1998 MERCEDES-BENZ S420 W140

Typical onset: 80,000-150,000 mi

Symptoms: Random misfires that move between cylinders, Hard starting or no-start when heat-soaked, Check engine light with multiple cylinder misfire codes, Rough idle that comes and goes, Fuel odor under hood from injector connector shorts

Fix: Complete engine harness replacement required — Mercedes used biodegradable insulation that crumbles into dust. 12-16 hours labor to remove intake manifold, fuel rails, and replace entire harness. Aftermarket harnesses available but OE quality varies. Cannot be reliably repaired with tape.

Estimated cost: $2,200-3,800

Typical onset: 150,000-200,000 mi

Symptoms: Blue smoke on cold start that clears after warmup, Consuming 1+ quart every 500-1000 miles, Carbon buildup fouling spark plugs repeatedly, Loss of compression on leakdown test, Eventually progresses to rod bearing knock if ignored

Fix: Requires complete engine teardown — rings, hone cylinders, inspect bearings. Often find scored cylinder walls requiring overbore or block replacement. 40-50 hours labor for proper rebuild with machine work. Many shops quote short block replacement instead at similar cost. If bearings are damaged, add crank grinding or replacement.

Estimated cost: $6,500-12,000

Typical onset: 100,000-180,000 mi

Symptoms: Harsh 2-3 or 3-4 shifts when cold, Limp mode (stuck in 2nd gear) intermittently, Delayed engagement into reverse, Transmission slipping between shifts, Fluid contamination from internal clutch wear

Fix: 722.6 five-speed transmission requires valve body removal and rebuild, replace conductor plate (circuit board), and new solenoids. 8-12 hours labor. If clutch packs are worn, full rebuild needed. Critical to use correct MB-spec fluid during repair or problems return quickly.

Estimated cost: $2,000-3,500

Typical onset: 90,000-140,000 mi

Symptoms: Rear end sags overnight or when parked, Suspension pump runs excessively (hear it cycling), Hydraulic fluid puddle under rear of car, Uneven ride height side-to-side, Warning light on dash for suspension fault

Fix: Rear accumulators (spheres) leak internally, rear struts develop external leaks at seals. Each rear strut is 3-4 hours labor, accumulators add another 2 hours. Pump failures less common but require 4-5 hours. Must bleed system properly after repairs. Some owners convert to coil springs to eliminate system entirely.

Estimated cost: $1,800-3,200

Typical onset: 120,000-180,000 mi

Symptoms: White smoke from exhaust on startup, Coolant loss with no external leaks visible, Milky oil on dipstick or oil cap, Overheating under load or in traffic, Combustion gases in coolant (bubbling in reservoir)

Fix: M119 heads must come off for proper gasket replacement — 18-24 hours labor per side if doing both. Must pressure-test heads for cracks and resurface. Often find warped heads on high-mileage engines. Head bolts are torque-to-yield, must replace. If coolant contaminated oil, risk of bearing damage exists.

Estimated cost: $4,500-7,500

Typical onset: 100,000-160,000 mi

Symptoms: Transmission fluid dripping from front of car, Low fluid level causes slipping or limp mode, Fluid visible on front crossmember or subframe, Burnt transmission smell if driven low on fluid

Fix: Hard lines corrode at fittings or flexible sections crack. Requires lifting car and accessing lines routed along subframe. 3-5 hours labor depending on which section failed. Must replace with OE or quality aftermarket — cheap lines fail quickly. Refill and check for proper shifting after repair.

Estimated cost: $600-1,200