GM LS-based V8 engines equipped with Active Fuel Management (AFM) can shut down half their cylinders under light load to save fuel. The system works well when it is healthy, but AFM lifter failures are a well-known repair on these engines. A common frustration for technicians is replacing failed AFM lifters, only to see the same symptoms return. In many cases, the lifters were not the root cause.
How AFM Works
AFM uses special deactivation lifters on selected cylinders. Inside each lifter is a locking pin that is released by engine oil pressure. When the pin is released, the lifter collapses instead of opening the valve, and that cylinder stops working.
Oil to these lifters is controlled by the Valve Lifter Oil Manifold (VLOM), an assembly with electronically controlled solenoids mounted on top of the engine block under the intake manifold. When the engine computer requests cylinder deactivation, the VLOM sends pressurized oil to the AFM lifters. When it shuts that oil off, the cylinders return to normal operation. Switching is designed to happen while the lifter is on the base circle of the cam lobe, so the change from eight to four cylinders is smooth.
Why Replacement Lifters Fail Again
According to a technical bulletin from lifter and oil pump manufacturer Melling, most AFM lifters returned for warranty analysis are found to have no defect. The more common causes are oil pressure and control problems elsewhere in the system:
- Low or unstable oil pressure. The AFM system operates within a defined oil pressure range, which Melling cites as roughly 27 to 66 psi. At low engine speeds, available pressure depends on the condition and output of the oil pump.
- Worn lifter bores. Melling notes a lifter bore specification of .843″–.844″ and that the deactivation lifters need about 22 psi to release their locking pins. A bore that is only slightly worn can leak enough oil to keep the lifter from operating correctly.
- A clogged VLOM filter. The VLOM has a small replaceable oil screen in its inlet passage. On high-mileage engines it is commonly found plugged, which can restrict oil to the lifters and contribute to misfire codes.
- VLOM timing faults. If the VLOM switches a lifter while it is on the cam ramp or lobe peak instead of the base circle, the lifter can be damaged or stuck collapsed.
A Complete AFM Lifter Repair
To prevent a repeat failure, treat AFM lifter replacement as a system repair rather than a single-part repair:
- Replace the lifter guides along with the lifters.
- Measure every lifter bore and address any that are out of specification.
- Test the VLOM for proper operation, or replace it.
- Replace the VLOM oil filter screen.
- Check engine oil pressure and the condition of the oil pump, pickup, and AFM pressure relief valve.
- Inspect the camshaft lobes for damage caused by the failed lifters.
Always confirm specifications and procedures for the exact engine in GM service information, as details can vary by engine and model year.
Evergreen Valvetrain and Lubrication Parts
Evergreen offers valve lifters, camshafts, and oil pumps for a range of domestic and import applications. Use the Evergreen eCatalog to find the right parts for your engine, and see our article on replacing the oil pump during a rebuild.
Source: Melling technical bulletin, “GM LS AFM Deactivation Lifter Issues.” Brand names are the property of their respective owners.
