Alright folks I cant find the problem on the ZJ. Stock 4.0 with the bad design (IMO) tension adjuster, long bolt through idler pulley. The belt keeps coming off and wedging itself behind the water pump pulley. I checked all alignment and all pulleys for wobble/loose etc. All seem OK. The part where I am stumped is it only happens when motor is shut off. I can fix it drive 1000 miles no problem then every third or fourth time the I shut it off it slips behind the water pump pulley. And only behind it stays put on everything else. What the heck!!!

 

Anybody?

 

Make sure its routed and tensioned correctly. All the edged pulleys need to run on the inside of the belt, and opposite for the flat ones. Post a pic if you can.

 

You might check your crankshaft pulley (dampener) if you didn't already.. they sometimes come apart at the rubber and can cause the belt to hop off.

 

Weird. Do you have a solid fan by chance? (no clutch) or is your clutch frozen? My thought, and a long shot it is, is that the fan (or something is over-running and puking the belt.
It would have to be something with some mass. For example, the idle pulley has little weight and little momentum thus stops quickly when the engine stops. A fan has a lot of rotating mass.
I would have someone watch as you shut off the motor and see what the heck is going on.
Let us know, this is a good one.