1997 MERCEDES-BENZ S-CLASS

Typical onset: 100,000-150,000 mi

Symptoms: Random misfires, multiple cylinder codes, Hard starting or no-start conditions, Fuel smell in engine bay from leaking injectors due to failed seals, Check engine light with seemingly unrelated codes

Fix: Mercedes used biodegradable insulation that turns to tar and exposes bare wires. Complete harness replacement requires 18-25 labor hours on V8, 30-40 hours on V12 due to packaging. Aftermarket improved harnesses available. Delay this and you risk internal engine damage from lean conditions.

Estimated cost: $3,500-6,000 V8, $7,000-12,000 V12

Typical onset: 80,000-120,000 mi

Symptoms: Vehicle sagging at one or more corners after sitting overnight, Compressor running constantly, ADS warning light illuminated, Harsh ride or wallowing handling

Fix: Air struts leak, compressor wears out, or valve block fails. Strut replacement is 2-3 hours each corner. Compressor is another 3-4 hours. Many owners convert to Arnott coil spring kit (8-10 hours) to eliminate system permanently. OEM air struts last 60-80k miles in practice.

Estimated cost: $800-1,500 per strut, $1,200-1,800 compressor, $2,000-3,000 coil conversion

Typical onset: 120,000-180,000 mi

Symptoms: Harsh 2-3 or 3-4 shifts, Slipping between gears, Limp mode activation, Delayed engagement into Drive or Reverse

Fix: The 5-speed 722.5 develops worn valve body bores and cracked conductor plate connectors. Requires transmission removal, disassembly, valve body replacement or refurbishment, new conductor plate. 12-16 hours labor. Use Sonnax upgrades if available. This trans was overworked in the heavy S-Class.

Estimated cost: $2,800-4,500

Typical onset: 80,000-130,000 mi

Symptoms: Excessive oil consumption, more than 1 qt per 500 miles, Blue smoke on startup or acceleration, Loss of power, misfires on multiple cylinders, Low compression readings on multiple cylinders

Fix: The M120 V12 has weak piston ring design and overbored cylinders. Rings flutter, glaze the bores, and oil control fails. Requires full engine-out rebuild with oversize pistons or sleeved block. 60-80 hours labor. Short block replacement is 45-55 hours. This is why you see 'engine rebuild' so common in repair databases for this year.

Estimated cost: $12,000-18,000 rebuild, $15,000-22,000 short block swap

Typical onset: 90,000-140,000 mi

Symptoms: Rear end sags when parked, rises when running, Green hydraulic fluid pooling under rear, SLS pump runs excessively, Rear bounces or floats over bumps

Fix: Hydraulic accumulators, struts, and lines leak. Strut replacement 2.5-3 hours each side. Accumulator spheres 1.5 hours each. Full system refresh with pump can hit 12 hours. Parts are expensive and aftermarket quality is hit-or-miss. Many convert to coil springs.

Estimated cost: $1,200-2,000 per strut, $2,500-4,000 full system service

Symptoms: Intermittent gauge cluster failures, COMAND or radio locks up or reboots, Climate control displays gibberish, Multiple control modules showing communication faults on scan tool

Fix: The W140 uses early fiber optic networking. Corrosion in connectors or kinked fiber loops cause cascading failures. Diagnosis is time-consuming, 3-6 hours tracing loop continuity. Repair involves cleaning optical connectors or replacing damaged fiber segments (0.5-2 hours per segment). Specialist knowledge required.

Estimated cost: $400-1,200 depending on location of break

Typical onset: 150,000+ mi

Symptoms: Rough idle, misfires after refueling, Fuel filters clogging every 5,000-10,000 miles, Black debris in fuel filter, Fuel pumps failing prematurely

Fix: Factory tank liner breaks down, sheds rubber particles that clog filters and injectors. Requires tank removal, cleaning or replacement, new pumps, filters, and injector service. Tank R&R is 6-8 hours, plus system cleaning adds 4-6 hours. Catch this early before injectors get damaged.

Estimated cost: $2,000-3,500