Anyone have any prior experience with an engine lifter that bleeds down while sitting in the hanger for several weeks? One partner fired up the engine and complained about a “noticeable
knock”. When I fired it up a few days later to see if I could hear it, the engine ran great. We’ve been speculating that perhaps a lifter had bled down prior to the event. He was sure it wasn’t a fouled plug, but that was my first thought. BTW, this is an IO-360A1B6 with 200 hrs since overhaul. We tested but didn’t replace the lifters at overhaul, all checked out fine. Any other opinions?
Bob Steward, A&P-IA:
If in fact you have a hydraulic lifter that is bleeding down, the tapping ought to immediately go away when oil pressure builds. If you can hear the tapping after the OP has come up (10-20 seconds after start), then you have additional problems. Can you still hear it after the engine warms up? Some problems with excess piston skirt clearance can make a “knocking” sound from the piston slapping the cylinder as it rocks in the bore until it warms up and expands enough to slowly muffle the sound.
During the Lycoming recommended SB 388B “Valve Wobble” check, one can and should consider checking the hydraulic lifters and checking and setting the “dry tappet clearance”, which unfortunately is a VERY misunderstood measurement. I have long lost count of the number of engines I’ve worked on that had a valve/tappet problem after a cylinder was installed in the field. In many of these cases the mechanic used the original pushrods without checking to see if the proper lifter pre-load was set.
There are other possible causes for a persistent tapping at idle and may go away as RPM builds. A failing impulse coupler can do this.