Did you use a genuine ford tensioner? If not, get that mofo out and put a real one in! Just spent the last two weeks dealing with what sounds like an identical problem. Also, did you prime the tensioner before putting it in?

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2002 XLT Supercab 4x4 4.0 Five Speed
Plasti-Dipped Rims
Painted Grille
T-Bars Cranked
Belltech 6400 Clones from Autozone