The other morning I started my '98 GT and I heard something under the hood. I checked it out, and the serpentine belt had started to fray. One loose thread was hitting my radiator hose (and anything else in it's path).

I shut it down and drove the '06 to work. I bought a replacement belt. My brother and I installed it today. But once we got it on and started the car, we noticed the crank pulley is wobbling, like it's loose. I shut it off, and saw that the belt had slipped off the pulley. It was seated properly before I turned it on. The pulley doesn't feel loose, but it also doesn't look like it's in alignment with the other pulleys.

Does anyone know what's causing this and how to fix it?

 

the pulley is made on to the harmonic balancer on the 4.6 the rubber around the center hub could be coming undone throwing the pulley off and since the 4.6 is an internal balance motor there shouldn't be any vibration if the balancer fails so check that