Luxury Cars Guide
đź“‹ In This Guide

Mercedes S550 M278 Reliability: The Flagship Gamble

Sun Feb 15 2026
Reliability Score: 64 /100

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) $15,000 - $25,000 (Long Block Replacement) Critical
Timing Chain Tensioners 60k - 120k miles Cold-start rattle (3-5 seconds) $2,000 - $6,000 High
Airmatic Suspension Struts 60k - 90k miles Car sagging, 'Suspension Fault' warning $1,500 - $2,500 per corner High
Turbo Coolant Lines 60k - 100k miles Coolant loss, pink residue around turbos $1,000 - $2,000 High
Magic Body Control (ABC) 70k - 100k miles Hydraulic leaks, pump noise, 'ABC Fault' $3,000 - $8,000 High
Oil Cooler / Oil-Coolant Mixing 80k - 120k miles Milky coolant, metallic debris in oil $2,000 - $3,000 (Cooler) / $15,000+ (Long Block) Critical

Reliability Verdict

The S550 (W222) with M278 V8 is a 449hp flagship with the same cylinder scoring and timing chain tensioner flaws as all M278 engines. If you buy a 2015+ model with compression test and no cold-start rattle, it's a fantastic luxury sedan. If not, you're gambling with a $25,000 long block replacement plus $8,000 ABC suspension repair.

Mercedes S550 M278 Reliability: The $25,000 Used Flagship with a $30,000 Repair Bill

The Mercedes S550 (W222) with the M278 4.7L biturbo V8 is the pinnacle of luxury sedans. 449hp, 0-60 in 4.8 seconds, and a cabin that makes a Bentley Flying Spur jealous.

But on the used market, it’s a financial trap. You can buy a $110,000 S550 for $28,000. Why? Because the cylinder bores score, the timing chain tensioners bleed down, and the ABC suspension leaks.

This guide breaks down exactly what fails on the W222 S550 with M278 V8.

1. The W222 S550: Two Generations

You must understand the difference between the early and late W222 models.

Early W222 S550 (2014-2015)

  • Engine: M278 4.7L biturbo V8 (449hp)
  • Risk Level: HIGH
  • Why: Early timing chain tensioners, higher cylinder scoring incidence.
  • Verdict: Buyable, but only with compression test and tensioner update verified.

Late W222 S550 (2016-2017)

  • Engine: M278 4.7L biturbo V8 (449hp)
  • Risk Level: MODERATE
  • Why: Updated tensioners with check valves, improved sealing.
  • Verdict: The smart buy. Still has cylinder scoring risk, but lower tensioner failure rate.

2. Common Failures & Costs

A. Cylinder Scoring (The Existential Risk)

The S550 shares the same Alusil cylinder scoring flaw as all M278 engines. See our Mercedes M278 V8 Pillar Page for the full technical breakdown.

  • Symptom: Rough idle, misfires (cylinders 5/1), blue smoke.
  • Test: Compression test. All cylinders should show 150+ PSI. If cylinder 5 or 1 is below 140 PSI, walk away.
  • Cost: $15,000 - $25,000 for long block replacement (higher on S-Class due to labor complexity).

[!WARNING] Critical: If the compression test shows low compression on cylinders 5 or 1, the car is a ticking time bomb. Walk away.

B. Timing Chain Tensioners

Early W222 S550s have hydraulic tensioners that bleed down at cold start, causing a 3-5 second rattle.

  • Symptom: Cold-start rattle, cam phaser codes.
  • Test: Start the car from cold. If you hear a rattle lasting more than 1 second, the tensioner update has not been done.
  • Cost: $2,000 - $6,000 (check valves to full chain kit).

C. Airmatic / ABC Suspension

The S550 uses either Airmatic (air suspension) or ABC (Active Body Control - hydraulic). Both fail.

  • Airmatic Symptoms: Car sagging on one corner, “Suspension Fault” warning.
  • ABC Symptoms: Hydraulic leaks, pump noise, “ABC Fault” warning.
  • Cost: $1,500 - $2,500 per corner (Airmatic) or $3,000 - $8,000 (ABC pump/accumulator).

D. Turbo Coolant Lines

The plastic turbo coolant lines crack and leak.

  • Symptom: Coolant loss, pink residue around turbos.
  • Cost: $1,000 - $2,000.

3. The “Bargain” Trap

Scenario: You see a 2015 Mercedes S550 with 75k miles for $28,000. It looks clean.

Reality: It likely needs:

  1. Compression test verification (if scoring: $20,000)
  2. Timing chain tensioner update ($2,500)
  3. Turbo coolant lines ($1,500)
  4. ABC suspension pump ($5,000)

Total Immediate Cost: $9,000 (or $29,000 if cylinder scoring has occurred).

You have effectively doubled the price of the car in the first month.

4. Maintenance Budget

Owning an S550 is not like owning an E-Class. It is a flagship-level maintenance schedule.

  • Oil Changes: Every 5,000 miles (Mandatory). Mercedes’ 10,000-mile interval will accelerate cylinder scoring.
  • Tires: Rear tires last 15,000-20,000 miles. Expect $2,000/set.
  • Brakes: Large rotors. Expect $2,500/axle.

Annual Expectation: $6,000 - $8,000 in non-routine repairs.

5. Should You Buy One?

YES, IF:

  1. You are looking at a 2016+ model.
  2. You can afford a $7,000/year maintenance budget.
  3. You have a trusted Mercedes specialist nearby.
  4. You get compression test verification and clean cold-start test.

NO, IF:

  1. You are stretching your budget to buy the car.
  2. You rely on the dealer for service.
  3. You need 100% reliable transportation.
  4. You cannot afford a potential $25,000 long block replacement or $8,000 ABC repair.

6. Verdict

The Mercedes S550 (W222) with M278 V8 is a flagship bargain if you know what you’re doing. It offers 449hp, a glorious exhaust note, and the prestige of the three-pointed star for the price of a used Camry.

But it requires a “flagship” budget to keep running. If you go in with open eyes and a full wallet, it’s a rewarding experience. If you treat it like a Toyota, it will bankrupt you.

Related Guides:

Expert Buying Advice

Only buy an S550 M278 with: 1) Compression test (all cylinders 150+ PSI), 2) No cold-start rattle, 3) Airmatic/ABC function verified, 4) 2015+ model year preferred. Budget $6,000/year for maintenance.