The W140 S420 with M119 V8 is a tank of a car—solid engineering, but engine bottom-end failures and transmission harness issues define ownership past 100k miles. When maintained, these run forever; when neglected, you're staring down a $10k engine rebuild.
M119 Engine Wiring Harness Biodegradation
Common · high severityTypical onset: 80,000-150,000 mi
Symptoms: rough idle or misfires, check engine light with multiple cylinder misfire codes, oil leaking onto harness from valve covers, brittle, cracking wire insulation
Fix: The biodegradable insulation on the engine harness disintegrates, causing shorts and misfires. Full harness replacement requires removing intake manifold and accessories. 12-16 hours labor, plus OEM or aftermarket harness.
Estimated cost: $2,800-4,500
M119 Valve Guide Wear and Oil Consumption
Occasional · medium severityTypical onset: 120,000-180,000 mi
Symptoms: blue smoke on cold start, burning a quart of oil every 500-1000 miles, fouled spark plugs, excessive crankcase pressure
Fix: Valve guides wear out, especially if oil changes were stretched. Head removal, machine work for new guides, valve job, new seals. 20-25 hours labor per bank if doing both heads.
Estimated cost: $4,000-7,000
M119 Bottom-End Failure (Rod/Main Bearings)
Occasional · high severityTypical onset: 150,000-200,000 mi
Symptoms: knocking or rattling from lower engine on cold start, metallic ticking that worsens with RPM, low oil pressure at idle, metal shavings in oil
Fix: Extended oil change intervals or low oil level destroys rod and main bearings. Requires full engine-out rebuild or short block replacement. 35-45 hours labor plus machine work, bearings, gaskets, seals. Some opt for used engine swap to save money.
Estimated cost: $8,000-14,000
722.6 Transmission Valve Body and Conductor Plate Failure
Common · high severityTypical onset: 100,000-150,000 mi
Symptoms: harsh or delayed shifts, limp mode (stuck in 2nd gear), slipping between gears, transmission fault codes for solenoids or pressure
Fix: The conductor plate (internal wiring harness) and valve body solenoids fail from heat and age. Transmission must come out or pan dropped for valve body R&R. 8-12 hours labor, remanufactured valve body or conductor plate replacement.
Estimated cost: $2,000-3,500
Front and Rear Suspension Self-Leveling System Leaks
Common · medium severityTypical onset: 80,000-130,000 mi
Symptoms: rear or front sags overnight, hydraulic fluid puddles under car, suspension warning light, compressor runs continuously
Fix: Hydraulic accumulators and lines age out, causing leaks. Common to replace front and rear accumulators, pressure lines, and check valve. 6-10 hours labor depending on how many components are shot.
Estimated cost: $1,800-3,200
Fuel Distributor and Injection System Issues
Occasional · medium severityTypical onset: 100,000-160,000 mi
Symptoms: hard starting when hot, stumbling or hesitation under load, rough idle after warm-up, fuel smell or visible leaks at distributor
Fix: The M119 uses CIS-E Motronic injection; fuel distributor O-rings harden and leak, causing fuel pressure loss. Distributor rebuild or replacement. 4-6 hours labor including fuel system depressurization and testing.
Estimated cost: $1,200-2,200
Biodegradable Transmission Wiring Harness
Common · high severityTypical onset: 90,000-140,000 mi
Symptoms: transmission stuck in limp mode, no gear display on dash, erratic shifting, transmission range sensor codes
Fix: Same biodegradable insulation issue as engine harness. External transmission harness from TCU to valve body disintegrates. Harness replacement takes 3-5 hours, accessed from underneath.
Estimated cost: $800-1,500
Buy one only if you have $5k-10k set aside for deferred maintenance or can wrench yourself—otherwise, find a pristine low-mile example with full records or walk away.
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.