The W140 S320 with M104 engine is mechanically solid but plagued by expensive age-related failures in wiring, hydraulics, and soft-mount systems. Engine harness biodegradation and transmission issues dominate the problem list after 100k miles.
Engine Wiring Harness Biodegradation
Common · high severityTypical onset: 80,000-150,000 mi
Symptoms: Random misfires, check engine light, Hard starting or stalling when warm, Erratic idle, limp mode engagement, Insulation crumbling off wires under hood
Fix: Complete engine harness replacement required. Mercedes used soy-based insulation that disintegrates. 8-12 hours labor for full harness swap including firewall connectors and injector wiring.
Estimated cost: $1,800-3,200
Transmission Valve Body and Conductor Plate Failure
Common · high severityTypical onset: 100,000-180,000 mi
Symptoms: Harsh or delayed shifts, especially 2-3 and 3-4, Limp mode (stuck in 2nd gear), Transmission slipping under load, Transmission fault codes stored
Fix: 722.6 five-speed transmission valve body rebuild or replacement with updated conductor plate. Often combined with full fluid flush and filter. 6-9 hours labor for pan drop, valve body R&R, and adaptation.
Estimated cost: $1,500-2,800
M104 Head Gasket Failure
Occasional · high severityTypical onset: 120,000-200,000 mi
Symptoms: White smoke from exhaust on cold start, Coolant loss with no visible leaks, Overheating under load, Oil milkshake in coolant reservoir
Fix: Head gasket replacement requires head removal, resurfacing, and new bolts. M104 straight-six layout makes this a 14-18 hour job. Must replace timing chain components while in there. Often reveals worn guides.
Estimated cost: $3,500-5,500
Hydraulic Self-Leveling Suspension Failure
Common · medium severityTypical onset: 90,000-140,000 mi
Symptoms: Rear end sagging overnight or after sitting, Pump runs constantly or not at all, Uneven ride height side-to-side, Hydraulic fluid leaks at accumulators or lines
Fix: Rear suspension spheres (accumulators) fail first, then pump and lines. Full system overhaul includes pump, both accumulators, and lines. 6-8 hours labor. Many shops convert to coil springs to avoid repeat failures.
Estimated cost: $2,000-3,500
Transmission and Engine Mount Collapse
Common · medium severityTypical onset: 80,000-120,000 mi
Symptoms: Heavy clunk on acceleration or deceleration, Vibration at idle in Drive, Excessive engine movement visible from driver seat, Transmission tunnel heat and noise
Fix: Hydraulic mounts deteriorate and leak fluid. Transmission mount most critical, engine mounts secondary. Replace all three mounts as a set. 4-6 hours labor with proper lift access.
Estimated cost: $1,200-2,000
Vacuum System Leaks
Common · low severityTypical onset: 100,000+ mi
Symptoms: Central locking inoperative or slow, Trunk won't lock or unlock, Soft brake pedal with hissing, Climate control flaps not responding
Fix: Vacuum lines become brittle and crack. Vacuum pump diaphragm tears. Chase leaks with smoke test, replace hard lines with silicone upgrade. Pump replacement 2 hours, full line replacement 6-8 hours if thorough.
Estimated cost: $600-1,800
Window Regulator and Motor Failure
Occasional · low severitySymptoms: Window drops into door or won't raise, Grinding or clicking noise from door, Window moves slowly or jams halfway, One-touch function stops working
Fix: Plastic rollers in regulator track break, motors burn out. Front regulators 3-4 hours each, rears 2-3 hours. Must remove door panel carefully to avoid trim damage. OE parts recommended over aftermarket.
Estimated cost: $600-1,000
Only buy if you have $5k cash reserve for deferred maintenance and can wrench yourself—otherwise these are money pits despite excellent road manners and build quality.
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.