What cylinders does an AFM shut down?
In GM's Active Fuel Management (AFM), four cylinders are shut down during light-load cruising to operate the engine as a 4‑cylinder unit. The exact cylinders vary by engine family, but on many LS‑family V8s the odd-numbered cylinders (1, 3, 5, 7) are deactivated, leaving cylinders 2, 4, 6, and 8 active.
Overview: AFM and its purpose
AFM, short for Active Fuel Management, is a displacement-on-demand system used by General Motors to boost fuel economy. When the engine is under light load and cruising, AFM temporarily disables four of the eight cylinders to reduce fuel consumption. When more power is needed—such as when accelerating or climbing—a switch back to all eight cylinders occurs automatically and quickly, often without the driver noticing. The system achieves this primarily by cutting fuel to the deactivated cylinders and by deactivating their valve/air intake in a way that minimizes disruption to the engine’s smooth operation.
Typical pattern and engine-family variations
Before presenting the pattern, note that cylinder numbering can vary slightly by engine design and model year. The following describes the pattern most commonly observed on GM’s LS‑based V8s, but就 verify with your vehicle’s documentation for your exact engine.
- Common pattern: deactivate four cylinders on one bank, typically the odd-numbered cylinders 1, 3, 5, 7, so that the active cylinders are 2, 4, 6, and 8.
Conclusion: While the odd-numbered cylinder shutdown pattern is widespread on many LS‑family V8s, some engines or model years may use a different numbering or activation scheme. Always consult the specific engine’s documentation or a GM service bulletin for your exact configuration.
What to expect when AFM engages
Drivers generally notice only subtle changes in noise or vibration during the transition between eight- and four-cylinder modes. The change is designed to be seamless, happening in response to driving conditions such as steady highway cruising at light throttle. If a vehicle experiences persistent misfires, rough idle, or a malfunction indicating AFM involvement, a professional diagnostic check is recommended to rule out injector or lifter issues affecting the deactivated cylinders.
Summary
AFM reduces fuel consumption by temporarily shutting down four cylinders in a V8 engine during light-load conditions. The most common pattern on many GM LS‑based engines is to deactivate the odd-numbered cylinders (1, 3, 5, 7), leaving the even-numbered cylinders (2, 4, 6, 8) active. However, exact cylinder shutdown can vary by engine family and model year, so always verify with the owner’s manual or a GM service bulletin for your specific vehicle.
