Originally Posted by Infinite

most modern automotive oiling systems don't have one way check valves. And per our FSM, none exist anywhere in the system. Oil pumps have a pressure check valve that circulates excess oil back around the pump to keep pressure in check.

All oil pumps have a minimum operating rpm (i haven't datalogged rpm vs. oil pressure so I can't give a concrete number) but lets say hypothetically that limit is 200 rpm. During shutdown as engine rpm drops past that number, the pump isn't spinning fast enough to pull oil from the pan and pressurize it when this happens. And the fact that the lifters on our cars are basically the last thing in the oil passage circuit, they lose pressure first (the most important thing to keep oiled regardless are the main and rod bearings, which is why they are almost always first in the circuit, they pretty much keep residual pressure during shutdown and the starter spins the motor fast enough during cranking to deliver small amounts of oil pressure until the motor turns over)

Hydraulic lifters use oil pressure to force a piston against a spring which pushes down on the roller taking up the lash between roller and valve.


This is a cutaway of an LS v8 HLA (hydraulic lifter assembly) but they are all the same in construction and basic principle of operation.

When oil pressure drops on shutdown, the spring forces the piston in the lifter to its full retracted position. This causes excess lash between the lifter and valve, causing the chatter you hear on shutdown.