The W124 E420 with M119 V8 is a supremely engineered platform, but the all-alloy engine develops catastrophic issues due to a nickel-silicon liner wear problem that Mercedes never officially recalled. When maintained perfectly and driven gently, they can be reliable; driven hard or neglected, expect complete engine failure.
Nikasil Cylinder Bore Wear & Engine Failure
Common · high severityTypical onset: 80,000-150,000 mi
Symptoms: Excessive oil consumption (>1 qt per 1,000 mi), Cold-start misfires that clear after warmup, Blue smoke on deceleration, Loss of compression across multiple cylinders, Rough idle and loss of power
Fix: The M119 used Nikasil bore coating that deteriorates from high-sulfur fuel. Requires complete engine rebuild with Locasil or sleeved cylinders, new pistons, rings, bearings, gaskets. 40-60 hours labor depending on shop familiarity with M119 platform.
Estimated cost: $8,000-15,000
Wiring Harness Biodegradation
Common · medium severitySymptoms: Intermittent electrical gremlins (gauges, windows, locks), Check engine light with multiple random codes, No-start conditions that resolve mysteriously, Crumbling insulation revealing bare copper wires, Engine running issues due to sensor failures
Fix: Mercedes used soy-based insulation that biodegrades after 20-30 years. Engine harness replacement is 12-16 hours; chassis harness adds another 20+ hours if you do everything properly. Many owners do engine-only and chase chassis issues as they arise.
Estimated cost: $2,500-6,000
Transmission Oil Cooler Line Failure
Common · high severitySymptoms: Transmission fluid leaking near radiator, Transmission slipping after warmup, Pink or milky transmission fluid (coolant contamination), Overheating transmission, Complete transmission failure if coolant mixes with ATF
Fix: The hard lines corrode and fail, and if the internal cooler in the radiator ruptures, coolant cross-contaminates ATF and destroys the 722.6 five-speed. Preventive replacement of lines and external cooler install is 3-4 hours. If transmission is contaminated, add rebuild at 18-25 hours.
Estimated cost: $800-1,200 (lines only), $3,500-5,500 (with trans rebuild)
Self-Leveling Rear Suspension (SLS) Failure
Occasional · medium severityTypical onset: 100,000-180,000 mi
Symptoms: Rear end sags, especially when loaded, Hydraulic fluid leaks at accumulators or lines, Uneven ride height side-to-side, Warning light on dash, Car sits nose-high when parked
Fix: The hydraulic system uses accumulators and valve blocks that fail over time. Most shops convert to conventional springs and shocks (4-6 hours) rather than repair the SLS system. OEM repair with new spheres and lines is 8-12 hours.
Estimated cost: $600-1,200 (coil conversion), $2,000-3,500 (OEM repair)
Head Gasket Failure (Secondary to Nikasil)
Occasional · high severityTypical onset: 100,000-160,000 mi
Symptoms: Coolant loss with no external leaks, White smoke from exhaust, Overheating under load, Milky oil or oil in coolant, Misfires on specific cylinders
Fix: Often occurs after Nikasil wear progresses or if engine overheats. Both head gaskets on M119 V8 require engine-out or extensive disassembly in-car. 25-35 hours labor, and if you're this deep, smart money does full rebuild inspection. Heads may need resurfacing.
Estimated cost: $4,500-7,500
Transmission Mounts and Flex Disc Wear
Common · low severityTypical onset: 80,000-140,000 mi
Symptoms: Clunk when shifting from P to D or R, Vibration at idle in gear, Driveline shudder on acceleration, Excessive driveshaft movement visible underneath
Fix: Rubber transmission mount and flex disc (giubo) at driveshaft deteriorate. Straightforward replacement, 2-3 hours for both. Do them together as labor overlaps.
Estimated cost: $500-900
Buy only if you find one with documented engine rebuild using Locasil/sleeved block, fresh harness, and comprehensive service records—otherwise you're buying someone else's $15k problem.
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.