The water pump tensions the timing belt? Do you have an official repair manual? On VWs with that tensioner setup, and other similar tensioners, you tighten the belt till you can only twist it some ammount, usually 90* for 8 valve engines and 45* for 16 valves and diesels.

-Jason

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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.