1998 MERCEDES-BENZ S500 W140

Typical onset: 80,000-150,000 mi

Symptoms: Random misfires that move cylinder to cylinder, Hard starting or no-start intermittently, Check engine light with multiple random codes, Fuel smell in engine bay, Rough idle that changes with engine temperature

Fix: Mercedes used biodegradable insulation that turns to dust. Complete engine harness replacement required—not a repair, full R&R. 12-16 hours labor for proper job including injector seals while you're in there. Aftermarket harnesses available but OE quality varies.

Estimated cost: $2,800-4,500

Typical onset: 120,000-180,000 mi

Symptoms: White smoke on cold start that clears, Coolant loss with no visible leaks, Slight rough idle when warm, Coolant in oil (milky dipstick) in severe cases, Overheating under load

Fix: M119 is an interference engine and both heads typically need work when one fails. Head gasket job requires heads off, resurface, valve stem seals while apart. 18-24 hours labor. Do timing chain components, water pump, thermostat at same time or you'll regret it.

Estimated cost: $4,500-7,500

Typical onset: 100,000-150,000 mi

Symptoms: Harsh 2-3 shift or flare on upshift, Limp mode (stuck in 2nd gear), Delayed engagement into Drive or Reverse, Erratic shifting when transmission is hot, Transmission fault codes stored

Fix: 722.6 five-speed transmission uses electrical conductor plate that cracks with heat cycles. Valve body typically needs replacement or rebuild simultaneously. Pan drop, 8-10 hours labor. Use OE Mercedes fluid only—aftermarket kills these. Do external filter and cooler flush at same time.

Estimated cost: $2,200-3,800

Typical onset: 90,000-130,000 mi

Symptoms: Hydraulic fluid puddles under car (greenish oil), Suspension warning light, Car sits low on one corner after sitting, Groaning noise over bumps, Firm or bouncy ride quality change

Fix: Active Body Control uses high-pressure hydraulics. Accumulators fail ($800 each, four corners), lines crack, pump seals leak. Typical repair is 2-3 components at once. Pump replacement is 6-8 hours. Pulsation damper another $600. You'll chase leaks forever on high-mileage examples.

Estimated cost: $1,800-5,000

Typical onset: 100,000-160,000 mi

Symptoms: Fuel smell in cabin or garage, Hard starting after sitting overnight, Rough running that improves as engine warms, Fuel pressure drops rapidly after shutdown, Visible fuel wetness on fuel distributor or lines

Fix: Fuel distributor (spider injection system) develops internal leaks, fuel pressure regulator fails, and rubber fuel lines under intake crack. Distributor alone is 8-10 hours because upper intake must come off. Do all fuel hoses, filter, and pressure damper while you're there. This generation still uses some rubber fuel line that hardens.

Estimated cost: $1,500-3,200

Typical onset: 80,000-140,000 mi

Symptoms: Rear end sags when parked overnight, Pump runs constantly (hear it cycling), Uneven rear ride height side-to-side, Warning light on dash, Rear suspension too firm or too soft

Fix: Rear accumulator sphere loses nitrogen charge, pump wears out from overwork. Accumulator is 2 hours, pump is 3-4 hours. Often both fail within 20k miles of each other. Not critical to driving safety but kills ride quality and wears pump if ignored.

Estimated cost: $800-1,800

Typical onset: 60,000-120,000 mi

Symptoms: Windows drop slowly or won't go up, Doors won't lock/unlock from vacuum system, Hissing sound when locking/unlocking, One window falls into door, Trunk won't unlock with key fob

Fix: Window regulators use plastic rollers that disintegrate. Door lock system uses vacuum lines that crack and leak. Regulator is 2-3 hours per door, vacuum line diagnosis is 1-2 hours but lines are cheap. Front regulators fail most. Not urgent but annoying daily.

Estimated cost: $400-900