As a data point, I have never lost a tensioner on any of my CDIs. One with 302k, that I've had since 130k. Water pumps have gone, but not the belt tensioner.

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Mark in MA
05 MB E320CDI 402k Granite Grey Metallic
05 MB E320CDI 267k Black
05 MB E320CDI 232k White
05 MB E320CDI 209k Tectite Grey
99 Dodge 2500 Cummins 5sp 148k
62 Jeep CJ-6 120k