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Mercedes GL550 M278 Issues: The Luxury SUV Reliability Guide

Sun Feb 15 2026
Reliability Score: 63 /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 - $22,000 (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 - $1,800 High
Oil Cooler / Oil-Coolant Mixing 80k - 120k miles Milky coolant, metallic debris in oil $2,000 - $3,500 (Cooler) / $15,000+ (Long Block) Critical
Transfer Case Leak 70k - 100k miles Oil dripping from transfer case $800 - $1,500 Medium

Reliability Verdict

The GL550 (X166) with M278 V8 is a 429hp luxury SUV 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 family hauler. If not, you're gambling with a $22,000 long block replacement plus $6,000 Airmatic repair.

Mercedes GL550 M278 Issues: The $22,000 Used SUV with a $28,000 Repair Bill

The Mercedes GL550 (X166) with the M278 4.7L biturbo V8 is a 429hp luxury SUV that can tow 7,500 lbs and seat seven in comfort.

But on the used market, it’s a financial trap. You can buy a $90,000 GL550 for $22,000. Why? Because the cylinder bores score, the timing chain tensioners bleed down, and the Airmatic suspension leaks.

This guide breaks down exactly what fails on the X166 GL550 with M278 V8.

1. GL550 X166: Early vs Late Models

The GL550 was produced from 2013-2016 before being renamed GLS550. You must understand the difference between early and late models.

Early X166 GL550 (2013-2014)

  • Engine: M278 4.7L biturbo V8 (429hp)
  • Risk Level: HIGH
  • Why: Early timing chain tensioners, higher cylinder scoring incidence.
  • Verdict: Buyable, but only with compression test and tensioner update verified.

Late X166 GL550 (2015-2016)

  • Engine: M278 4.7L biturbo V8 (429hp)
  • 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 GL550 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 - $22,000 for long block replacement (higher on GL due to SUV 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. Airmatic Suspension

The GL550 uses Airmatic air suspension. The air springs dry rot and leak.

  • Symptom: Car sagging on one corner, “Suspension Fault” warning.
  • Risk: The rubber air bags dry rot and leak, especially in cold climates.
  • Cost: $1,500 - $2,500 per corner (OEM). Arnott aftermarket struts are $800/corner.

C. Transfer Case Leak

The GL550 is 4MATIC (AWD). The transfer case can develop leaks.

  • Symptom: Oil dripping from transfer case, whining noise.
  • Mileage: 70,000 - 100,000 miles.
  • Fix: Transfer case seal replacement or rebuild.
  • Cost: $800 - $1,500.

3. Maintenance Budget

Owning a GL550 is not like owning a GLE. It is a luxury-SUV-level maintenance schedule.

  • Oil Changes: Every 5,000 miles (Mandatory). Mercedes’ 10,000-mile interval will accelerate cylinder scoring.
  • Tires: Rear tires last 20,000-25,000 miles. Expect $1,800/set.
  • Brakes: Large rotors. Expect $2,000/axle.

Annual Expectation: $5,000 - $7,000 in non-routine repairs.

4. The “Bargain” Trap

Scenario: You see a 2014 Mercedes GL550 with 72k miles for $22,000. It looks clean.

Reality: It likely needs:

  1. Compression test verification (if scoring: $18,000)
  2. Timing chain tensioner update ($2,500)
  3. Turbo coolant lines ($1,500)
  4. Airmatic struts (2 corners: $4,000)

Total Immediate Cost: $8,000 (or $26,000 if cylinder scoring has occurred).

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

5. Should You Buy One?

YES, IF:

  1. You are looking at a 2015+ model.
  2. You can afford a $6,000/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 $22,000 long block replacement or $6,000 Airmatic repair.

6. Verdict

The Mercedes GL550 (X166) with M278 V8 is a luxury SUV bargain if you know what you’re doing. It offers 429hp, a glorious exhaust note, and the prestige of the three-pointed star for the price of a used Camry.

But it requires a “luxury SUV” 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.

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Expert Buying Advice

Only buy a GL550 M278 with: 1) Compression test (all cylinders 150+ PSI), 2) No cold-start rattle, 3) Airmatic function verified, 4) 2015+ model year preferred. Budget $5,000/year for maintenance.