BULLETIN .Manufacturers have noticed an increase in the issues surrounding the replacement of deactivation lifters in GM LS engines with Active Fuel Management or AFM. After installing new lifters the original issue may not have been corrected. Most lifters returned for analysis are found to be good. We have found that most lifter faults are caused by oil pressure issues, or control issues. The AFM activation and deactivation is controlled by the Valve Lifter Oil Manifold or VLOM. The VLOM consists of 4 solenoids and is bolted to the top of the engine block beneath the intake manifold assembly. It directs the flow of pressurized engine oil to the active fuel management intake and exhaust valve lifters. VLOM applies pressurized oil to the AFM lifters when cylinder deactivation is requested, and shuts off that supply of oil to reactivate those cylinders. Cylinder activation and deactivation are both supposed to occur on the base circle of the cam lobe, making the transition from four to eight cylinder mode unnoticeable to the driver. To control contamination a small replaceable oil filter is located in the VLOM inlet oil passageway. The AFM oil pressure relief valve regulates the oil supplied to the VLOM and is located in the oil pan near the oil filter housing. The AFM system has an operating range from 27 PSI to 66 PSI of oil pressure. At higher engine speeds the high side of this operating range is controlled by the AFM oil pressure relief valve. At low engine speeds the low side of the operating range will depend on the engines ability to produce oil pressure using the flow of oil from the oil pump. The AFM lifter bores in these engines have a spec of .843-.844, and the deactivation lifters require 22 PSI of pressure to release the locking pins. Taking these two things into consideration a lifter bore that is even slightly worn could bleed off enough oil pressure to prevent the lifter from unlocking. and bad oil pump can do the same.
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