You replace the serp belt when its worn. If you aren't sure when it was done last... just replace it. A quality belt is only $30 or so (same as a GM 3800 V6, so they're easy to find). Also give your belt tensioner a long hard look. It should have no axial play, and the damper should still be in good shape.

-J

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1991 350SDL. 230,000 miles (new motor @ 150,000). Blown head gasket

Tesla Model 3. 205,000 miles. Been to 48 states!
Past: A fleet of VW TDIs.... including a V10,a Dieselgate Passat, and 2 ECOdiesels.
2014 Cadillac ELR
2013 Fiat 500E.