The 2009 S63 AMG with the naturally-aspirated 6.2L M156 V8 is a high-performance luxury sedan with notorious engine self-destruction issues and typical high-end Mercedes transmission and electronics concerns. The M156 engine's head bolt and bore wear problems can turn catastrophic, making pre-purchase inspection and maintenance history absolutely critical.
M156 Engine Head Bolt Failure and Cylinder Bore Wear
Common · high severityTypical onset: 60,000-100,000 mi
Symptoms: coolant consumption without external leaks, white smoke from exhaust on cold start, misfires and rough idle, coolant in oil or oil in coolant, catastrophic overheating
Fix: M156 head bolts stretch and allow coolant seepage into cylinders, accelerating bore scoring. Requires both cylinder heads removed, new head bolts, bore inspection, and often a complete engine rebuild or replacement with updated sleeves. 40-60 hours labor for rebuild, 20-25 hours for head bolt replacement if caught early and bores are acceptable.
Estimated cost: $18,000-35,000
Connecting Rod Bearing Failure (M156)
Occasional · high severityTypical onset: 70,000-120,000 mi
Symptoms: knocking noise from engine bay especially when cold, metallic rattling at idle, oil pressure warning light, sudden catastrophic engine failure
Fix: Undersized rod bearings from factory wear prematurely, especially with extended oil change intervals or spirited driving. Requires complete engine disassembly, new bearings, crankshaft inspection/machining. Many shops recommend full short block replacement. 35-50 hours labor.
Estimated cost: $15,000-28,000
7-Speed Automatic Transmission Oil Cooler and Conductor Plate Failure
Common · medium severityTypical onset: 80,000-130,000 mi
Symptoms: harsh shifting or delayed engagement, transmission slipping between gears, check engine light with transmission codes, transmission overheating warnings, pink fluid under car from cooler leak
Fix: The 722.9 transmission's internal oil cooler develops leaks mixing coolant and ATF, destroying the transmission if not caught early. Conductor plate (valve body electronics) also fails causing erratic shifts. Requires transmission removal, cooler replacement, new fluid, often conductor plate. 12-16 hours labor.
Estimated cost: $4,500-7,500
Airmatic Air Suspension Compressor and Strut Failure
Common · medium severityTypical onset: 70,000-110,000 mi
Symptoms: vehicle sitting low on one or more corners, suspension warning light, compressor running constantly, bouncy or harsh ride quality, compressor noise from front or rear
Fix: Air struts develop leaks at seals, compressor wears out from overwork. Each strut 2-3 hours, compressor 3-4 hours. Expect to do multiple corners if one fails. Many owners convert to coilovers ($3,000-5,000) rather than ongoing air suspension repairs.
Estimated cost: $1,800-3,200 per strut, $2,500-3,500 compressor
Front Engine Mounts Collapsing
Common · low severityTypical onset: 60,000-90,000 mi
Symptoms: clunking when accelerating or decelerating, excessive engine movement visible from engine bay, vibration at idle in drive, harsh shifts felt through cabin
Fix: Hydraulic engine mounts fail allowing excessive drivetrain movement. Front mounts most common. Requires lifting engine slightly, 3-4 hours per side.
Estimated cost: $1,200-2,000
ABC (Active Body Control) Hydraulic System Leaks
Occasional · medium severityTypical onset: 80,000-120,000 mi
Symptoms: ABC warning light, vehicle sagging, hydraulic fluid puddles (green fluid), loss of ride quality adjustment, harsh ride or excessive body roll
Fix: If equipped with ABC instead of Airmatic, hydraulic lines, pulsation dampers, and pump develop leaks. Hoses 2-4 hours, pump 6-8 hours, tandem pump 8-12 hours. ABC repairs are extremely expensive.
Estimated cost: $2,000-8,000 depending on component
Rear Differential Carrier Bearing and Seal Leaks
Occasional · low severityTypical onset: 90,000-140,000 mi
Symptoms: whining noise from rear on acceleration, clunking from rear on direction changes, gear oil leak at rear differential, vibration at highway speeds
Fix: Carrier bearings wear and seals leak on rear differential. Requires differential removal, bearing replacement, seal service, new fluid. 8-12 hours labor.
Estimated cost: $2,500-4,200
Only buy if you have $10,000-15,000 set aside for the inevitable M156 engine work or can verify recent rebuild with upgraded parts—otherwise this is a financial grenade with the pin pulled.
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.