1998 MERCEDES-BENZ G320 W463

Typical onset: 120,000-180,000 mi

Symptoms: White smoke from exhaust on cold start, Coolant loss with no visible leaks, Overheating or erratic temperature gauge, Milky residue in oil cap or dipstick

Fix: Head gaskets on both banks require complete top-end disassembly. Budget 16-20 hours labor. While you're in there, replace timing chain guides, tensioners, and valve stem seals. Head resurfacing often needed. If neglected, leads to cracked head or complete engine failure.

Estimated cost: $3,500-6,000

Typical onset: 80,000-150,000 mi

Symptoms: ATF pooling under vehicle near radiator area, Transmission slipping or delayed engagement when fluid is low, Pink or red fluid mixing with coolant in overflow tank (internal cooler failure)

Fix: External lines rust through and blow out; internal radiator cooler can fail and cross-contaminate coolant and ATF, which kills the transmission. Replace all cooler lines preventively, external cooler is 3-4 hours. If radiator internals failed, full transmission flush/fluid replacement mandatory, sometimes new trans needed. Total 6-8 hours with trans service.

Estimated cost: $800-2,500

Symptoms: Intermittent stalling or no-start conditions, Check engine light with multiple random codes, Rough idle, misfires that move between cylinders, Visible cracked or sticky insulation on injector or ignition wires

Fix: Mercedes used soy-based insulation in this era that literally rots and gets eaten by rodents. Engine harness replacement is 10-14 hours of labor-intensive disassembly. OE harness is discontinued; aftermarket or used are risky. Some techs rebuild in place with heat shrink and patience, but it's tedious. This can strand you.

Estimated cost: $1,800-3,500

Typical onset: 100,000-140,000 mi

Symptoms: Clunking on takeoff or deceleration, Excessive vibration at idle in gear, Visible engine movement when revving in park, Shifter feels sloppy or notchy

Fix: Rubber mounts age out and tear, letting the heavy M104 move excessively. Transmission mount is the worst offender. Replace all engine and trans mounts as a set—about 4-6 hours labor depending on access. Improves drivability dramatically.

Estimated cost: $900-1,600

Typical onset: 100,000+ mi

Symptoms: Oil drips from bell housing area onto crossmember, Oil coating underside of engine and transmission, Persistent oil smell after driving, Low oil level between changes

Fix: Rear main seal requires transmission removal—8-10 hours labor minimum. Oil pan gasket is easier at 3-4 hours but often overlooked until it's a mess. Both are common on high-mileage M104s. Do them together if trans is already out for clutch or other work.

Estimated cost: $1,200-2,800

Typical onset: 80,000-120,000 mi

Symptoms: Hard starting, especially when hot, Rough idle or hesitation under acceleration, Black smoke or strong fuel smell, Check engine light with lean/rich codes

Fix: Fuel filters clog if not changed every 20-30k miles; in-tank pump screens rust and collapse. Injectors gum up from ethanol and old fuel. Full injector cleaning or replacement is 4-6 hours plus parts. Replace fuel filter, tank strainer, and consider fuel distributor rebuild if issues persist.

Estimated cost: $800-2,200

Typical onset: 180,000+ mi

Symptoms: Knocking noise on cold start that fades with warmth, Heavy knocking under load or acceleration, Metallic rattling from bottom end, Low oil pressure at idle

Fix: M104s are generally durable, but neglected oil changes or overheating events kill bearings. Requires engine-out rebuild: crank inspection, bearing replacement, possibly rod resizing. 30-40 hours labor if you're doing a full rebuild with machine work. Often cheaper to swap in a low-mileage used engine (15-20 hours).

Estimated cost: $5,000-9,000