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ATF = Automatic Transmission Fluid
a lifter sits between a camshaft lobe and a valve. cars with solid lifters must be frequently adjusted for valve clearance. cars with hydraulic lifters use oil pressure to 'pump' a lifter up to proper clearance.
when an engine ages, hydraulic lifters can gum up a bit, causing them to sag. this creates a gap between the lifter and the camshaft. this gap is the source of 'valve tick.' the ATF can, in some circumstances, clean the lifter to a point where it functions properly again.
additionally, saggy lifters can be caused by low oil pressure (worn out oil pump, worn bearings, etc).
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