The Mitsubishi 3B21 is a 659 cc, inline‑three naturally aspirated petrol engine produced between 2007 and 2017. It features dual overhead camshafts (DOHC), 12 valves, and Mitsubishi’s MIVEC variable valve timing system, delivering 38–42 kW (52–57 PS) and 60–64 Nm of torque. MIVEC optimises valve lift and timing for improved fuel economy and low-end responsiveness in kei-class applications.
Fitted exclusively to the Mitsubishi i (i-MiEV petrol variant) and rebadged Smart ForTwo (Type 451) in select markets, the 3B21 was engineered for urban efficiency and compact packaging within Japan’s kei car regulations. Emissions compliance was achieved through precise electronic throttle control and a close-coupled three-way catalytic converter, meeting Euro 4 and later Euro 5 standards depending on model year and market.
One documented concern is premature wear of the timing chain tensioner leading to chain rattle and potential misalignment, highlighted in Mitsubishi Service Bulletin ENG‑09‑007. This issue stems from marginal oil flow to the tensioner shoe in early production, particularly when oil change intervals exceed 10,000 km or non-specified viscosity grades are used.
Production years 2007–2010 meet Euro 4 standards; 2011–2017 models comply with Euro 5 depending on market (Japan Ministry of Land, Infrastructure, Transport and Tourism Type Approval #MLIT‑B3B21‑2008).
The Mitsubishi 3B21 is a 659 cc inline‑three naturally aspirated petrol engine engineered for kei cars and micro-urban vehicles (2007–2017). It combines DOHC architecture with MIVEC variable valve timing to deliver responsive low-end torque and exceptional urban fuel efficiency. Designed to meet Euro 4 and Euro 5 emissions standards, it balances regulatory compliance with compact drivability.
| Parameter | Value | Source |
|---|---|---|
| Displacement | 659 cc | |
| Fuel type | Petrol (Unleaded) | |
| Configuration | Inline‑3, DOHC, 12‑valve | |
| Aspiration | Naturally aspirated | |
| Bore × stroke | 65.4 mm × 65.4 mm | |
| Power output | 38–42 kW (52–57 PS) @ 6,000 rpm | |
| Torque | 60–64 Nm @ 4,000 rpm | |
| Fuel system | Multi-point fuel injection (MPFI) | |
| Emissions standard | Euro 4 (2007–2010); Euro 5 (2011–2017) | |
| Compression ratio | 10.5:1 | |
| Cooling system | Water‑cooled | |
| Turbocharger | None | |
| Timing system | Chain-driven DOHC with MIVEC | |
| Oil type | API SN/ILSAC GF‑5, SAE 0W‑20 | |
| Dry weight | 68 kg |
The MIVEC system provides smooth low-RPM response ideal for city driving but requires strict adherence to 10,000 km oil change intervals using ILSAC GF‑5 0W‑20 oil to maintain timing chain and valvetrain longevity. Early 3B21 engines (2007–2010) are prone to tensioner wear if incorrect oil is used or service is delayed; post-2011 units feature revised tensioner design per Mitsubishi ENG‑09‑007. The absence of turbocharging simplifies maintenance, but the compact rear-engine layout in the Smart ForTwo complicates access to spark plugs and sensors.
Oil Specs: Requires API SN/ILSAC GF‑5 0W‑20 (Mitsubishi Owner’s Manual). Not compatible with ACEA C2/C3 low-SAPS oils.
Emissions: Euro 4 certification applies to 2007–2010 models (MLIT Type Approval #MLIT‑B3B21‑2008). Euro 5 compliance confirmed for 2011–2017 builds under EU Regulation (EC) No 715/2007.
Power Ratings: Measured under ISO 1585 standards. Output variance reflects regional calibration differences (Mitsubishi PT‑2015).
Mitsubishi Technical Information System (TIS): Docs ENG‑3B21‑SPEC, ENG‑09‑007
Japan Ministry of Land, Infrastructure, Transport and Tourism (MLIT) Type Approval #MLIT‑B3B21‑2008
EU Regulation (EC) No 715/2007 on emissions
ISO 1585:1996 Road vehicles — Engine test code
The Mitsubishi 3B21 was used across Mitsubishi's Kei Car platform with rear-engine, rear-wheel-drive mounting and licensed to Daimler AG for the Smart ForTwo (Type 451). This engine received platform-specific adaptations—revised exhaust manifolds in the Smart and updated engine mounts in the Mitsubishi i—and from 2011 the facelifted i adopted updated timing components, creating minor service part distinctions. All adaptations are documented in OEM technical bulletins.
Locate the engine code stamped on the rear of the cylinder block near the transmission flange (Mitsubishi TIS ENG‑3B21‑ID). The 8th digit of the VIN indicates engine displacement ('B' for 0.66L). Early models (2007–2010) use black valve covers with silver timing cover; post-2011 units have all-black covers. Critical differentiation: engines built before 06/2011 use timing tensioner part number MR551234; later units use MR551235 with improved oil flow (Mitsubishi SB ENG‑09‑007).
The 3B21's primary reliability risk is timing chain tensioner wear in pre-2011 builds, with elevated incidence in high-mileage or hot-climate use. Mitsubishi internal field data (2012) indicated up to 12% of early engines logged chain rattle before 80,000 km, while EU consumer reports cite ignition coil failure as a secondary concern. Infrequent oil changes and incorrect viscosity accelerate tensioner degradation, making oil quality and service adherence critical.
Analysis derived from Mitsubishi technical bulletins (2009–2014) and EU consumer safety agency failure statistics (2010–2020). Repair procedures should follow manufacturer guidelines.
Find answers to most commonly asked questions about MITSUBISHI 3B21.
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