Quick, not-very-exciting update: Bodhi is still with my mechanic. I left it with him over the weekend so he could drive it a bit, look it over and give me his opinion. When I called this morning I talked to one of his assistants and he relayed that my mechanic had not found anything wrong, but he did think it wandered a little. He said he wants to keep it for another day to try something. So, I'm glad he didn't find anything terribly wrong, but on the other hand frustrated not to have an answer.
Of course if I know nothing major is wrong, I can proceed with tweaking when I get it back. I'll be interested to hear all my mechanic's thoughts though.

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1987 300D, arctic white/palomino--314,000 miles
1978 240D 4-speed, Euro Delivery, light ivory/bamboo--370,000 miles
2005 Jeep Liberty CRD Limited, light khaki/slate--140,000 miles
2018 Chevy Cruze diesel, 6-speed manual, satin steel metallic/kalahari--19,000 miles
1982 Peugeot 505 diesel, 4-speed manual, blue/blue, 130,000 miles
1995 S320, black/parchment--34,000 miles (Dad's car)