What is AFM/DFM, and should I delete it?

What it is: Active Fuel Management (AFM, earlier "Displacement on Demand", later DFM on Gen V) shuts off both valves on four cylinders at light load to save fuel. Special lifters collapse on command, fed by an oil-control manifold (LOMA/VLOM) in the valley cover. GM introduced it in 2005 on SUVs like the Tahoe.

Why people delete it: the deactivation lifters are the common failure point. Symptoms include a single-cylinder misfire, a persistent tick, rough idle, low power and a cylinder-specific misfire code. Left alone, failures can progress to bent pushrods, a worn cam lobe and metal in the oil. On Gen IV engines you also must delete AFM to run an aftermarket cam.

A proper delete includes:

1. A non-AFM camshaft (the AFM cam has different lobes on the deactivating cylinders).
2. All 16 conventional roller lifters, not just the 8 AFM lifters, plus new lifter trays.
3. Blocking the deactivation oil circuit: a non-AFM valley cover or the correct plugs.
4. Head gaskets, head bolts and exhaust gaskets, because the heads must come off to reach the lifters.
5. A GM cap (12639759) over the oil pan's pressure-relief valve, and a new pickup O-ring.
6. A tune to stop the PCM commanding deactivation (plus idle, fuel and spark changes for an aftermarket cam).

Don't just plug the oil passages and keep the AFM lifters. The collapsible mechanism remains, and oil can back-feed and collapse them later. AFM cams are single-bolt; a three-bolt performance cam needs a matching gear and chain.