Still appreciate the response even though (or because) it's moved into the philosophical (or methodological). I love the paradoxical and counter-intuitive. There is some sense in which the more we know the less we understand. My interest in this issue devives from my experience with our family's 10 123s. The one that sounds the best, is the slowest, is basically driven around town, has cost the least in repairs over the last two years is my daughter's 240D. It's the one that started burning up the last glow plug because (according to Bosch tech) of carbon build up. Real solution? Remove the pre chambers and clean or replace. Short fix? Newly redesigned fast glow plug kit. It's been fine for a year now while she putters around town. There is always that tension between what we know and what actually happens. Kind of the art and science of life.
Thanks again for taking a shot at an ongoing question.
I'm sure we'll talk more about other issues.

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Dan Stadt
'85 300D turbo
'79 300SD
'73 220D