Reliability? Some consider the 606 the most reliable/durable of the 60x series, and it certainly is the most powerful. Issues can include stuck glowplugs owing to the DOHC head (GP's are extra long), however, in actuality it isn't a rampant problem.

On the other hand, you will most likely encounter leaks from the engine bay fuel lines. Plenty of reading material here on that. Make sure you test drive it first, there are worlds of difference between the 606 and 617.

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99 E300 TD -- sold
01 540i 6 spd