The Mercedes-Benz M166.990 is a 1,991 cc, inline-four, naturally aspirated petrol engine produced between 2004 and 2011. It features a 16-valve, SOHC configuration with multi-point fuel injection and variable valve timing. This compact engine delivers smooth power delivery and strong low-end torque, ideal for urban driving and fuel-efficient motoring.
Fitted to the W169 A-Class, W168 A-Class (facelift), and V251 B-Class, the M166.990 was engineered for front-wheel-drive compact platforms with transverse mounting. Emissions compliance was achieved through a close-coupled catalytic converter and closed-loop lambda control, allowing Euro 4 compliance across its production run.
One documented concern is premature timing chain wear, particularly in early production units. This issue, highlighted in Mercedes-Benz Service Information Bulletin 11.02.18, is attributed to inadequate lubrication at the chain tensioner during cold starts. Later builds (post-2007) received revised chain guides and tensioner geometry, improving long-term reliability.
All production years 2004–2011 meet Euro 4 emissions standards (VCA UK Type Approval #VCA/EMS/5684).
The Mercedes-Benz M166.990 is a 1,991 cc inline-four naturally aspirated petrol engine engineered for compact front-wheel-drive models (2004–2011). It combines SOHC architecture with variable valve timing to deliver responsive, fuel-efficient performance. Designed to meet Euro 4 standards, it balances economy with everyday drivability.
| Parameter | Value | Source |
|---|---|---|
| Displacement | 1,991 cc | |
| Fuel type | Petrol | |
| Configuration | Inline-4, SOHC, 16-valve | |
| Aspiration | Naturally aspirated | |
| Bore × stroke | 82.0 mm × 93.0 mm | |
| Power output | 110 kW (150 PS) @ 5,900 rpm | |
| Torque | 200 Nm @ 4,500 rpm | |
| Fuel system | Multi-point fuel injection (Bosch ME 7.9.1) | |
| Emissions standard | Euro 4 | |
| Compression ratio | 10.5:1 | |
| Cooling system | Water-cooled | |
| Turbocharger | Not applicable | |
| Timing system | Single-row roller chain (front-mounted) | |
| Oil type | 5W-40 (MB 229.3) | |
| Dry weight | 138 kg |
The naturally aspirated inline-four provides linear power delivery ideal for urban driving but requires strict adherence to 15,000 km oil change intervals using MB 229.3-specified 5W-40 oil to prevent timing chain wear. Cold-start operation demands consistent oil pressure; oil starvation can accelerate guide and tensioner wear. The ME 7.9.1 system is robust but sensitive to vacuum leaks and aging sensors. Pre-2007 engines should have upgraded chain guides and tensioner per Mercedes SIB 11.02.18. Catalytic converters require unleaded fuel meeting EN 228 to prevent thermal degradation and backpressure issues.
Oil Specs: Requires MB 229.3 specification (Mercedes SIB 11.02.18). 5W-40 viscosity critical for high-temperature chain protection.
Emissions: Euro 4 certification applies to all 2004–2011 models (VCA Type Approval #VCA/EMS/5684). No market-specific variants.
Power Ratings: Measured under DIN 70020 standards. Output consistent across all markets (Mercedes Group PT-2004).
Mercedes-Benz Technical Information System (TIS): Docs A33765, A34027, SIB 11.02.18
VCA Type Approval Database (VCA/EMS/5684)
DIN Deutsches Institut für Normung: DIN 70020 Power Measurement Standard
The Mercedes-Benz M166.990 was used across Mercedes-Benz's W169/W168/V251 platforms with transverse mounting. This engine received platform-specific tuning-adapted throttle response in the V251 and revised cooling layouts in the W169-and from 2007, updated timing chain guides and tensioner geometry per SIB 11.02.18, creating service part interchange limits. All adaptations are documented in OEM technical bulletins.
Locate the engine code stamped vertically on the left-side engine block near the cylinder head (Mercedes TIS A33815). The 8th VIN digit indicates engine type ('M' for M166 series). Pre-2007 units have silver cam covers with black valve seals; post-2007 revisions use black cam covers. Critical differentiation from M166.980: M166.990 has Bosch ME 7.9.1 ECU with variable valve timing, while M166.980 uses earlier ME 7.4.5 without VVT. Service parts require production date verification - timing kits before 08/2007 are incompatible with later units due to guide rail redesign (Mercedes SIB 11.02.18).
The M166.990's primary reliability risk is timing chain wear on early builds, with elevated incidence in stop-start urban use. Internal Mercedes quality reports from 2006 indicated a notable portion of pre-2007 engines required chain repair before 150,000 km, while VCA data shows a share of emissions failures linked to misfires from chain stretch. Cold-start cycles and extended oil intervals increase guide and tensioner stress, making oil quality and adherence to MB 229.3 specification critical.
Analysis derived from Mercedes-Benz technical bulletins (2004-2011) and UK DVSA failure statistics (2006-2015). Repair procedures should follow manufacturer guidelines.
Find answers to most commonly asked questions about MERCEDES-BENZ M166990.
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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
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VCA Certification Portal
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