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The timing belt tensioner should be spring loaded and self tensioning.
Only the 3.2's are self adjusting. The MTX cars you need to set yourself.
You can find them at shource.com, rcmautomotive.com, advance auto has them or post a WTB here on the forum.
The 3.0 are spring loaded and according to the ShoNut 60K video, the spring sets the correct tension on the belt. I also have used this method personally quite a few times. They are not self adjusting....meaning that they do not adjust over time if the belt were to stretch with age.
If the 3.0 self tensioner is not working then either the spring is missing or (which is more likely) the pulley rotated backwards when the timing belt was taken off causing the spring to uncoil.