2009 MERCEDES-BENZ ML550 W164

Typical onset: 70,000-120,000 mi

Symptoms: Metallic rattling on cold start that may disappear when warm, Increasing oil consumption (1qt per 500-1000 miles), Check engine light with cylinder misfire codes, Metal shavings visible in oil during changes, Catastrophic failure: sudden loss of power, knocking, white smoke

Fix: The M273 balance shaft gears wear prematurely, sending metal through the engine and scoring cylinder walls. Once piston scoring begins, repair requires full engine rebuild or replacement. Short block replacement is 25-35 hours; full rebuild with head work is 35-45 hours. Many owners opt for remanufactured long blocks to avoid repeat failures.

Estimated cost: $12,000-18,000

Typical onset: 80,000-130,000 mi

Symptoms: Transmission fluid leaking at radiator connections, Transmission overheating warnings on dash, Harsh shifting or delayed engagement when hot, Pink or milky transmission fluid (coolant contamination), Limp mode activation during highway driving

Fix: The 722.9 seven-speed trans has a cooler integrated into the radiator that clogs or leaks externally at the quick-connect fittings. If coolant mixes with ATF, the transmission is toast. Proper fix is external trans cooler installation plus fluid flush (6-8 hours), or transmission replacement if contaminated (18-22 hours).

Estimated cost: $1,200-2,500 for cooler; $5,500-8,000 with transmission

Typical onset: 90,000-140,000 mi

Symptoms: Vehicle sags at one or more corners overnight, Airmatic Visit Workshop message on startup, Compressor runs constantly or for extended periods, Clunking over bumps from worn strut mounts, Suspension lowering and vehicle stuck in access mode

Fix: Front struts leak first, followed by compressor failure from overwork. Each strut is 2-3 hours; compressor is 3-4 hours. Valve block can also fail (6-7 hours). Many owners convert to Arnott or Strutmasters aftermarket systems or delete Airmatic entirely with coil conversion kits.

Estimated cost: $1,800-3,200 per strut OE; $2,200-3,500 compressor; $3,500-5,500 coil conversion

Typical onset: 60,000-100,000 mi

Symptoms: Clunking from front end over bumps, Steering wander or vague on-center feel, Inner tire edge wear, Vibration through steering wheel at highway speeds, Failed state inspection for excessive play

Fix: The forward lower control arm bushings deteriorate and ball joints develop play. Mercedes sells only complete arms, not serviceable bushings. Front lower control arms are 2.5-3.5 hours each side including alignment. Often both sides plus upper arms and links needed simultaneously.

Estimated cost: $1,400-2,400 for both lowers with alignment

Typical onset: 70,000-120,000 mi

Symptoms: 4MATIC warning light illuminated, Grinding or whining from center of vehicle, Inability to select 4WD modes, Intermittent loss of AWD function, Fault codes for transfer case actuator position

Fix: The electric actuator motor that engages 4WD modes fails internally or the encoder loses calibration. Actuator motor replacement is 3-4 hours and requires a Star Diagnostics programming session. Transfer case removal for internal repairs is 10-14 hours and often not worth it on high-mileage examples.

Estimated cost: $1,200-1,800 actuator motor; $3,500-5,000 transfer case overhaul

Typical onset: null

Symptoms: Backup camera intermittent or non-functional, Rear wiper stops working sporadically, Third brake light or license plate lights out, Tailgate will not lock or unlock electrically, Multiple electrical faults appear together

Fix: Wires fatigue and break in the rubber boot between body and tailgate due to repeated flexing. Diagnosis is 1-1.5 hours; repair involves removing trim and either splicing wires or replacing the entire harness section (3-4 hours total). Mercedes updated the harness design in later years.

Estimated cost: $450-850