Understanding The GM Active Fuel Management System

Learn How The GM Active Fuel Management (AFM) System Works

Let me break down GM Active Fuel Management (AFM) for you in simpler terms. This system is designed to save fuel by temporarily “turning off” half of the engine’s cylinders when they aren’t needed. It’s like putting your engine on a diet during light driving. Here’s how it works and what you should know:

What Does GM Active Fuel Management Do?

When you’re cruising down the highway or driving with a light load, your vehicle doesn’t need all eight cylinders firing at full power. The GM Active Fuel Management system kicks in and turns your V8 engine into a V4 by deactivating four cylinders—specifically cylinders 1, 7, 4, and 6. When you accelerate or need more power (like climbing a hill), the system switches back to all eight cylinders seamlessly.

To learn more about common problems with the GM AFM system, see this post.

AFM System Components

• Valve Lifter Oil Manifold (VLOM)
• Eight special AFM valve lifters, 2 per deactivating cylinder
• The engine oil pressure regulator valve for cylinder deactivation operation
• Gen IV cylinder deactivation engine block
• The ECM

GM’s VLOM and AFM lifters

How Does AFM Work?

This system relies on some clever engineering:

Cylinder Deactivation: AFM uses special hydraulic valve lifters. When AFM is active, these lifters temporarily stop the valves from opening and closing, effectively “shutting down” certain cylinders.

Switching Modes:
In V8 mode, all eight cylinders are active, which happens when starting, idling, or when you need more power.
In V4 mode, four cylinders deactivate to save fuel. The switch between modes takes less than 250 milliseconds—so fast you won’t even notice it.

The AFM Lifters Are Activated and Deactivated by Oil Pressure: The system uses engine oil pressure to move small spring-loaded locking pins in the lifters. This action enables or disables the lifter, which then enables or disables valve operation. Each lifter is built with two oil ports: one for activating the locking pin and the other for deactivating the locking pin. The oil flow is managed by a component called the Valve Lifter Oil Manifold (VLOM), which works together with the Engine Control Module (ECM).

To initiate cylinder deactivation, The ECM cuts the fuel injector to each deactivated cylinder and commands a solenoid within the VLOM to send oil pressure to the deactivation ports on both the intake and exhaust lifters. The deactivation occurs during the intake stroke. Even though the solenoid pressurizes both lifters at the same time, the exhaust valve deactivates first. Here’s how that happens.

The deactivation pressure applies to the intake lifter pin and spring. However, the cam lobe is still applying pressure to the lifter, so the pin can’t move until the cam lobe passes the base circle. At that point, the pressure is off, and the locking pin can move and deactivate the intake valve. Since the exhaust valve is closed during the intake stroke, its locking pin activates immediately, allowing some exhaust to remain in the cylinder.

Why Capture Exhaust Gases In The Deactivated Cylinders?

When switching to V4 mode, the system purposely traps some exhaust gas in the inactive cylinders. This helps:

• Reduce noise and vibrations.
• Lower oil consumption.
• Minimize emissions.

The spark plugs in the deactivated cylinders still fire, but there’s no fuel to burn. Firing the spark plugs helps maintain smooth engine operation.

What Causes the AFM To Not Work

1 ) Low Oil Pressure: If your oil pressure or temperature isn’t within the correct range the locking pins won’t move.
2) AFM doesn’t engage when you’re accelerating hard or driving with a heavy load.
3) During certain engine conditions like overheating or when there’s a mechanical fault.

GM Active Fuel Management is a smart, fuel-saving feature that works quietly in the background to improve efficiency without compromising performance. It’s been carefully designed to be reliable and trouble-free, so you can focus on enjoying the drive instead of worrying about your engine.