The 2018 GLS63 AMG with the M157 5.5L twin-turbo V8 is a high-performance SUV that suffers from catastrophic engine failure issues related to defective piston weld rings, alongside typical high-mileage transmission and cooling concerns that can cost as much as a used economy car to repair.
M157 Engine Piston Ring Failure (Catastrophic)
Common · high severity
Typical onset: 40,000-80,000 mi
Symptoms: Excessive oil consumption (1 qt per 500-1000 miles), Blue smoke on startup or acceleration, Cylinder misfires and rough idle, Low compression on multiple cylinders, Complete engine seizure in severe cases
Fix: The M157 has a known defect where piston skirt weld rings separate, causing catastrophic damage. Fix requires complete engine rebuild with updated pistons or short block replacement. 40-60 hours labor. Mercedes extended warranty to 10yr/155k mi for some VINs, but many 2018s fall outside coverage window. Verify warranty status before purchase.
Estimated cost: $25,000-45,000
Transmission Oil Cooler and Line Leaks
Common · medium severity
Typical onset: 60,000-90,000 mi
Symptoms: Transmission fluid puddles under vehicle, Low transmission fluid warning light, Harsh shifting or slipping when fluid level drops, Visible fluid seepage at cooler connection points
Fix: The 7-speed AMG Speedshift transmission cooler and lines develop leaks at connection points and through-body corrosion. Requires cooler replacement, lines, and often full transmission fluid flush. 6-9 hours labor including fluid service.
Estimated cost: $2,200-3,800
Turbocharger Wastegate Actuator Failure
Occasional · high severity
Typical onset: 70,000-110,000 mi
Symptoms: Check engine light with P0299 (underboost) or P0234 (overboost) codes, Sudden loss of power and limp mode activation, Audible wastegate rattle at idle or low RPM, Overboosting causing rich conditions and potential engine damage
Fix: Wastegate actuators stick or fail, causing boost control issues. Turbos are integrated units, so actuator failure typically means complete turbocharger replacement per side. 12-16 hours labor for both turbos, 8-10 for single side. Engine-out access required.
Estimated cost: $6,500-9,500 per side, $12,000-17,000 both
Transmission Mount Collapse
Common · medium severity
Typical onset: 50,000-80,000 mi
Symptoms: Excessive drivetrain clunk on acceleration or deceleration, Vibration felt through chassis during shifts, Visible transmission sag when inspected on lift, Metallic knocking from transmission tunnel area
Fix: The transmission mount deteriorates from heat and stress of the high-torque AMG powertrain. Requires transmission support and mount replacement. Often accompanied by engine mount replacement for best results. 4-6 hours labor for transmission mount, add 3-4 hours if doing engine mounts simultaneously.
Symptoms: "Airmatic Visit Workshop" warning message, Vehicle sitting low at one or more corners, Compressor runs excessively or constantly, Rough ride quality or reduced damping effectiveness, Air leak hissing audible near wheels
Fix: Air struts develop leaks at bladder and compressor fails from overwork. Common to replace struts in pairs (front or rear). Compressor is separate job. 3-4 hours per strut, 2-3 hours for compressor. Valve block failures also occur but less common.
Estimated cost: $2,800-4,200 per axle pair, $1,800-2,500 compressor
Fuel Injector Carbon Buildup and Failure
Occasional · medium severity
Typical onset: 60,000-100,000 mi
Symptoms: Rough idle with cylinder-specific misfires, Poor fuel economy and reduced power, Check engine light with P030X misfire codes, Hard starting when engine is hot, Fuel smell from excessive injector leakage
Fix: Direct injection engines accumulate carbon on injector tips and injectors fail internally. Requires removal for cleaning or replacement. Often done with walnut blasting intake valves simultaneously. 8-12 hours labor for all injectors plus valve cleaning.
Estimated cost: $3,500-5,500
Owner tips
Verify engine warranty status for M157 piston issue before purchase - some VINs covered to 155k miles, others not at all
Change transmission fluid every 40k miles regardless of "lifetime" claim - prevents cooler clogs and extends life
Use only MB-approved 0W-40 oil and change every 5k miles maximum to reduce carbon buildup on direct injection system
Budget $3,000-5,000 annually for maintenance and repairs after warranty - these are $120k+ SUVs when new
Pre-purchase inspection must include compression test, leak-down test, and borescope inspection for piston condition
Only buy if you have comprehensive service records showing engine rebuild with updated pistons or absolute confirmation of extended warranty coverage - otherwise this is a $15k SUV with a $30k engine time bomb.
AI-assisted summary drawn from NHTSA recall data, our labor-times database, and platform knowledge. Not a substitute for a pre-purchase inspection on a specific vehicle.
Fitment notes: AGM battery required; located under hood on right side; high-performance V8 BiTurbo requires premium AGM specification
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Every control module on the 2018-2020 Mercedes-Benz GLS63 AMG — where it lives, replacement time, and what it takes to program a replacement. Modules marked dealer / factory tool won't work after a part swap alone — budget for programming.
Transmission Control Unit (TCU)2.5 hr R&Rsecurity gateway +1.0 hr▸ programming details
⚠️ Trailer hitch and lighting control. Requires trailer detection calibration.
Aftermarket tool coverage varies by software version and vehicle build — treat "aftermarket tool" rows as "usually possible" and verify against your tool maker's coverage list before promising a customer. Spot a wrong location or hour? Tell us — corrections ship fast here.
Size-standard part numbers — verify your connector type before buying. Rear blades are model-specific; check the package's vehicle list.
Fuel economy figures are EPA data via fueleconomy.gov (median across matching trims). Performance figures are compiled estimates for the 2018 Mercedes-Benz GLS63 AMG 5.5L V8 BiTurbo M157 and can vary by trim.
🔧 Database maintained under the daily editorial review of Chris Hackleman · Master Technician · 20+ years and Jeff Moore · Master Lexus & Toyota Mechanic · 20+ years.