The 2004 Diamante with the 3.5L 6G74 V6 is a comfortable FWD sedan plagued by catastrophic engine failures and aging automatic transmission issues. When these problems hit, repair costs often exceed the vehicle's value.
Catastrophic Engine Failure (Piston Ring / Bearing Collapse)
Common · high severityTypical onset: 80,000-140,000 mi
Symptoms: Excessive oil consumption (1+ quart per 500 miles), Blue smoke on startup or acceleration, Knocking or rattling from bottom end, Sudden oil pressure loss and seizure
Fix: The 6G74 V6 suffers piston ring land failure and bearing wear due to inadequate oiling in the design. Once symptoms appear, you're looking at either complete engine rebuild (30-40 hrs) or used engine swap (12-16 hrs). Most shops recommend replacement over rebuild given the core design flaw. Parts availability is declining.
Estimated cost: $3,500-6,500
Automatic Transmission Oil Cooler Line Leaks
Common · medium severityTypical onset: 90,000-150,000 mi
Symptoms: ATF puddles under front of engine bay, Transmission overheating on highway drives, Low fluid level warnings or slipping shifts, Pink residue near radiator area
Fix: Steel cooler lines rust through where they connect to the radiator and transmission. Requires replacement of hard lines and sometimes the rubber hoses. Labor is 2-3 hrs due to tight access. If fluid ran low before detection, internal transmission damage may follow within 5,000-10,000 miles.
Estimated cost: $350-650
Transmission Mount Collapse
Occasional · medium severityTypical onset: 100,000-160,000 mi
Symptoms: Severe clunk when shifting from Park to Drive, Vibration at idle that smooths out when shifted to Neutral, Visible engine movement when revving, Driveline shudder during acceleration
Fix: The front transmission mount deteriorates and allows excessive powertrain movement. Replacement is straightforward (1.5-2 hrs) but often reveals that other mounts are also failing. Recommend inspecting all engine mounts simultaneously to avoid repeat labor charges.
Estimated cost: $250-450
Head Gasket Failure (External Leaks)
Occasional · medium severityTypical onset: 120,000-180,000 mi
Symptoms: Coolant seepage at cylinder head mating surfaces, Sweet smell from engine bay, Slow coolant loss without visible radiator leaks, Oil contamination in coolant (less common but catastrophic)
Fix: The 6G74 develops external head gasket weeps over time. Both heads should be done simultaneously (16-20 hrs) and heads resurfaced. If coolant has mixed with oil, the engine damage described above accelerates rapidly. Parts are still available but machine shop turnaround adds weeks.
Estimated cost: $2,200-3,800
Fuel Filter Clogging / Fuel Pump Strainer Issues
Occasional · low severityTypical onset: null
Symptoms: Hard starting after sitting overnight, Intermittent stalling at idle, Hesitation or stumbling under load, Check engine light with fuel trim codes
Fix: The in-tank fuel pump strainer clogs with sediment, especially in vehicles that sat or used questionable fuel. The inline filter is often neglected. Pump access requires dropping the tank (2-3 hrs). Replace both filter and pump assembly together to avoid comebacks.
Estimated cost: $450-750
Crankshaft Position Sensor Failure
Occasional · high severityTypical onset: null
Symptoms: No-start condition with cranking but no spark, Sudden stalling while driving with no restart, Intermittent stumbling that becomes permanent, Check engine light may or may not set code
Fix: Sensor fails due to heat cycling near the crankshaft. When it dies, the engine stops immediately and won't restart. Replacement is 1-1.5 hrs, but getting stranded is the real cost. Always carry a spare if keeping this car long-term.
Estimated cost: $180-320
Only buy if under 80,000 miles with meticulous maintenance records and priced under $2,500 — the engine time-bomb makes this a gamble, not transportation.
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.