You may be attacking the wrong part of the problem. With the 19mm retainer bolt loose and you cannot get the adjuster to increase the tension, you may have either broken the adjuster off of the collar at the rear of the tensioner or...the rubber insert inside the tensioner body has torn loose. First is easy to fix about $25 for adjuster and new collar, second will require a new tensioner body About $115 or so. Good luck..

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Jeff Lawrence
1989 300e
2000 Dodge Grand Caravan SE
No matter what you fix, there will always be something else to fix..
"Warranty" is just another way of postponing the inevitable.