Mercedes M278 Reliability: The Last Great Non-AMG V8?

The Mercedes-Benz M278 (4.6L / 4.7L V8 Biturbo) is the engine that powered the absolute peak of modern Mercedes luxury—the W222 S-Class, the W212 E-Class, and the C218 CLS. It replaced the naturally aspirated M273, bringing twin-turbo torque (516 lb-ft!) and effortless power.

But owning an M278 out of warranty is a high-stakes game. While it avoids the catastrophic reputation of the BMW N63, it hides its own set of “wealth killers.” From cylinder scoring that scraps the block to oil wicking that fries the ECU, the M278 demands respect and deep pockets.

This guide is the internet’s only honest, technical deep-dive into the M278’s failure points, specifically written for second and third owners.

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1. M278 Engine Architecture & Applications

The M278 is a 90-degree V8 with:

• Displacement: 4,663 cc (marketed as 4.7L or “550” / “500”).
• Induction: Twin Honeywell turbochargers (0.9 bar boost).
• Injection: 3rd Gen Piezo Direct Injection (multi-spark).
• Block: Aluminum with Silitec (Silicon-Aluminum) cylinder liners.

Affected Models (North America)

• S550 (W221 / W222): 2011 – 2017
• E550 (W212 / A207): 2012 – 2017
• CLS550 (C218): 2012 – 2018
• GL550 / GLS550 (X166): 2013 – 2019
• SL550 (R231): 2013 – 2020
• CL550 (C216): 2011 – 2014

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2. The “Silent Killer”: Camshaft Sensor Oil Wicking

[!WARNING] CRITICAL DESIGN FLAW A $50 sensor can ruin your $2,500 Engine Control Unit (ECU).

This is the most insidious failure on the M278 (and related M157 AMG). The Camshaft Position Sensors and Camshaft Solenoids develop internal leaks. The oil does not drip onto the ground; it gets pushed through the electrical connector pins.

The Mechanism

1. Leak: Oil breaches the sensor’s internal seal.
2. Capillary Action: Oil travels up the copper wiring inside the insulation, like a straw.
3. Migration: It travels through the main engine harness, all the way to the ECU pins.
4. Failure: The oil shorts the ECU, O2 sensors, or even the fuel injector harness.

Prevention

• Inspect: Pull the connectors off your cam sensors today. If they are wet with oil, you have the problem.
• Fix: Replace all 4 sensors and 4 solenoids immediately.
• Sacrificial Harness: Mercedes sells small “pigtail” extensions (Part # A2711502733) that act as an oil break. Install these on every M278.

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3. Cylinder Wall Scoring (Silitec Failure)

The M278 does not use cast iron sleeves. It uses Silitec—an Alusil-like process where silicon crystals are exposed on the cylinder wall to provide a hard sliding surface for the piston rings.

Why It Fails

Unlike the Porsche/Audi Alusil issues, M278 scoring is often link to:

1. Thermal Distortion: The M278 runs hot. Localized hotspots can cause the piston skirt to touch the wall.
2. Injector Wash: A leaking piezo injector washes the oil film off the wall, causing metal-on-metal contact.
3. Carbon: Hard carbon chunks breaking off valves and scouring the cylinder.

Symptoms

• Knocking: A rhythmic “thock-thock-thock” that speeds up with RPM (Piston Slap).
• Oil Consumption: Burning 1 quart every 500-800 miles.
• Misfires: Fouled plugs in one specific cylinder.

The Reality

If your M278 has deep scoring, the engine is totaled. You cannot easily bore/sleeve these blocks economically. A used engine is the only fix ($6,000+).

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4. Timing Chain & Tensioner Rattle

Mercedes timing chains used to be forever. On the M278, they are an 80k-120k mile wear item.

The “Check Valve” Issue

Early M278s (Pre-2014) lacked check valves in the chain tensioners. When the engine sits, oil drains out of the tensioner.

• Startup: On cold start, the chain is loose for 2-3 seconds until oil pressure builds.
• Damage: The loose chain slaps the guides and stretches over time.

Check Engine Light: P0016 / P0017

If the chain stretches enough, the camshafts go out of sync with the crank.

• The Fix: You need a new chain, new tensioners, and to verify the cam adjusters (phasers) aren’t destroyed.
• Cost: $4,000 - $6,000 (Engine-out or front-clip-off procedure).

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5. Turbo Coolant Lines (The Plastic Fail)

The twin turbos are fed coolant to keep them alive. Mercedes used plastic lines that run directly through the hottest part of the engine valley.

• Failure: The plastic turns to dust after 6-7 years.
• Result: Rapid coolant loss.
• Upgrade: Replace these with the updated lines or aftermarket metal solutions during any major service.

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6. Maintenance Schedule for Longevity

To keep an M278 alive past 150k miles, you must ignore the factory 10k mile oil intervals.

Service	Interval	Notes
Oil Change	5,000 miles	Use 5W-40 MB 229.5 spec. 10k is suicide for timing chains.
Spark Plugs	40,000 miles	Multi-spark ignition eats plugs.
Air Filters	20,000 miles	Turbos need to breathe.
Transmission	40,000 miles	7G-Tronic Plus needs fluid refresh.
Cam Sensors	Inspect Every Oil Change	Check for oil in connectors.

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7. M278 vs. The Competition

Feature	Mercedes M278	BMW N63 (Pre-TU)	Audi 4.0T (EA824)
Architecture	4.7L V8 Biturbo	4.4L V8 Biturbo (“Hot V”)	4.0L V8 Biturbo (“Hot V”)
Reliability	Medium	Very Low	High
Fatal Flaw	Cylinder Scoring	Valve Stem Seals / Everything	Turbo Oil Screen
Repair Costs	$$$$	$$$$$	$$$$
Power Potential	High (Tune-only 500whp)	High	Very High

The M278 is significantly more reliable than the early BMW N63, but arguably less robust than the Audi 4.0T (once the Audi’s oil screen is deleted).

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8. 100k Mile Ownership Budget

If you buy a 2014 S550 with 80,000 miles for $20,000, expect to spend the following in the first 3 years:

• Cam Sensor Fix: $300 (DIY) / $800 (Shop)
• Coolant Lines: $1,200
• Air Suspension (2 Struts): $2,000
• Engine Mounts: $1,200 (Labor intensive)
• Brakes/Tires: $2,500
• Contingency (Chain/Injector): $4,000

Total Projected Cost: ~$11,000 on top of purchase price.

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9. Conclusion: Is the M278 Worth It?

The M278 offers the kind of “sovereign” power that modern EQ-Boost hybrid 6-cylinders cannot match. It is smooth, torque-rich, and sounds like money.

Verdict:

• Buy if you can verify the service history (5k drain intervals) and can inspect for scoring/oil-wicking.
• Walk Away if you hear a rattle on startup or see oil in the ECU connectors.
• Best Year: 2015+ models have the updated timing check valves and tensioners.

Related Guides:

• Mercedes S550 (W222) Reliability
• Mercedes E550 (W212) Problems
• Mercedes CLS550 (C218) Reliability
• Mercedes GL550 (X166) Problems
• Mercedes M157 AMG Engine Guide