Modern piston engines maintain 95% of their compression for the life of the engine. In fact, piston/cylinder/ring design technology has come so far that this part of the engine just doesn't wear out anymore. These days, you can tear down a modern engine that has logged 300,000 miles of use and find that the cylinders still have the machining marks on the walls—they often look virtually brand-new, which is truly amazing. Thus, a compression test is not a very good measure of the overall health of the engine (or a measure of how much "life" is left). That is a common misunderstanding, plain and simple. Good compression test results indicate that the engine does not have a compression-related problem at the moment—and that's about all.
The thing is this: Any mechanic worth his weight in salt should already know that something is wrong with the engine prior to doing a compression test. The engine will not be running smoothly—especially at idle—and the check engine light will be on, if so equipped.
Testing compression is really nothing more than a way to figure out which cylinder has a problem (because you already know there is a problem!). Once you determine which cylinder has a problem, then a leak down test is run to further pinpoint the issue. The three possibilities for low compression in a particular cylinder are:
1. Past the rings: damaged piston and/or cylinder and/or rings
2. Past the valve: burned valve
3. Past the head gasket: blown head gasket
Equipped with this information, the mechanic can figure out how next to proceed.
The next time somebody tells you, "It has good compression," you can answer, "So what?" If the vehicle was manufactured after 1990, and has a working check engine light, don't waste your time on a compression test. Just take it to a mechanic who knows his ass from a hot rock—that is a much better bet.