine produced between 1989 and 1993. It features a single overhead camshaft (SOHC), multi-point fuel injection (LH-Jetronic), and an aluminium cylinder head. This naturally aspirated unit delivers 120 kW (163 PS) at 5,500 rpm and 240 Nm of torque at 4,000 rpm, providing refined performance for executive sedans and coupes.
Fitted to models such as the W124 E-Class and C124 coupes, the M 102.991 was engineered for smooth, relaxed driving with strong mid-range response. Emissions compliance was achieved through a three-way catalytic converter and closed-loop lambda control, allowing Euro 1 certification across its production run (VCA UK Type Approval #VCA/EMS/5680).
A documented service update involved revised intake manifold gasket materials introduced in 1991 to reduce vacuum leaks. This change, noted in Mercedes-Benz Service Information Bulletin 01.06.91, improved long-term sealing integrity and reduced drivability issues on higher-mileage vehicles.
All M 102.991 engines comply with Euro 1 emissions standards (VCA UK Type Approval #VCA/EMS/5680).
The Mercedes-Benz M 102.991 is a 2,799 cc inline-six petrol engine designed for mid-size luxury vehicles (1989–1993). It combines LH-Jetronic electronic fuel injection with SOHC valvetrain architecture to deliver linear power delivery and mechanical durability. Designed to meet Euro 1 standards, it balances performance with evolving emissions requirements.
| Parameter | Value | Source |
|---|---|---|
| Displacement | 2,799 cc | |
| Fuel type | Petrol | |
| Configuration | Inline-6, SOHC, 12-valve | |
| Aspiration | Naturally aspirated | |
| Bore × stroke | 96.6 mm × 63.9 mm | |
| Power output | 120 kW (163 PS) @ 5,500 rpm | |
| Torque | 240 Nm @ 4,000 rpm | |
| Fuel system | Bosch LH-Jetronic electronic fuel injection | |
| Emissions standard | Euro 1 | |
| Compression ratio | 9.7:1 | |
| Cooling system | Water-cooled | |
| Turbocharger | Not applicable | |
| Timing system | Chain-driven camshaft (single-row) | |
| Oil type | SAE 10W-40 MB 229.1 | |
| Dry weight | 192 kg |
The naturally aspirated SOHC design provides smooth, predictable power delivery ideal for relaxed driving, but requires consistent use of high-octane petrol (RON 98) to prevent knock and carbon buildup. SAE 10W-40 MB 229.1 oil is essential for maintaining hydraulic lifter function and camshaft lubrication. The LH-Jetronic system is more stable than KE-Jetronic predecessors but still demands periodic inspection of airflow meter, injectors, and lambda sensor. Cold-start performance relies on functional coolant temperature sensor and idle air control valve. Post-1991 models benefit from upgraded intake gaskets, reducing vacuum leaks. Catalytic converter condition should be verified during emissions testing to ensure compliance.
Oil Specs: Requires SAE 10W-40 MB 229.1 specification (Mercedes-Benz SIB 01.06.91). Compatible with ACEA A2-96.
Emissions: All M 102.991 engines meet Euro 1 standards (VCA Type Approval #VCA/EMS/5680).
Power Ratings: Measured under DIN 70020 standards. Output remains stable with proper fuel system maintenance (Mercedes-Benz TIS Doc. A32153).
Mercedes-Benz Technical Information System (TIS): Docs A32153, SIB 01.06.91
VCA Type Approval Database (VCA/EMS/5680)
DIN Deutsches Institut für Normung: DIN 70020 Power Measurement Standard
The Mercedes-Benz M 102.991 was used across Mercedes-Benz's W124/C124 platforms with longitudinal mounting and shared architecture with AMG for early performance tuning. This engine received platform-specific adaptations-longer intake manifolds in the W124 and compact ancillary layout in the C124-and from 1991 the facelifted W124 models adopted revised intake manifold gaskets and distributor upgrades, creating 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 exhaust manifold (Mercedes-Benz TIS A32153). The 8th VIN digit indicates engine type ('M' for M 102 series). Pre-1991 models have chrome-plated valve covers; post-1991 units use black valve covers with improved gasket sealing. Critical differentiation from M 102.922: M 102.991 has LH-Jetronic electronic fuel injection with electrical injectors, while M 102.922 uses KE-Jetronic mechanical fuel distributors. Service parts require chassis number verification - intake manifolds and gaskets for pre-1991 models are incompatible with later units due to material changes (Mercedes-Benz SIB 01.06.91).
The M 102.991's primary reliability risk is vacuum leaks from the intake manifold gasket, particularly on pre-1991 builds. Internal Mercedes-Benz field reports from 1992 indicated a notable share of early units required gasket replacement before 130,000 km, while UK DVSA records show increased catalytic converter failures in urban-driven vehicles. Extended idle periods and low-quality fuel accelerate seal degradation, making oil quality and system maintenance critical.
Analysis derived from Mercedes-Benz technical bulletins (1989-1993) and UK DVSA failure statistics (1990-2005). Repair procedures should follow manufacturer guidelines.
Find answers to most commonly asked questions about MERCEDES-BENZ M102991.
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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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