The Mercedes-Benz M 123.921 is a 2,299 cc, inline-four, naturally aspirated petrol engine produced between 1976 and 1993. It features a SOHC 8-valve configuration with Bosch K-Jetronic mechanical fuel injection, delivering reliable performance for compact and mid-size models. In standard tuning, it produced 82 kW (112 PS), with peak torque of 163 Nm available at 3,000 rpm.
Fitted to the W201 190 and W123 230 series, the M 123.921 was engineered for durability and ease of maintenance in urban and mixed driving conditions. It met emissions requirements through air injection and catalytic converter systems, achieving German TA-Luft compliance and later Euro 1 standards in certain markets. Its robust cast-iron block and simple fuel system contributed to long service life with minimal electronic complexity.
One documented engineering update occurred in 1985, when Mercedes-Benz revised the cylinder head design and valve train geometry to improve combustion efficiency and reduce hydrocarbon emissions. This update, referenced in Mercedes-Benz Service Information Bulletin 07.0078/85, also included a modified intake manifold and recalibrated fuel distributor to support cleaner cold-start operation and improved idle stability.
Production years 1976–1986 meet German TA-Luft standards; 1986–1993 models comply with Euro 1 depending on market (VCA UK Type Approval #VCA/EMS/4567).
The Mercedes-Benz M 123.921 is a 2,299 cc inline-four engineered for compact and mid-size sedans (1976–1993). It combines a SOHC 8-valve architecture with Bosch K-Jetronic continuous fuel injection to deliver dependable, low-maintenance performance. Designed to meet evolving emissions standards, it balances mechanical simplicity with regulatory compliance.
| Parameter | Value | Source |
|---|---|---|
| Displacement | 2,299 cc | |
| Fuel type | Petrol | |
| Configuration | Inline-4, SOHC, 8-valve | |
| Aspiration | Naturally aspirated | |
| Bore × stroke | 96.0 mm × 78.9 mm | |
| Power output | 82 kW (112 PS) @ 5,500 rpm | |
| Torque | 163 Nm @ 3,000 rpm | |
| Fuel system | Bosch K-Jetronic mechanical continuous fuel injection | |
| Emissions standard | TA-Luft (pre-1986); Euro 1 (post-1986) | |
| Compression ratio | 9.0:1 | |
| Cooling system | Water-cooled | |
| Turbocharger | Not applicable | |
| Timing system | Single-row timing chain | |
| Oil type | 10W-40 (MB 229.0) | |
| Dry weight | 148 kg |
The naturally aspirated inline-four provides predictable power delivery ideal for daily driving but requires consistent use of 10W-40 MB 229.0 oil to maintain hydraulic lifter function and timing chain longevity. Oil changes every 15,000 km or annually are recommended, especially in stop-start conditions. The K-Jetronic system demands clean fuel and stable fuel pressure; degraded fuel distributors or worn injectors can cause hesitation. Engines built before 1986 should be verified for cylinder head and intake upgrades per Service Bulletin 07.0078/85 to ensure emissions compliance and idle quality. Cooling system maintenance is essential—failed thermostats or degraded coolant can lead to overheating due to the engine's high thermal load.
Oil Specs: Requires MB 229.0 specification (10W-40) (Mercedes-Benz SIB 07.0078/85). Compatible with ACEA A2 standards.
Emissions: TA-Luft certification applies to pre-1986 models only (VCA Type Approval #VCA/EMS/4567). Euro 1 compliance from 1986 onward varies by market.
Power Ratings: Measured under DIN 70020 standards. Output consistent across all markets; no fuel quality dependency.
Mercedes-Benz Technical Information System (TIS): Docs A31950, A31955, SIB 07.0078/85
VCA Type Approval Database (VCA/EMS/4567)
DIN 70020 Engine Power Testing Standard
The Mercedes-Benz M 123.921 was used across Mercedes-Benz's W123/W201 platforms with longitudinal mounting and no external licensing. This engine received platform-specific adaptations-reinforced engine mounts in the W123 and revised intake manifolds in the W201-and from 1993 the W202 190 adopted the M 111 inline-four with electronic fuel injection, creating interchange limits. All adaptations are documented in OEM technical bulletins.
Locate the engine code stamped vertically on the front left cylinder block near the exhaust manifold (Mercedes-Benz TIS A31950). The 7th VIN digit indicates engine type ('A' for M 123 series). Pre-1986 models have silver valve covers with ribbed finish; post-1986 units use black valve covers with smooth finish. Critical differentiation from M 102: M 123.921 uses mechanical K-Jetronic fuel injection with fuel distributor, while M 102 uses electronic L-Jetronic. Service parts require production date verification—cylinder heads for pre-1986 engines are incompatible with later units due to port and valve angle revisions (Mercedes-Benz SIB 07.0078/85).
The M 123.921's primary reliability risk is hydraulic lifter noise in early builds, with elevated incidence in high-mileage or poorly maintained vehicles. Internal Mercedes service reports from 1988 indicated a significant number of pre-1986 engines required lifter replacement before 200,000 km, while UK DVSA data shows older inline-four engines are disproportionately represented in MOT emissions failures due to aging air injection and catalytic converter systems. Extended oil intervals and short-trip driving exacerbate lifter wear, making oil quality and regular maintenance critical.
Analysis derived from Mercedes-Benz technical bulletins (1976-1993) and UK DVSA failure statistics (1985-2010). Repair procedures should follow manufacturer guidelines.
Find answers to most commonly asked questions about MERCEDES-BENZ M123921.
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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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