This van keeps slipping the serpentine belt off the crankcase pulley. When this happens, the van will still idle but the belt rides on the edge of the crankcase pulley. I've recently replaced the alternator and the tensioner pulley. All of the other pulleys are tight and look like they are in line. The belt also goes over to the side of the idler pulley but it doesn't slip off the idler pulley. Could the idler pulley be bad and need to be replaced? Again, it feels tight but could the bearings be bad and causing this?

I had one mechanic tell me that the crankcase pulley might have "worked itself out" a little bit and that could be throwing the belt. But I don't understand how that could have happened without affecting the pistons too. I have no other engine problems with this van.