In my experience with diesels and living at a high altitude, the thinner air brings out the worst in diesels which as you know primarily rely on compression. If you have lower than optimal compression or injector spray pattern, you may not notice anything at lower altitudes but you probably will higher up. Sometimes all you can do is live with it. I would not recommend adjusting the timing just for temporary changes in altitude.

For the record I didn't read or even see your previous 2 threads.

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2004 VW Jetta TDI (manual)

Past MB's: '96 E300D, '83 240D, '82 300D, '87 300D, '87 420SEL