The 2004 SLK320 R170 with the M112 V6 is a stylish roadster that suffers from serious engine durability issues related to block porosity and bearing failures, plus typical Mercedes transmission and hydraulic top concerns. When they go wrong, repair costs can exceed the car's value.
M112 Engine Block Porosity and Coolant-Oil Cross-Contamination
Common · high severity
Typical onset: 80,000-140,000 mi
Symptoms: Milky oil on dipstick or oil cap, Unexplained coolant loss with no external leaks, White smoke from exhaust on cold start, Overheating with clean cooling system, Coolant smell from oil filler
Fix: M112 blocks from this era suffer from micro-porosity allowing coolant into oil galleries. No head gasket leak—it's the block itself. Requires complete engine replacement or short block swap. 18-24 labor hours for R&R plus machining if salvageable core available. Most shops recommend used low-mileage engine swap due to parts availability.
Estimated cost: $4,500-8,000
Main and Rod Bearing Failure (M112 V6)
Occasional · high severity
Typical onset: 100,000-150,000 mi
Symptoms: Deep knocking from lower engine on cold start that may disappear when warm, Metallic rattling under acceleration, Low oil pressure warning intermittently or constantly, Metal shavings in oil during changes, Sudden catastrophic failure with no prior warning in some cases
Fix: The M112 has marginal oiling to mains and rods, especially with extended oil change intervals or low-quality oil. Bearing wear leads to knock and eventual spun bearing. Requires complete teardown, crank polishing or replacement, new bearings, plus address any scoring. 20-28 labor hours. Often totals the car given values.
Estimated cost: $5,000-9,500
5-Speed Automatic Transmission Conductor Plate and Solenoid Failures
Common · medium severity
Typical onset: 90,000-130,000 mi
Symptoms: Harsh 2-3 or 3-4 shifts, Limp mode with transmission fault codes, Slipping between gears under light throttle, No upshifts past 2nd or 3rd gear, Intermittent flaring during shifts
Fix: The 722.6 five-speed auto uses a conductor plate (wiring harness inside the valve body) that cracks from heat cycling. Solenoids also fail. Requires valve body removal, new conductor plate, 13-pin connector, solenoids as needed, fresh fluid and filter. 6-9 labor hours. Sometimes requires full transmission rebuild if internal damage occurred.
Estimated cost: $1,800-3,500
Transmission Oil Cooler Line Leaks and Cooler Failure
Common · medium severity
Typical onset: 70,000-110,000 mi
Symptoms: Red transmission fluid pooling under engine bay center, Transmission overheating warnings, Burnt transmission fluid smell, Low fluid level on dipstick, Lines visibly wet or corroded at radiator connections
Fix: Steel cooler lines rust through at the radiator, and the cooler itself can crack internally. External leaks are obvious; internal cooler failures mix coolant and ATF (destroys transmission). Replace lines, external cooler, and flush system. 3-5 labor hours for lines, more if radiator or transmission contaminated.
Estimated cost: $800-2,200
Hydraulic Convertible Top Pump and Cylinder Leaks
Occasional · medium severity
Symptoms: Top operates slowly or stops mid-cycle, Hydraulic fluid leaking in trunk near pump or cylinders, Top won't latch or unlatch electrically, Groaning or laboring sounds during top operation, Top control illuminated or flashing warning light
Fix: The vario-roof system uses hydraulic pump and cylinders that leak from aged seals or develop internal wear. Requires pump rebuild or replacement and cylinder seal kits. Top must be manually positioned and system bled after repair. 4-7 labor hours depending on which components fail. Fluid mess in trunk is diagnostic.
Estimated cost: $1,200-2,800
Crankshaft Position Sensor and Camshaft Position Sensor Failures
Occasional · high severity
Typical onset: 80,000-140,000 mi
Symptoms: No-start condition with cranking but no firing, Intermittent stalling when hot, Long crank time before starting, Check engine light with P0335 or P0340 codes, Engine dies while driving and restarts after cooling
Fix: Hall-effect sensors fail from heat exposure on the M112. Crank sensor is behind the starter (remove starter for access), cam sensors at cylinder head front. Diagnosis requires scanner to confirm codes. 1.5-3 labor hours depending on which sensor and access. Parts are cheap; labor varies by location.
Estimated cost: $300-700
Front Suspension Lower Control Arm Bushings and Ball Joints
Common · low severity
Typical onset: 60,000-100,000 mi
Symptoms: Clunking over bumps from front end, Steering wander or vague on-center feel, Uneven inside tire wear, Visible cracking or tearing of rubber bushings, Excessive play in ball joints during inspection
Fix: R170 front lower control arms use pressed bushings that wear and crack. Ball joints also fail. Mercedes typically replaces entire arms versus pressing bushings (faster, guaranteed fitment). Need alignment after. 3-4 labor hours for both sides plus alignment.
Estimated cost: $900-1,600
Owner tips
Use only Mercedes-approved 229.5 spec oil and change every 5,000 miles maximum—the M112 is unforgiving with extended intervals
Inspect for coolant-oil mixing at EVERY oil change; early detection can sometimes save the engine
Flush transmission fluid every 40,000 miles with genuine Mercedes fluid to extend conductor plate life
Check transmission cooler lines annually for rust; $50 in preventive line replacement beats a $4,000 transmission
Budget $1,500/year for unexpected repairs once past 100,000 miles; these are money pits when neglected
Only buy if under 80,000 miles with immaculate service records and you have a $5,000 emergency fund—the M112 engine is a ticking time bomb, and repair costs routinely exceed the car's $8,000-12,000 market value.
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: Battery located in trunk on right side; DIN standard H8 also compatible
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Every control module on the 2001-2004 Mercedes-Benz SLK320 R170 — 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.
📍 Under driver and passenger seats, mounted to seat frame rails
🔧 Star Diagnosis or Autel
⚠️ Controls power seat adjustment and memory. Basic adaptation usually sufficient. Separate modules for driver and passenger seats.
Bi-Xenon Control Unit (XCU)0.5 hr R&Rrelearn only +0.1 hr▸ programming details
📍 Behind each headlight assembly, integrated with ballast
🔧 Self-learning procedure
⚠️ Optional Xenon equipment. Auto-leveling calibration via ignition cycle procedure. Separate units per side.
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 2004 Mercedes-Benz SLK320 R170 3.2L V6 M112 and can vary by trim.
🔧 Database maintained under the daily editorial review of Chris Hackleman · Master Technician · 20+ years and Jeff Moore · Master Mechanic · 20+ years.