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Mercedes M256 3.0 L Inline‑6 Turbo Engine (2017–Present): Specs, Issues & Maintenance
The M256 Engine marks Mercedes-Benz’s return to the straight-six petrol engine offering the balance and smoothness of an inline layout, paired with modern features like 48V electric supercharging (eBooster), Integrated Starter-Generator (ISG), and twin-scroll turbo systems. It debuted in 2017 on the S-Class and CLS and also fits the E-Class, GLE, and AMG variants.
M256 engine is also notable for its improved reliability and engineering refinement, but like any engine, it has its known issues such as occasional coolant leaks or electronic component faults. In this article, we explore the engine’s specifications, oil capacity, and how it compares to the M276 V6.
We’ll also explain the difference between straight-6 and inline-6 configurations, offering a full understanding of what makes the M256 a key part of Mercedes-Benz’s modern powertrain lineup.
Key Specifications
| Specification | Details |
|---|---|
| Displacement | 2,998 cc |
| Configuration | Inline‑6, DOHC, 4 valves/cylinder |
| Power Output | 270–320 kW (362–429 hp) |
| Torque | 500–520 Nm |
| Bore × Stroke | 83 × 92 mm |
| Compression Ratio | 10.5:1 |
| Boost System | Twin‑scroll turbo + 48 V eBooster |
| Fuel System | Direct injection + OPF |
| Oil Capacity | ~7.5 L synthetic (MB229.51) |
| Service Interval | Oil: ~10,000 mi; spark plugs: ~60,000 mi |
M256 engine reliability
When it comes to reliability, the M256 engine has shown itself to be robust, thanks to its advanced engineering. The use of a 48-volt electrical system allows for improved start-stop functionality, quicker turbo spool-up, and reduced turbo lag.
The engine’s design emphasizes durability, and many users report that with proper maintenance, it remains reliable over long periods. However, as with any engine, the longevity of the M256 is highly dependent on regular maintenance, including timely oil changes and addressing any emerging issues promptly.
M256 engine problems & Recommended Fixes
The Mercedes-Benz M256 engine, while renowned for its advanced technology and performance, isn’t without its challenges. Like any sophisticated powertrain, the M256 has encountered some issues that owners and potential buyers should be aware of.
These problems, ranging from electrical faults in the 48-volt system to common wear and tear on the turbocharger, can affect the engine’s performance and reliability over time. Understanding these issues below is crucial for maintaining the M256 engine’s longevity and ensuring that it continues to deliver the exceptional driving experience Mercedes-Benz is known for.
Misfire In Engine Mercedes : Case Study & Solution
How to Tell If Timing Chain Is Bad: Here Is Solution
| Issue | Symptoms | Diagnosis & Fix |
|---|---|---|
| Oil consumption | Needing top-ups between services (~60k mi) | Check bore scoring and PCV system; top with quality oil |
| Coolant pump & 48V fractures | Erratic cooling or warning lights | Inspect and test the electric coolant pump and hybrid components |
| Timing chain timber chain noise | Rattle on cold start | Check chain tensioners; replace if noisy |
| OPF / intake valve soot buildup | Rough idle, misfires | Clean intake via walnut blasting (~60k mi) |
| Sensor/eBooster glitches | Fault codes or reduced performance | Update software; inspect BorgWarner eBooster unit |
| Cylinder scoring (rare) | Compression loss, misfires | Perform compression/leak-down tests; inspect bores |
Recommended Maintenance Schedule
| Maintenance Task | Interval |
|---|---|
| Oil & filter change | Every 10,000 miles |
| Walnut-blast intake cleanup | Around 60,000 miles |
| Spark plug replacement | ~60,000 miles |
| Timing chain and tensioner check | From ~100,000 miles |
| Inspect 48V system & e-booster | At major service intervals |
| Compression test | If performance declines |
M256 Engine Oil Capacity
The M256 engine has an oil capacity of approximately 7.5 liters (around 8 quarts), though it’s essential to consult your vehicle’s manual for the exact specification. Regular oil changes are crucial for maintaining the engine’s health, and using high-quality synthetic oil is recommended to ensure optimal performance.
Mercedes M256 vs. M276: A Comparative Analysis
The M256 and M276 engines represent two different approaches to engine design by Mercedes-Benz. The M276 is a V6 engine, while the M256 is an inline-six. Here are some key differences:
- – Power and Efficiency: The M256, with its inline-six configuration, tends to be smoother and more balanced than the V6 M276. The inclusion of a mild-hybrid system in the M256 also gives it an edge in fuel efficiency and reduced emissions.
- – Technology: The M256 benefits from newer technology, including the 48-volt electrical system and electric auxiliary compressor, which enhance performance and responsiveness.
- – Reliability: Both engines are reliable, but the M256 may offer better long-term durability due to its simplified inline-six design, which inherently has fewer components than a V6.
Which Mercedes Models Are Equipped with the M256 Engine?
The M256 engine is found in several high-end Mercedes-Benz models, including:
Mercedes-Benz GLS (X167)
These models showcase the engine’s versatility, offering a blend of luxury, performance, and efficiency
Straight-6 vs. Inline-6: Understanding the Terminology
The terms “straight-6” and “inline-6” are often used interchangeably. Both refer to an engine with six cylinders arranged in a straight line along the crankcase. This configuration is known for its smooth operation and inherent balance, which minimizes vibrations and enhances the driving experience.
The straight-6 (or inline-6) design has historically been favored for its mechanical simplicity and balance, making it an ideal choice for luxury vehicles like those in the Mercedes-Benz lineup.
EQ Boost in the M256 Engine
EQ Boost is a key feature of the Mercedes-Benz M256 engine, contributing significantly to its performance, efficiency, and overall driving experience. EQ Boost refers to a 48-volt mild-hybrid system integrated into the engine, providing several important functions:
Instant Power Assistance:
- – EQ Boost acts as an electric motor that can provide up to 21 horsepower and 250 Nm of torque instantaneously. This additional power comes in handy during acceleration, reducing turbo lag and delivering a smoother, more responsive driving experience.
Regenerative Braking:
- – The system captures energy during braking and deceleration, which is then stored in a lithium-ion battery. This recovered energy can be used later to power the electric motor, further enhancing efficiency.
Start-Stop Functionality:
- – EQ Boost allows for a smoother and quicker start-stop operation. The engine can turn off during coasting, idling, or when the vehicle is at a standstill, and restart almost instantaneously when needed. This feature helps in reducing fuel consumption and emissions.
Electric Auxiliary Compressor:
- – The EQ Boost system powers an electric auxiliary compressor, which helps in spooling up the turbocharger more quickly. This reduces turbo lag and improves engine response, particularly at lower RPMs.
Efficiency Improvements:
- – By assisting the internal combustion engine with electric power, EQ Boost helps reduce the engine’s workload, leading to better fuel economy and lower emissions. It also enables the M256 engine to operate more efficiently across a wide range of driving conditions.
Incorporating EQ Boost into the M256 engine not only enhances performance but also aligns with the growing demand for more eco-friendly and fuel-efficient vehicles. This system represents Mercedes-Benz’s commitment to innovative engineering, offering a bridge between traditional internal combustion engines and fully electric powertrains.
M256 Mercedes Engine Presentation video
The Mercedes-Benz M 256 engine represents a significant advancement in automotive engineering, combining performance, efficiency, and reliability. Whether you’re considering a vehicle with this engine or simply curious about its capabilities, understanding its strengths and potential issues will help you make an informed decision.
The M256 stands out not just for its technological innovations but also for the driving experience it delivers across various Mercedes models.
Frequently Asked Questions
Is the M256 engine reliable?
Generally yes, it’s smooth and advanced. Watch for oil consumption, timing-chain noise, and OPF buildup.
When should intake valves be cleaned?
At ~60,000 miles to avoid carbon buildup that causes misfires and rough idle.
Do hybrids in M256 require special maintenance?
Yes, the 48V electric compressor and coolant pump need diagnostics and occasional software updates.
How much oil does the M256 consume?
Many owners top off between services around 60,000 miles; check PCV and bore condition.
