The W202 C280 with M104 2.8L inline-six is a solid platform when maintained, but suffers from predictable age-related failures in the transmission cooling system, engine mounts, and internal wiring harnesses. The M104 engine itself is durable, but neglected cooling system maintenance leads to catastrophic head gasket failures.
Head Gasket Failure from Overheating
Common · high severityTypical onset: 120,000-180,000 mi
Symptoms: White smoke from exhaust, Coolant disappearing with no visible leaks, Oil contamination in coolant reservoir, Overheating under load, Rough idle when cold
Fix: Both head gaskets, machining heads flat, new head bolts, full cooling system refresh. 14-18 labor hours. If overheating wasn't caught early, expect warped heads requiring replacement or extensive machining.
Estimated cost: $2,800-4,500
Transmission Oil Cooler Line Failure
Common · high severityTypical onset: 80,000-150,000 mi
Symptoms: Transmission fluid leaking near radiator, Pink fluid under car, Transmission slipping or delayed engagement, Milky transmission fluid (if coolant mixing)
Fix: Replace steel cooler lines that rust through where they pass frame rails. Often requires dropping subframe for access. If coolant mixed into trans, full flush and possibly rebuild. Lines alone: 4-6 hours. With transmission service: 6-8 hours.
Estimated cost: $800-1,400
Engine and Transmission Mount Collapse
Common · medium severityTypical onset: 100,000-140,000 mi
Symptoms: Clunk when shifting from Park to Drive, Excessive vibration at idle, Engine rocks visibly under throttle, Shifter vibration
Fix: Hydraulic mounts fail internally. Front engine mount and transmission mount are priorities. Rear mount less critical. Replace all three for best results. 3-4 hours total.
Estimated cost: $600-950
Wiring Harness Deterioration
Common · medium severitySymptoms: Intermittent no-start, Check engine light with multiple random codes, Rough idle or misfires that move between cylinders, Electrical gremlins (windows, locks)
Fix: Biodegradable insulation on engine bay harness crumbles from heat. Repair means engine harness replacement or painstaking rewrap with new insulation. Full replacement: 8-12 hours. Partial repair possible but temporary.
Estimated cost: $1,200-2,200
Mass Air Flow Sensor Contamination
Occasional · medium severityTypical onset: 90,000-130,000 mi
Symptoms: Hesitation on acceleration, Poor fuel economy, Black smoke under hard throttle, Rough idle, Check engine light with lean/rich codes
Fix: Hot-wire MAF gets contaminated by oil vapors from worn crankcase vent system. Clean MAF with proper cleaner first, then replace if no improvement. Also inspect crankcase breather hoses. 0.5-1 hour diagnostic and cleaning.
Estimated cost: $150-400
Automatic Transmission Valve Body Wear
Occasional · medium severityTypical onset: 150,000-200,000 mi
Symptoms: Harsh 2-3 shift, Delayed engagement when cold, Flare on 3-4 upshift, Won't downshift under load
Fix: 722.6 five-speed experiences bore wear in valve body, especially if fluid changes were skipped. Valve body rebuild or replacement required. Remove transmission, disassemble, resurface bores or replace. 10-14 hours.
Estimated cost: $1,800-2,800
Crankshaft Position Sensor Failure
Occasional · high severitySymptoms: No-start with no crank, Stalling while driving with no restart, Intermittent cutting out, Tachometer drops to zero while driving
Fix: Hall-effect sensor behind harmonic balancer fails from heat. Must remove balancer for access. Keep spare in glovebox—common stranding issue. 2-3 hours with proper puller.
Estimated cost: $350-600
Buy one only if it has documented head gasket replacement or verified pristine cooling system history—these are excellent highway cruisers when the fatal flaws are addressed, but neglected examples are money pits.
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.