The R129 500SL with the M119 5.0L V8 is a beautifully-engineered roadster that suffers from biodegradable wiring harnesses, failing engine wiring harness insulation, and wear issues in both the hydraulic top system and valve train at higher miles. When maintained, they're remarkably durable, but deferred maintenance turns expensive quickly.
Engine Wiring Harness Deterioration
Common · high severityTypical onset: 80,000-150,000 mi or 25+ years regardless of miles
Symptoms: Random misfires, rough idle, or no-start conditions, Check engine light with multiple cylinder misfire codes, Cracked, sticky, or disintegrating wire insulation—looks like melted tar, Intermittent electrical gremlins affecting injectors and ignition coils
Fix: Complete engine harness replacement is the only real fix. This is 12-16 hours of labor to remove the intake manifold, strip the old harness, and install OEM or aftermarket replacement. DIY-friendly if you're methodical, but fiddly work.
Estimated cost: $1,800-3,200
Head Gasket Failure (M119 V8)
Occasional · high severityTypical onset: 120,000-180,000 mi
Symptoms: White smoke from exhaust, especially on cold start, Coolant loss with no visible external leaks, Milky oil on dipstick or oil cap, Overheating or fluctuating coolant temperature
Fix: Both head gaskets on the M119 require removing intake manifold, valve covers, timing chain components, and both heads. Figure 24-30 hours labor. Always resurface heads, replace head bolts, and do timing chain guides while you're in there. Engine-out makes it easier but not mandatory.
Estimated cost: $4,500-7,500
Hydraulic Convertible Top Failures
Common · medium severityTypical onset: 90,000-140,000 mi or age-related
Symptoms: Top moves slowly, jerks, or stops mid-cycle, Hydraulic fluid leaks visible under rear seat area or trunk, Top control switch unresponsive or partially functional, Cylinder rods pitted or seals weeping
Fix: Hydraulic cylinders (there are multiple) and pump/valve block are common culprits. Cylinders can be rebuilt or replaced—figure 6-10 hours depending on which ones fail. Pump replacement is 4-6 hours. Micro-switches in the latch mechanism also fail and require disassembly of trim.
Estimated cost: $1,200-3,500
Valve Guide Wear and Oil Consumption
Occasional · medium severityTypical onset: 150,000+ mi
Symptoms: Blue smoke on deceleration or startup, Burning a quart of oil every 800-1,200 miles, Fouled spark plugs on one or more cylinders, Rough idle that smooths out when warm
Fix: Worn valve guides allow oil past the seals into the combustion chamber. Proper fix requires head removal, guide replacement, valve job, and new seals—20-26 hours for both banks. Some try seal replacement as a Band-Aid (8-10 hours) but it rarely holds long-term.
Estimated cost: $3,800-6,000
Transmission Wiring Harness and Conductor Plate Failure
Common · medium severityTypical onset: 100,000-160,000 mi
Symptoms: Harsh or delayed shifts, especially 2-3 or 3-4, Limp mode—transmission stuck in second gear, No upshifts or erratic shift points, Transmission fault codes for solenoids or speed sensors
Fix: The 722.5 transmission's internal wiring harness and conductor plate degrade over time. Requires dropping the transmission pan, replacing harness, conductor plate, speed sensors, and filter. 5-7 hours labor. Do the valve body gaskets and all 13-pin connector while you're in there.
Estimated cost: $1,200-2,200
Crankshaft Position Sensor Failures
Occasional · high severityTypical onset: null
Symptoms: Sudden no-start with cranks but won't fire, Intermittent stalling, usually when hot, No RPM reading on instrument cluster during crank, Check engine light with crank sensor code
Fix: The crank position sensor on the M119 mounts at the bellhousing. Replacing it is only 1-2 hours, but it often fails without warning and leaves you stranded. Always carry a spare—they're cheap insurance.
Estimated cost: $250-450
Biodegradable Underhood Insulation and Sound Deadening
Common · low severityTypical onset: null
Symptoms: Foam insulation on hood disintegrating into black dust, Sound deadening material in engine bay falling off in chunks, Excessive underhood heat affecting plastics and hoses, Material clogs drainage channels and sticks to engine
Fix: Remove all old insulation manually—it's messy but not hard. Replace with modern aftermarket insulation or leave bare. Budget 2-3 hours for thorough cleanup and reinstallation. Prevents future damage to wiring and hoses.
Estimated cost: $200-600
Buy one if you can wrench or have a trusted independent Mercedes specialist—sorted examples are magical, but neglected ones will bankrupt you fast.
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.