The 1999 G350d W463 is a virtually indestructible diesel workhorse, but the OM642 engine listed is incorrect for this year—1999 models used the OM603 or OM606 inline-6 diesels. These older mills are legendary for durability but can suffer catastrophic failures from neglected maintenance, particularly oil starvation and contaminated fuel.
Engine Bottom-End Failure (Rod Bearings / Main Bearings)
Occasional · high severityTypical onset: 200,000-300,000 mi
Symptoms: heavy knocking on cold start that persists, metallic rattling under load, low oil pressure warnings, metal shavings in oil filter
Fix: Complete engine rebuild or short block replacement required—30-40 hours labor depending on access and parts availability. Often triggered by extended oil change intervals or running low on oil. Prevention is everything here.
Estimated cost: $6,000-12,000
Transmission Oil Cooler Failure
Common · high severityTypical onset: 120,000-180,000 mi
Symptoms: transmission slipping or harsh shifts, milky transmission fluid (coolant contamination), engine overheating, transmission failure after coolant mixes with ATF
Fix: Replace oil cooler and flush both cooling and transmission systems—8-12 hours. If coolant has contaminated transmission, expect full transmission rebuild. This is a known weak point on W463 automatics.
Estimated cost: $1,200-2,500 (cooler only), $4,500-8,000 (with transmission rebuild)
Fuel System Contamination / Injector Failure
Common · medium severitySymptoms: hard starting or no-start condition, rough idle and misfires, black smoke on acceleration, loss of power, check engine light with fuel system codes
Fix: These old diesels are sensitive to fuel quality. Water or debris in fuel requires tank drop, fuel system flush, new fuel filter, and potentially injector replacement or rebuild—6-10 hours labor. Always run quality diesel and replace fuel filter every 10,000 mi.
Estimated cost: $800-2,500
Cylinder Head Gasket Failure
Occasional · high severityTypical onset: 180,000-250,000 mi
Symptoms: white smoke from exhaust, coolant loss with no visible leaks, overheating, oil contamination in coolant reservoir, rough running
Fix: Head gasket replacement on OM606 requires removing both heads, resurfacing, and new head bolts—18-24 hours labor. Often caused by overheating events. While heads are off, inspect for cracks.
Estimated cost: $3,500-6,000
Transmission Mounts Deterioration
Common · low severityTypical onset: 100,000-150,000 mi
Symptoms: clunking when shifting from park to drive, excessive vibration at idle, driveline shudder on acceleration, visible sagging or cracked rubber mounts
Fix: Replace transmission mount and inspect engine mounts while there—2-3 hours labor. Cheap fix that dramatically improves drivability. Old G-Wagens beat mounts to death.
Estimated cost: $300-600
Glow Plug System Failures
Common · medium severitySymptoms: extended cranking in cold weather, hard starting below 40°F, white smoke on cold start, glow plug warning light, rough running until warm
Fix: Individual glow plugs fail or the relay/controller goes bad. Diagnosis requires ohm-testing each plug. Replacement is 3-5 hours depending on accessibility—some require manifold removal. Critical for cold-weather operation.
Estimated cost: $600-1,200
Buy one if you can verify religious maintenance history and budget for a potential engine rebuild—when maintained properly, these are 500,000-mile trucks, but one neglected owner can destroy the bottom end.
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.