All this leads me to the question: what causes compression to reduce and eventually disappear (rendering the engine dead)? Expansion and contraction of the combustion chamber walls through the years? Can anyone here clearly explain?

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1999 Mercedes-Benz S600, 103K miles - garage queen
1988 Mercedes-Benz 560SEL, 89K miles - daily driver
2007 Hyundai Sonata Limited, 31K - daily driver