My ex's 1999 Subaru Legacy (4 cyl, 126K miles) began making some nasty
noises in the motor a week ago. She drove it 2 blocks and it's been sitting
in the driveway ever since, while she ponders what to do. Except for
warranty work, It's been maintained by one very competent mechanic
throughout its life. He's hesitant to mess with it because he says that
evaluation could require major engine disassembly which could cost so much
that no sane owner would want to even go that far just to find out that the
car is toast. Car is in otherwise excellent condition.

So, ***if*** we were dealing with a spark plug which was broken due to
incorrect length, wouldn't that have happened almost immediately after the
plugs were installed? Assuming yes, what else are we left with? Right
length, wrong plug (temperature)? Wrong torque? Defective plug? Seems like
some things could point to the mechanic, as unlikely as that seems based on
his track record. Or, manufacturer.

Any other thoughts on this?