1995 MERCEDES-BENZ S420 W140

Typical onset: 80,000-150,000 mi

Symptoms: Engine stalling or rough idle, Transmission erratic shifting or limp mode, Random electrical gremlins—windows, climate control, gauges, Check engine light with multiple sensor codes, Crumbling wire insulation visible in engine bay

Fix: Mercedes used soy-based insulation 1992-1996 that degrades into sticky residue. Engine and transmission harnesses both fail. Full engine harness replacement is 12-16 hours, trans harness adds another 8-10 hours. Many owners do both simultaneously. Requires total engine bay tear-down.

Estimated cost: $3,500-6,500

Typical onset: 100,000-180,000 mi

Symptoms: Harsh 2-3 or 3-4 shifts, Transmission slipping under load, Delayed engagement into Drive or Reverse, Limp mode—stuck in 2nd or 3rd gear, Metal shavings in fluid on drain

Fix: The electrohydraulic valve body and 13-pin conductor plate wear out, causing shift solenoid failures. Proper fix is removal, disassembly, replacement of valve body assembly and conductor plate, plus fluid flush. Transmission doesn't need to come out—pan-drop job. 8-12 hours labor. Use genuine or OE-equivalent parts only.

Estimated cost: $2,000-3,800

Typical onset: 100,000-200,000 mi

Symptoms: Rough idle when engine is hot, Misfires on multiple cylinders, Poor fuel economy, Difficulty starting when warm, Visible cracking on injector connector boots

Fix: Separate from the main engine harness—this is the section that runs over the valve covers to injectors and coils. Heat deteriorates insulation. Can be replaced without doing full engine harness. 6-8 hours labor, accessible with intake plenum removal.

Estimated cost: $1,200-2,200

Typical onset: 80,000-140,000 mi

Symptoms: Clunk when shifting from Park to Drive or Reverse, Vibration at idle, especially with A/C on, Visible sagging of engine when inspected from below, Transmission tunnel heat or vibration felt in cabin

Fix: Hydraulic mounts fail—engine has two, transmission has one substantial mount. All three should be done together. Engine mounts are 2-3 hours each, transmission mount is 4-6 hours due to exhaust and driveshaft work. Not complicated but time-consuming.

Estimated cost: $1,500-2,800

Symptoms: White smoke from exhaust, Coolant loss with no visible leaks, Oil looks milky or has coolant contamination, Overheating after previous cooling system neglect, Rough idle and misfires on one bank

Fix: M119 head gaskets are extremely durable unless overheated. Caused by failed water pump, clogged radiator, or ignored thermostat. Each head is 16-20 hours labor—full timing chain, cam adjustment, and valve cover work required. Often both heads done simultaneously if one fails. Machine shop work adds cost and time. This is a $6k-10k job that totals most W140s.

Estimated cost: $6,000-10,000

Typical onset: 120,000-200,000 mi

Symptoms: Rear suspension sags overnight or after sitting, Compressor runs excessively or constantly, Suspension warning light on dash, Rear end bottoms out over bumps with passengers

Fix: Rear self-leveling spheres develop leaks, or the hydraulic pump fails. Spheres are 2-3 hours each, pump is 4-5 hours. Many owners convert to Arnott coil springs ($800-1,200 installed) and delete the system entirely—reliable long-term solution.

Estimated cost: $1,800-3,200

Typical onset: 100,000-180,000 mi

Symptoms: Hard starting after sitting overnight, Fuel smell in cabin or garage, Visible fuel weeping at distributor under hood, Loss of fuel pressure—long crank before start

Fix: Fuel distributor (not CIS—this is returnless EFI) and accumulator develop leaks. Distributor is plastic and cracks with age. Both are in the engine bay. 3-5 hours labor combined, but parts are expensive from Mercedes. Aftermarket options exist but quality varies.

Estimated cost: $800-1,600