The W140 S500 with M119 V8 is a magnificent over-engineered sedan that suffers from a catastrophic design flaw: biodegradable engine wiring harnesses that fail predictably, plus the typical transmission cooler and mount issues common to all W140s. When the wiring goes, you'll chase phantom issues for weeks before realizing the insulation has turned to sticky tar.
Biodegradable Wiring Harness Failure
Common · high severityTypical onset: 80,000-150,000 mi or 20-30 years age
Symptoms: Intermittent no-start or stalling, Check engine light with multiple random codes, Rough idle or misfires that move between cylinders, Sticky tar-like residue on wiring insulation, Corroded connectors and exposed copper wire
Fix: Complete engine harness replacement requires 12-16 hours labor. Must remove intake manifold and accessories for access. Aftermarket harnesses available, OEM discontinued. Cannot be repaired reliably—full replacement only solution.
Estimated cost: $2,000-3,500
M119 Engine Head Gasket Failure
Occasional · high severityTypical onset: 120,000-180,000 mi
Symptoms: White smoke from exhaust on cold start, Coolant loss with no visible leaks, Oil in coolant or coolant in oil (milky dipstick), Overheating under load, Rough running after sitting overnight
Fix: Both heads must come off—this is not a one-side job on the M119. 18-24 hours labor including machine shop time for head resurfacing. Common to find warped heads. Many shops recommend full timing chain, guides, and tensioner replacement while apart. Often combined with wiring harness if not already done.
Estimated cost: $4,500-7,000
Transmission Oil Cooler Line Failure
Common · high severityTypical onset: 60,000-120,000 mi
Symptoms: Transmission fluid puddle under front of vehicle, Slipping or delayed engagement when hot, Burnt transmission fluid smell, Rapid fluid loss leading to no drive, Lines corroded at crimped fittings or radiator connections
Fix: Steel lines rust through at crimp points or where they connect to radiator-mounted cooler. Requires custom line fabrication or expensive OEM parts. 3-5 hours labor including fluid flush. Check auxiliary electric cooler if equipped—fans fail. Replace both lines preventively when one fails.
Estimated cost: $800-1,400
Transmission Mount Collapse (Rear Mount)
Common · medium severityTypical onset: 80,000-140,000 mi
Symptoms: Clunk when shifting from Park to Drive or Reverse, Vibration at idle in gear, Excessive driveline movement visible under throttle, Transmission tunnel heat after highway driving
Fix: Rear transmission mount is fluid-filled and fails from age and heat. Front mount less common but check both. 2-3 hours labor, requires transmission support and subframe access. OEM Lemforder or Corteco recommended—cheap parts fail quickly.
Estimated cost: $400-700
M119 Valve Cover Gasket and Spark Plug Tube Seal Leaks
Common · low severityTypical onset: 60,000-100,000 mi
Symptoms: Oil pooling in spark plug wells causing misfires, Burning oil smell from engine bay, Oil seepage visible on sides of valve covers, Smoke from exhaust manifolds after sitting
Fix: Both valve covers must be resealed. Spark plug tube seals often overlooked and cause oil in plug wells. 4-6 hours labor for both sides. Use OEM gaskets only—aftermarket leak within months. Replace spark plugs while apart—oil-fouled plugs won't recover.
Estimated cost: $800-1,200
Self-Leveling Suspension (SLS) Accumulator and Line Failure
Occasional · medium severityTypical onset: 90,000-150,000 mi
Symptoms: Rear sags overnight or when parked, Suspension pump runs constantly, Firm or harsh ride quality, Warning light on dash, Hissing sound from rear suspension area
Fix: Accumulators lose pressure, lines crack. System uses mineral oil not ATF. 2-4 hours labor per accumulator, more for lines. Some owners delete system and convert to coils—not recommended as it affects handling. System requires specialized bleeding procedure.
Estimated cost: $1,200-2,000
Fuel Distributor and System Seals (CIS-E System)
Occasional · medium severityTypical onset: 100,000-160,000 mi
Symptoms: Hard starting when hot, Fuel smell in engine bay, Rough idle with black smoke, Poor fuel economy suddenly worsening, Visible fuel weeping from distributor or injector lines
Fix: CIS-E mechanical injection uses rubber seals that age out. Distributor rebuild or replacement required. Injectors can leak at seals. 4-8 hours labor depending on how deep you go. This is old-school tech—many younger techs won't touch it. Fuel pressure must be tested with analog gauge.
Estimated cost: $1,000-2,200
Buy only if wiring harness is already done, you have a trusted independent W140 specialist nearby, and you're comfortable with $3-5k annual maintenance—otherwise this magnificent dinosaur will bankrupt you with its complexity.
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.