Failure mode - 6.6L V8 Gasoline (GM HD, L8T 2020+)

Intake-valve carbon buildup (direct injection, no port wash)

Reliability tier: aggregated - verify before ordering, see /disclosure/

Symptoms

gradual rough idle, hesitation, efficiency loss over high mileage - the generic DI intake-deposit picture

Compare with the other causes of the same sign on this engine: misfire, rough idle, hesitation or surging (4 causes)

Root cause

Direct injection sprays fuel after the intake valves, so nothing washes them: oil-vapor and blow-by deposits accumulate over high mileage. This is a generic DI-architecture risk, not a documented L8T-specific defect - the engine is too young for the deposit record to exist publicly.

Diagnosis

borescope the intake valves when idle-quality complaints accumulate on a high-miler
distinguish carbon deposits from ignition and injector causes (same symptoms)

Fixes

walnut-blast or chemical cleaning of intake valves; catch-can/oil-separator kits are the community preventive ()

Prevalence

occasional