How to tell the difference between a slipping liner and a duff lifter?
By listening? Or is there a test?
Its easy, A tappet noise will occure from a cold engine. A loose liner will not (normally) and will only be audible when the engine has become hot, its this expansion of the Alluninium block that allows the steel liner not only adequate movement clearances but also allow it to tempoalaly break free.
A Loose liner will only be noticable (audible) when the engine reaches a specific temperature, the sound is likely to be pesistant, sounds slightly heavier than a tappet noise and also is less random. When the RPM is increaded the noise will stop as the piston out - accelerates the liner.
When the engine is cold, the process is restarted, otherwise returning to low rpm will always re-produce the sound.
Order of merit (Chances of loose liner)
4.0 and 4.6 1999 to 2002 ---- very very likely
4.0 and 4.6 1984 to 1999 ---- Very Likely
3.9 and 4.2 1989 to 1996 ---- Likely
All 3.5 engines ---- Less likely However such engines are now very old so possability level is rising.