The W140 S320 with M104 inline-six is generally robust, but by now most survivors have 150k+ miles and face age-related failures in wiring harnesses, suspension hydraulics, and biodegradable engine wiring that can strand you. The transmission cooler and harness issues are the real gotchas.
Biodegradable Engine Wiring Harness Failure
Common · high severityTypical onset: 100,000-200,000 mi or 20+ years regardless of miles
Symptoms: Intermittent misfires, rough idle, or no-start conditions, Check engine light with random cylinder misfire codes, Crumbling insulation visible on ignition coil and injector wiring, Fuel smell from cracked fuel injector wiring
Fix: Complete engine harness replacement, 8-12 hours labor. Mercedes used soy-based insulation that degrades. Aftermarket harnesses available but OE-quality matters here. Must be done eventually on every survivor.
Estimated cost: $1,200-2,200
Transmission Oil Cooler Line Failure
Common · high severityTypical onset: 120,000-180,000 mi
Symptoms: Transmission fluid leaking onto exhaust, visible smoke, Sudden loss of transmission fluid, slipping or no movement, Burnt transmission fluid smell, Fluid pooling under center of vehicle
Fix: Replace metal cooler lines from transmission to radiator, 3-5 hours. Lines rust through where they run along frame rails. If ignored, you'll destroy the 722.6 transmission which is a $4k+ replacement. Preventive replacement recommended on high-milers.
Estimated cost: $600-1,100
M104 Head Gasket Failure (Both Banks)
Occasional · high severityTypical onset: 150,000-250,000 mi
Symptoms: White smoke from exhaust on cold start, Coolant loss with no visible external leaks, Overheating under load or in traffic, Milky oil on dipstick or oil cap (advanced failure), Rough idle and loss of power
Fix: Both head gaskets, resurface heads if warped, 16-22 hours labor. M104 is an interference engine, so do timing chain components and tensioner while in there. Budget expands fast if heads need machining or valve work. Some shops quote engine-out for access.
Estimated cost: $3,500-6,000
Automatic Transmission Mount Collapse
Common · medium severityTypical onset: 80,000-140,000 mi
Symptoms: Clunk or jolt when shifting from park to drive or reverse, Vibration at idle that smooths out under acceleration, Visible sagging of transmission tailshaft, Shifter feels sloppy or delayed engagement
Fix: Replace rear transmission mount, 2-3 hours labor. Rubber deteriorates and tears. While under there, inspect front engine mounts and conductor plate/13-pin connector inside trans pan (common 722.6 electrical issue). Many techs do mounts and fluid service together.
Estimated cost: $400-700
Hydraulic Self-Leveling Suspension (SLS) Failure
Common · medium severityTypical onset: 100,000-180,000 mi or age-related
Symptoms: Rear suspension sags overnight or after sitting, SLS warning light on dash, Uneven ride height side-to-side, Whining pump noise from right rear on startup, Hydraulic fluid leaks at accumulators or lines
Fix: Rear accumulators, hydraulic lines, or pump. Accumulators are $300-500 each plus 3-4 hours. Pump is 4-6 hours. System uses mineral oil hydraulic fluid. Many owners convert to coil springs ($800-1,200) to eliminate the system permanently. SLS shares fluid with brakes—don't ignore leaks.
Estimated cost: $1,200-2,500
Crankshaft Position Sensor Failure
Occasional · high severityTypical onset: 120,000-200,000 mi
Symptoms: No-start condition, engine cranks but won't fire, Intermittent stalling at operating temperature, Tachometer drops to zero while driving, Check engine light with P0320 code
Fix: Replace crankshaft position sensor at front of engine, 1.5-2.5 hours. Sensor fails hot, so car may start when cold then die when warmed up. Keep a spare in the trunk on high-mileage cars—it's cheap insurance against a tow.
Estimated cost: $250-450
Fuel System Contamination from Tank Rust
Occasional · medium severityTypical onset: Age-related, 20+ years
Symptoms: Repeated fuel pump failures (less than 50k miles between pumps), Rough running, hesitation, or stalling after refueling, Fuel filter clogs quickly, turns brown/orange, Check engine light with lean codes
Fix: Steel fuel tank rusts internally on aged cars, especially if stored or driven infrequently. Requires tank removal, cleaning or replacement, new pump, filter, lines inspected. 8-12 hours labor. If you're replacing a second fuel pump in under 80k miles, suspect the tank.
Estimated cost: $1,800-3,200
Window Regulator Failure
Common · low severityTypical onset: 90,000-150,000 mi
Symptoms: Window drops into door or moves slowly, Clicking or grinding noise when operating window, Window tilts or jams in track, One-touch auto feature stops working
Fix: Replace window regulator assembly, 2-3 hours per door. Plastic clips and metal rails fatigue. Driver's door fails first. Aftermarket regulators are hit-or-miss; genuine or OEM-quality recommended. Budget for multiple doors on high-milers.
Estimated cost: $500-800 per door
Buy one if the harness and transmission cooler lines are already done and you have records of fluid services; otherwise, budget $3k-5k in deferred maintenance within the first year. Solid platform once sorted, but these are 25+ year-old flagship luxury cars—parts and labor aren't Civic money.
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.