The W124 E420 with the M119 4.2L V8 is a solid, over-engineered Mercedes, but the engine has a catastrophic weak point: biodegradable wiring harnesses that disintegrate and nickel-silicon alloy cylinder liners prone to bore scoring under specific conditions. When these fail, you're looking at full engine rebuilds.
Engine Wiring Harness Deterioration
Common · high severityTypical onset: any mileage, age-dependent (25+ years)
Symptoms: rough idle or misfires, check engine light with multiple cylinder misfire codes, engine won't start or runs on fewer than 8 cylinders, visible cracked or crumbling insulation on engine bay wiring
Fix: Complete engine harness replacement, 8-12 hours labor. Mercedes used biodegradable insulation that turns to dust. You must replace the entire harness, not patch it. Aftermarket harnesses available but OE-style replacement is safest.
Estimated cost: $1,200-2,200
Cylinder Bore Scoring and Piston Ring Failure (M119 Nikasil Issue)
Occasional · high severityTypical onset: 80,000-150,000 mi
Symptoms: excessive oil consumption (1 quart per 500-1000 miles), blue smoke on startup or under load, loss of compression in one or more cylinders, rattling noise from pistons with excessive clearance
Fix: Engine rebuild or short block replacement, 25-35 hours labor. Early M119 engines had Nikasil cylinder liners that score when exposed to high-sulfur fuel. Later blocks (post-1996 casting dates) have Alusil liners. Requires bore inspection, re-sleeving or block replacement, new pistons, rings, bearings.
Estimated cost: $8,000-15,000
Transmission Oil Cooler and Line Leaks
Common · medium severityTypical onset: 100,000-180,000 mi
Symptoms: transmission fluid puddles under vehicle, low transmission fluid level, harsh or delayed shifts when fluid is low, visible corrosion or seepage at cooler lines near radiator
Fix: Replace transmission oil cooler lines and/or external cooler, 2-4 hours labor. The steel lines rust through, especially in salt states. Cooler itself can develop pinhole leaks. Flush system and replace fluid after repair.
Estimated cost: $400-800
Transmission Mounts (Engine and Trans)
Common · medium severityTypical onset: 80,000-120,000 mi
Symptoms: clunk or thud when shifting from park to drive or reverse, excessive drivetrain vibration at idle in gear, visible sagging or torn rubber in mounts
Fix: Replace all motor and transmission mounts, 3-5 hours labor. The W124 has multiple hydraulic and rubber mounts that degrade. Do them all at once to avoid repeat labor.
Estimated cost: $600-1,200
Head Gasket Leaks (Both Banks)
Occasional · high severityTypical onset: 120,000-200,000 mi
Symptoms: external oil or coolant seepage at cylinder head mating surface, overheating or coolant loss with no external leaks, white smoke from exhaust, oil in coolant or vice versa in severe cases
Fix: Head gasket replacement both banks, 18-24 hours labor. V8 requires removal of intake manifolds, exhaust manifolds, and extensive disassembly. Machine shop inspection of head flatness recommended. Replace all head bolts, valve cover gaskets, coolant hoses while open.
Estimated cost: $4,500-7,000
Fuel System Issues (Filter, Pump, Accumulator)
Occasional · medium severityTypical onset: 100,000+ mi or neglected maintenance
Symptoms: hard starting, especially when hot, stumbling or hesitation under acceleration, engine stalling after sitting, whining noise from fuel tank area
Fix: Replace fuel filter (every 30k), fuel pump, and fuel accumulator if needed. Filter is 0.5 hours, pump is 2-3 hours (tank drop). The CIS fuel system uses an accumulator that maintains pressure; when it fails you get hot-start issues.
Estimated cost: $300-1,000
Rear Self-Leveling Suspension (SLS) Failure
Common · low severityTypical onset: any mileage, age-dependent
Symptoms: rear end sags when parked, uneven rear ride height, SLS warning light on dash, hissing from rear suspension when engine running
Fix: Replace SLS accumulators, lines, or pump. Accumulators are 1-2 hours each, pump is 2-3 hours. Many owners convert to coil springs to eliminate system entirely (cheaper long-term).
Estimated cost: $800-2,000 for repair, $400-600 for coil conversion
Buy only if the wiring harness has been replaced and you have documented proof the engine doesn't burn oil — otherwise you're one bore-scope away from a $12k engine rebuild on a $5k car.
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.