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How to tell if a timing belt tensioner is bad 
Listen. 
A bad tensioner will usually be accompanied by noise. This noise could be some type of squealing or rattling coming from the timing cover area.
Also, when your timing belt is loose, it will cause some drivability issues usually under high load or high rpm. If your timing belt is not properly tensioned it will not keep the valves timed properly to the lower end and this will cause misfires, loss of power, bucking, and could even cause a no start condition.