Tech Tips: Replacing LS Active Fuel Management Lifters With Melling

When it comes to performance engine builds, most enthusiasts we know opt to delete GM’s Displacement-On-Demand (also known as Active Fuel Management) system for simplicity’s sake. Even those who opt to not remove the hardware, often disable the system through ECM tuning. However, not everyone tosses the system into the scrap bin, and that’s where this tech video from Melling Engine Parts comes into play.

Since replacing a failed DOD/AFM lifter is a more involved process that swapping out a traditional lifter, Melling has seen a large number of failures due to improper replacement methods; hence this video. Because the operating pressure of the AFM lifter is a relatively narrow window of 27-66psi, even being slightly out of spec on the lifter bore diameter — either due to wear of the bore or an undersized lifter body — can cause enough pressure loss that they will no longer function properly. This is the case for a large number of the warranty returns Melling receives — the returned lifter is completely operational, the user’s system was just unable to reach that 27psi operating pressure.

In addition to the lifter bores themselves, Melling points out that the entire DOD oiling system needs to be maintained at the same time as lifter replacement, since often it is the source of problem to begin with. That means that every time a lifter is changed, the lifter guides must be replaced, the Valve Lifter Oil Manifold (or VLOM) filter needs to be replaced, and the VLOM itself tested (at the minimum) to ensure proper oil flow for the AFM system.

The low side of the VLOM’s operating pressure is determined by the engine’s ability to make oil pressure. The tolerance range on the AFM lifter bore is 0.8430-0.8440 inches. If there is excessive clearance, oil pressure could be low enough to not unlock the lifter. Unfortunately the fix for an out-of-spec bore is to replace the block, or have the bore machined and sleeved.

The VLOM is a critical component to the entire system, as it regulates and distributes the pressurized oil needed to deactivate and reactivate the lifters as the AFM system commands. Melling has found, through its warranty replacement program, that an unusually high number of lifter failures aren’t actually failures of the lifters themselves, but rather are due to either oil pressure or oil control issues, most commonly related to the previously outlined factors of lifter bore clearance or a clogged/restricted VLOM system.

For the returns they have received due to actual failures, a large percentage of those failures can be traced to lifters which are deactivated by the system off of the cam lobe’s base circle, and as such are subjected to acceleration forces outside of the lifter’s operational envelope, causing them to bind and seize. These failures can usually be traced back to an oil supply issue due to a malfunctioning or clogged VLOM causing improperly timed activation and deactivation.

If you haven’t caught on yet, the main point of Melling’s tech tip video is that the oiling system associated with the AFM lifters is an absolutely critical component and needs to be checked an maintained when an issue arises, as it is the source of a large number of the failures seen in the modern Displacement-On-Demand-equipped LS engines.

By identifying and treating the root cause of the failure (or possibly preventing it in the first place, if you happen to be overhauling an engine with AFM) you are solving the actual problem. If you simply replace a failed or worn lifter, you are only treating a symptom, and will likely do so multiple times, until the root cause is addressed.

The active fuel management system is designed to disengage the lifter at the point of least load in the cycle of camshaft’s rotation, which would be while the lifter is riding on the base circle of the lobe. If a VLOM filter is clogged, it could cause a delayed building of pressure, getting to the unlocking pressure late in the cycle, while the lifter is being accelerated on the cam lobe. Decoupling anywhere other than the base circle can cause a violent collapse of the lifter instead of just disengaging the internal lock, causing the whole assembly to bind and seize, creating a collapsed lifter situation.

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About the author

Greg Acosta

Greg has spent over a decade in automotive publishing as Senior Editor of Race Pages magazine. In his free time, he is a firearms instructor and volunteer in the police armory.
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