The 1991 G350d W463 is a solidly-built military-derived platform, but the OM642 diesel (actually introduced 2005+, so this is likely misidentified—early G-Wagens ran OM603/OM606 inline-6 diesels) suffers from specific chronic issues around fuel injection, transmission cooling, and in higher-mileage examples, catastrophic lower-end failures that can total the engine.
Transmission Oil Cooler Failure Leading to Contamination
Common · high severityTypical onset: 80,000-140,000 mi
Symptoms: Transmission fluid in coolant (milky reservoir or expansion tank), Coolant in transmission (delayed engagement, slipping, burnt ATF smell), Overheating transmission or engine
Fix: Replace oil cooler, flush both cooling system and transmission completely (sometimes requires multiple flushes), inspect transmission internals for damage. If contamination was severe, transmission rebuild often necessary. 6-10 hours labor depending on extent of damage.
Estimated cost: $1,200-4,500
Catastrophic Lower-End Engine Failure (Crankshaft, Rod Bearings, Pistons)
Occasional · high severityTypical onset: 150,000-250,000 mi
Symptoms: Severe knocking or metallic rattling from crankcase, Loss of oil pressure, Metal shavings in oil or filter, Sudden loss of power or engine seizure
Fix: Engine rebuild or replacement required—crankshaft regrind/replacement, all main and rod bearings, pistons, rings, honing cylinders. Often combined with head gasket job if engine is apart. 40-60 hours labor for full rebuild in-chassis, or 25-35 for short-block swap if available.
Estimated cost: $6,500-12,000
Fuel Injection System Degradation and Injector Failure
Common · medium severityTypical onset: 100,000-180,000 mi
Symptoms: Hard starting, especially when cold, Rough idle or misfires, Black smoke under load, Loss of power and fuel economy
Fix: Replace fuel injectors (often all at once for consistent performance), new fuel filters, possibly injection pump refurbishment. 4-8 hours labor depending on injector accessibility and pump work.
Estimated cost: $1,800-3,500
Transmission Mounts Collapse
Common · medium severityTypical onset: 60,000-100,000 mi
Symptoms: Excessive drivetrain clunk on throttle application or deceleration, Vibration at idle in gear, Visible transmission sag or misalignment
Fix: Replace transmission mount(s)—G-Wagen has multiple mounts and requires chassis lift or support for access. 2-4 hours labor.
Estimated cost: $400-800
Head Gasket Failure (Often Both Cylinder Banks if OM642 V6)
Occasional · high severityTypical onset: 120,000-200,000 mi
Symptoms: White smoke from exhaust (coolant burning), Coolant loss with no external leaks, Overheating or fluctuating temperature gauge, Oil contamination (milky oil cap/dipstick)
Fix: Head gasket replacement both banks, machine heads if warped, new head bolts. Often discover additional issues (cracked head, worn guides) once opened. 20-30 hours labor.
Estimated cost: $4,000-7,000
Chronic Fuel Filter Clogging from Tank Contamination
Common · low severitySymptoms: Intermittent stalling or loss of power, Difficulty maintaining speed under load, Rough running after sitting
Fix: Replace fuel filter(s)—often multiple inline filters in these systems—and inspect/clean fuel tank if contamination is chronic. Some owners install auxiliary filter/water separator. 1-2 hours labor per incident.
Estimated cost: $150-400
Buy only if you find one with documented lower-end rebuild or low original miles with proof of meticulous maintenance—otherwise budget $10k+ for inevitable engine work.
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.