Mercedes M278 V8 Reliability: The Definitive Guide (4.7L Biturbo)
Common Failure Points & Costs
| Component | Failure Mileage | Symptom | Est. Cost (USD) | Risk Level |
|---|---|---|---|---|
| Cylinder Scoring (Alusil Bores) | 70k - 130k miles | Rough idle, misfires, low compression (cylinders 5/1), blue smoke | $10,000 - $20,000 (Long Block Replacement) | Critical |
| Timing Chain Tensioners (Hydraulic) | 60k - 120k miles | Cold-start rattle (3-5 seconds), chain noise, cam phaser codes | $1,500 - $6,000 (Check Valves to Full Chain Kit) | High |
| Turbo Coolant Lines (Plastic) | 60k - 100k miles | Coolant loss, pink residue around turbos, overheating | $800 - $1,800 | High |
| Oil Cooler / Oil-Coolant Mixing | 80k - 120k miles | Milky coolant, metallic debris in oil filter, coolant loss warning | $1,500 - $3,000 (Cooler) / $10,000 - $20,000 (Long Block if Contaminated) | Critical |
| Oil Leaks (Timing Cover, Cam Plugs) | 70k - 100k miles | Oil dripping from front/top of engine | $500 - $1,500 | Medium |
| Carbon Buildup (Direct Injection) | 50k - 80k miles | Rough idle, hesitation, misfires | $600 - $1,200 (Walnut Blasting) | Medium |
| Ignition Coils | 40k - 60k miles | Misfire, check engine light | $400 - $800 (Set of 8) | Medium |
| High-Pressure Fuel Pump | 70k - 100k miles | Long crank, rough idle, fuel pressure codes | $800 - $1,500 | Medium |
Reliability Verdict
The Mercedes M278 4.7L biturbo V8 is fundamentally strong but has two catastrophic design flaws: Alusil cylinder scoring (especially cylinders 5/1) and timing chain tensioners that bleed down at cold start. If you buy a 2015+ model with documented tensioner update and compression test, it's a solid engine. If not, budget for a $20,000 long block replacement.
Mercedes M278 V8 Reliability: The $15,000 Used Sedan with a $20,000 Engine Replacement
The Mercedes M278 4.7L biturbo V8 is a 402-455hp hot-V twin-turbo masterpiece that powers the S550, E550, CLS550, and GL550. It is one of the smoothest, most refined V8s ever built.
But there’s a problem: Mercedes used Alusil cylinder bores that score, and hydraulic timing chain tensioners that bleed down at cold start. These two flaws can turn a $20,000 used S550 into a $40,000 repair nightmare.
This guide breaks down exactly what fails, when it fails, and how to buy an M278 without regret.
1. The M278 Engine Family: Architecture & Applications
The M278 is part of Mercedes’ modular V8 family, a hot-V twin-turbo design with turbos nestled in the V of the engine.
Key Specifications
- Displacement: 4.7L (4,663cc)
- Configuration: 90° V8, DOHC, 4 valves per cylinder
- Turbo Layout: Hot-V (turbos in the V of the engine)
- Bore Material: Alusil (aluminum-silicon alloy)
- Fuel Delivery: Direct Injection (FSI)
- Power Output:
- Standard: 402-435hp
- BiTurbo (later models): 449-455hp
Applications
- W222 S550 (2014-2017): 449hp flagship sedan
- W212 E550 (2012-2016): 402hp sport sedan
- C218 CLS550 (2012-2017): 402-449hp four-door coupe
- X166 GL550 (2013-2016): 429hp luxury SUV
- W166 ML550 (2012-2015): 402hp mid-size SUV
The early models (2011-2013) are the highest risk. The 2015+ models have improved tensioners but are not immune to cylinder scoring.
2. The Existential Risk: Cylinder Scoring (Alusil Bores)
Cylinder scoring is the single biggest financial risk of owning an M278.
What is Alusil?
Alusil is an aluminum-silicon alloy used for cylinder liners. It is lightweight and thermally efficient, but it has a critical weakness: the bore surface can degrade under high heat and inadequate lubrication.
How Scoring Happens
- Long Oil Change Intervals: Mercedes’ original 10,000-mile oil change intervals allow oil to break down, reducing lubrication.
- High Intake Charge Temps: The turbos sit in the V of the engine, creating chronically high intake temperatures that increase piston crown and ring temperature, thinning the oil film.
- Piston Ring Issues: Some techs suspect marginal oil control ring design plus Alusil wear.
- Bore Surface Degradation: The Alusil surface degrades, allowing metal-to-metal contact between the piston and cylinder wall.
Which Cylinders Fail?
Cylinders 5 and 1 are the most common low-compression cylinders, with visible scoring on teardown.
Symptoms
- Rough Idle: Especially when cold.
- Misfires: Specific cylinders (5 or 1) fail due to low compression.
- Blue Smoke: On startup or overrun.
- Fouled Spark Plugs: Oil burning causes carbon buildup on plugs.
Diagnosis
A compression test or leak-down test is the only way to confirm scoring. Healthy cylinders should show 150+ PSI. Scored cylinders will show 90-120 PSI.
Repair Cost
- Long Block Replacement: $10,000 - $20,000 (Mercedes does not sleeve these blocks in normal dealer practice).
[!WARNING] Cylinder scoring is NOT covered by warranty. Mercedes considers it a “maintenance issue” rather than a manufacturing defect, despite the widespread nature of the problem.
3. Timing Chain Tensioner Failures
The M278 has a primary chain plus two secondary chains, each with its own hydraulic tensioner (total of three tensioners).
The Problem
Early M278s (2011-2013) have hydraulic tensioners that bleed down when parked and refill too slowly at startup. This causes a cold-start rattle lasting 3-5 seconds.
Root Cause
The tensioners lack an internal check valve in the oil feed, allowing oil to drain back when the engine is off. This leaves slack in the chain at first crank, accelerating guide wear and chain stretch.
Symptoms
- Cold-Start Rattle: A loud rattling noise for 3-5 seconds on cold starts.
- Cam Phaser Codes: P0016, P0017 (correlation codes).
- Stretched Chains: If ignored, the chains stretch and can cause timing errors.
The Fix
Mercedes issued a TSB describing installation of press-in check valves in the cylinder head oil galleries feeding the tensioners. Updated tensioner part numbers (A 278 050 23 00 / 24 00) became standard from engine no. 278 9xx 30 035186 (roughly MY2014).
Repair Cost
- Check Valves + Updated Tensioners: $1,500 - $2,500 at independents.
- Full Chain Kit (Primary + Secondary Chains, Guides, Tensioners, Cam Phasers): $3,500 - $6,000.
[!TIP] Buying Advice: If you hear a cold-start rattle lasting more than 1 second, walk away. The tensioner update has not been done, and you’re looking at a $2,500+ repair.
4. Turbo Coolant Line Failures
The M278 uses plastic turbocharger coolant lines and several plastic fittings around the turbos. These crack and leak as the plastic becomes brittle with age and heat.
Symptoms
- Slow Coolant Loss: The coolant level drops over time.
- Pink/White Residue: Dried coolant around the rear or top-side turbo coolant pipes.
- Dripping onto Transmission: Coolant drips onto the transmission or underside.
- Overheating: If ignored, the engine can overheat.
Repair Cost
- Turbo Coolant Line Replacement: $800 - $1,800 (depending on chassis and whether the engine needs to be partially lowered).
5. Oil-Coolant Mixing (Oil Cooler Failure)
At high mileage, the engine oil cooler or oil-to-coolant interface seals can fail, allowing oil and coolant to mix.
Symptoms
- Milky Coolant: The coolant in the expansion tank looks milky or contaminated.
- Metallic Debris in Oil Filter: The oil filter contains metallic debris.
- Coolant Loss Warning: The cluster shows a coolant loss warning.
Consequences
If run like this, the contaminated oil can destroy bearings and require a full long block replacement ($10,000 - $20,000+).
Repair Cost
- Oil Cooler Replacement (if caught early): $1,500 - $3,000 (including cooler, seals, and extensive flushing).
- Long Block Replacement (if bearings are damaged): $10,000 - $20,000+.
6. Maintenance Schedule: The “Survival Guide”
If you want your M278 to survive past 100,000 miles, you must follow an enthusiast maintenance schedule.
| Service | Mercedes Interval | Enthusiast Interval | Cost (Indie) |
|---|---|---|---|
| Engine Oil | 10,000 miles | 5,000 - 7,500 miles | $150 |
| Compression Test | Never | 70,000 miles | $300 |
| Timing Chain Tensioners | Never | 60,000 miles (if rattle present) | $2,000 |
| Turbo Coolant Lines | Never | 80,000 miles | $1,200 |
| Spark Plugs | 60,000 miles | 40,000 miles | $300 |
| Ignition Coils | As needed | 60,000 miles | $600 |
| Carbon Cleaning | Never | 60,000 miles | $800 |
| Coolant Flush | 60,000 miles | 40,000 miles | $200 |
| Transmission Fluid | Lifetime | 60,000 miles | $400 |
[!IMPORTANT] Oil Change Interval is Critical: The single most effective way to prevent cylinder scoring is to change oil every 5,000 - 7,500 miles with high-quality synthetic.
7. Early vs Revised Engines (2011-2013 vs 2015+)
There are important running changes within the M278 family.
Early M278 (2011-2013)
- Risk Level: HIGH
- Why: No check valves in tensioners, higher cylinder scoring incidence.
- Verdict: Buyable, but only with compression test and tensioner update verified.
Revised M278 (2015+)
- Risk Level: MODERATE
- Why: Updated tensioners with check valves, improved coolant/oil sealing.
- Verdict: The smart buy. Still has cylinder scoring risk, but lower tensioner failure rate.
8. Mercedes M278 vs BMW N63 vs Audi 4.0T vs Porsche 4.8L
How does the Mercedes M278 stack up against its German rivals?
| Feature | Mercedes M278 | BMW N63 | Audi 4.0T | Porsche 4.8L |
|---|---|---|---|---|
| Major Flaw | Cylinder Scoring | Valve Stem Seals | Turbo Oil Screen | Bore Scoring (Alusil) |
| Repair Cost | $10k - $20k | $5k - $9k | $5k - $10k | $10k - $20k |
| Frequency | Moderate (70-130k mi) | High (60-90k mi) | Moderate (40-80k mi) | Moderate (70-100k mi) |
| Preventability | Partially (oil changes) | No | Yes (screen service) | Partially (oil changes) |
| Longevity | Good (if no scoring) | Poor (OG N63) | Good (if recall done) | Good (if no scoring) |
Verdict: The Audi 4.0T (with recall completed) is the safest long-term bet. The Mercedes M278 and Porsche 4.8L are in the middle—both have Alusil scoring risk. The BMW N63 is the riskiest.
9. 100,000-Mile Ownership Cost Projection
Here is a realistic budget for owning an M278 from 50,000 to 150,000 miles:
| Mileage | Expected Repairs | Cost |
|---|---|---|
| 50k - 60k | Timing chain tensioner check valves | $2,000 |
| 60k - 70k | Carbon cleaning, ignition coils | $1,400 |
| 70k - 80k | Compression test, turbo coolant lines | $1,500 |
| 80k - 100k | Oil cooler seals, spark plugs | $2,000 |
| 100k - 120k | Transmission service, brakes | $2,500 |
| 120k - 150k | Injectors, misc leaks | $2,000 |
Total (50k - 150k miles): $11,400
Annual Average: $1,140/year (excluding routine oil changes and tires).
[!CAUTION] If cylinder scoring occurs, add $10,000 - $20,000 to the total.
10. Buying Advice: How to Buy an M278 Without Regret
If you want an S550 or E550 and MUST have the M278:
- Compression Test: Demand a compression test. All cylinders should show 150+ PSI. If cylinder 5 or 1 is below 140 PSI, walk away.
- Cold-Start Test: Start the car from cold. If you hear a rattle lasting more than 1 second, the tensioner update has not been done.
- Turbo Coolant Lines: Ask if the plastic turbo coolant lines have been replaced. If not, budget $1,500.
- Avoid 2011-2013 Models: The early cars are the highest risk.
- Buy a 2015+ Model if Possible: The revised models have improved tensioners and sealing.
11. Conclusion
The Mercedes M278 V8 is a tragedy of engineering. It performs brilliantly, sounds incredible, and offers 449hp in the S550. But the Alusil cylinder scoring and timing chain tensioner bleed-down are catastrophic flaws that can turn a dream car into a financial nightmare.
Owning one is a calculated risk. If you buy a car with a clean compression test, no cold-start rattle, and a commitment to 5,000-mile oil changes, it can be a rewarding experience. If you ignore the warning signs, it will bankrupt you.
Related Guides:
- Mercedes S550 M278 Reliability
- Mercedes E550 Engine Problems
- Mercedes CLS550 Common Issues
- Mercedes GL550 M278 Issues
- BMW N63 vs Mercedes M278 Comparison
- Audi 4.0T vs Mercedes M278 Comparison
- Cylinder Scoring Guide
- Timing Chain Tensioner Failure
- Turbo Coolant Leak
- Oil-Coolant Mixing
- BMW N63 Comparison
- Audi 4.0T Comparison
- Porsche 4.8L Comparison
Expert Buying Advice
Only buy an M278 with: 1) Compression/leak-down test showing all cylinders above 150 PSI, 2) No cold-start rattle (tensioner update completed), 3) Turbo coolant lines replaced, 4) 2015+ model year preferred. Budget $4,000/year for maintenance.