Still no answer to this problem but was unsuccessful in loosening or tightening up serpentine belt. Apparently the owner before me tried to do this w/o loosening the 19 mm. tensioner bolt. After struggling and getting it loose, the 13 mm hex nut just spins.
Suspect the hex head on the threaded shaft is stripped, but before I take off the air cleaner to get the nut off and ordering another, does anyone has any experience with whether I can expect the treaded shaft to be stripped too? If so, I'm in for a much harder project.