The 2018 GMC Sierra represents the final year of the K2 generation, featuring V8 engines with Active Fuel Management for improved efficiency. When your Sierra displays P0304 indicating a cylinder 4 misfire, considering age-related wear and AFM system characteristics helps target the diagnosis.
Understanding P0304 in Your 2018 Sierra
At this age, your Sierra may have accumulated significant mileage where component wear becomes a factor. The V8 engines use Active Fuel Management (predecessor to DFM) to deactivate cylinders for fuel economy. Cylinder 4 is on the right bank and is one of the cylinders that deactivates during AFM operation.
AFM System on Aging Vehicles
The AFM lifters in 2018 Sierras have a documented history of wear-related issues on higher-mileage trucks. These lifters can stick or fail, causing misfires particularly during AFM transitions. Oil maintenance history significantly affects AFM lifter longevity. Lifter tick or knock sounds often accompany AFM-related misfires.
Comprehensive Diagnosis
Given the vehicle's age, take a thorough approach. Inspect all ignition components—if one is failing, others may be close behind. Listen for lifter noise indicating AFM issues. Check compression to identify mechanical wear. Verify oil level and condition meet specifications.
Repair Considerations
Replace all spark plugs as a set if approaching service interval. Test ignition coils and replace any showing weakness. If AFM lifter failure is confirmed, options include lifter replacement or AFM delete kit installation. Ensure oil changes use correct Dexos1-approved oil.