Anyone Else Have Issues With Drive Belt Coming Off

[OpenRoad]

I have had my serpentine drive belt come off the pulleys on my 4.0L V6 engine three times now.  Every time it occurs at engine shutdown, i.e. everything is fine during a normal drive but discover the problem next time I go to start the engine.  Luckily it hasn't happened while driving as I would loose power steering assist and the situation could become very dangerous.  I replaced the drive belt about 20k miles ago as it was making noise (thought it was a bearing issue but turned out to be the drive belt).  The issue with the drive belt coming off has only occurred since the new replacement was installed.  Visually the drive belt looks in excellent shape and the drive belt auto-tensioner pulley is within spec per the shop manual.  It makes no sense to me why this has only occurred at engine shutdown but I am also happy this is the case and not while driving.  The first time it happened I found a dead bird in the engine compartment and the second time a chunk of ice.  So I attributed the belt jumping off the pulleys  to foreign objects somehow getting under the belt and throwing it off.  This last time there was no evidence of anything strange in the engine compartment.

Obviously this is a concerning issue for safety reasons.  I am going to replace the drive belt even though it looks fine.  Anyone else have drive belt issues or thoughts on the issue?

Thanks.

[radin2son]

Why not replace the tensioner first? 

[OpenRoad]

I was thinking about that too.  There is an indicating line and scale on the side of the tensioner that shows whether it is keeping the belt tight.  The line is well within the range of the scale and also the tensioner seems to flex smoothly when I have had to rotate it to put the belt back into place.  So it seems good but as long as I am messing with replacing the belt I might as well replace it also.  I am baffled as to why this only happens at engine shutdown but should count my lucky stars that this is the case and not while driving in heavy traffic on the interstate.  Thanks for your response.

[crusty]

Check for worn bearings (fan, idler, tensioner, alternator, etc.)

 

When an engine shuts down, sometimes the engine spins back a little bit.  A cylinder coming up on compression but doesn't get past top dead center.  The compressed air spins the engine backwards a little bit.  maybe a quarter of a turn?  If a bearing is off and when moving backwards it may cock sideways enough to toss a belt.

[OpenRoad]

Thanks.  When I have the belt off I was going to check all the pulleys to see if there is any play in them.  When the engine is running there are no odd sounds and visually all the pulleys seem to run true, i.e no discernible wobble.  Probably won't be able to get to it until after Christmas.  Was planning on replacing the belt, tensioner and idler assembly.  These are the low hanging fruit items, easy to replace and relatively low cost so around <$200 for OEM equipment.  May be awhile before I find out if this makes a difference as 1000's of miles and months pass between when the belt comes off.