I replaced a worn belt that was slipping with a Gates belt from NAPA along with the idler shortly after I got the truck (with around 143,000 miles at the time) and it frayed and came off the pulleys after about 900 miles. The replacement belt lasted about 2700 miles before doing the same a few days ago. After a visual inspection and turning them by hand, the pulleys appear to be properly aligned and in good condition – except possibly the power steering pulley. It has about 1/16 inch of play along the shaft; I can turn it by hand, but it does not spin freely like the other pulleys.

1/16 of an inch doesn't sound like much, but could that little bit of play cause the belt to ride up over the edge of the pulley over time? If that pulley is the likely culprit, I presume the issue is worn bearings in the pump itself and not just the pulley, in which case the power steering pump would have to be replaced together with the pulley. I can do the work; it doesn't look like much. I just want to be sure before buying the parts and getting my hands dirty. On the other hand, if it's not likely the PS pulley, then I'll take the truck to a local independent garage and have them check it with a laser. My eyeballs aren't calibrated as well as they used to be, so maybe one of the other pulleys is out of alignment.

 

I don't think 1/16 inch of play is enough to hop the belt. I remember have a small amount of play with my P.S.pump and it had some fluid drag to it as I rotated it. I don't think that is your problem.
Many years ago I had the same problem with my 4.9. I would look closely at the tensioning capability, the pulley bearing slop and alignment at the belt tensioner which is where my problem was. If that tensioner is original, it may be the problem.

 

Pulley issue. One of them is wobbling and walking the belt. The best way to observe this is with the vehicle running.

 

Tighten the torsion pulley...as tight as you think it is...it needs to be tighter.
Use a prybar to place tension on the belt/pulley then tighten it up.

 

The 4.9 uses an automatic tensioner. There's no prying involved in tensioning the belt. If its pulley is loose, that means the bearing in it is spent.

 

The tensioner was replaced in January 2021 with an OEM part before the belt I mentioned above that lasted 900 miles. I had ordered a Motorcraft tensioner and Gates belt from Rockauto when the old belt started slipping. The replacement belt was a closeout item and shredded within 25 miles of driving (really old stock I presume). I got a refund from Rockauto for the belt and then bought another from a local NAPA auto parts store, assuming at the time that the OEM tensioner was fine. The old belt that started slipping shortly after I got the truck had been on the truck for a long time and by the looks of it, the tensioner too. I inherited the truck from my uncle, so I am familiar with its history. The problem of belts coming off the pulleys didn't begin until after I replaced the belt and tensioner.

Is there any way to tell whether the tensioner is failing? It's a Motorcraft part, which you would expect to be good quality and last tens of thousands of miles. But I suppose it could be faulty.

 

in the first post it says you replaced an idler - what pulley is this, exactly? Sure it was exactly the same and its not spaced differently?

id fire the truck up with a new belt and watch it track from each side of the engine bay - you should be able to see where its walking, might even see wear marks with the belt off.

The tensioner will be moving but it shouldnt be bouncing all over the place.

 

I misspoke when I wrote "idler" in my first post on this topic. What I meant was the automatic tensioner. The old tensioner was replaced with an OEM part a few days before the belt I mentioned in my first post above that frayed after 900 miles.

 

I installed a new belt and observed it with the engine idling. From both sides and above all the pulleys look to be spinning smoothly and as far as I can tell the belt is tracking uniformly and not walking back and forth on the tensioner pulley. However, from up from under the truck the crankshaft pulley looks to be either slightly off-kilter or a bit wobbly. Does that point to a worn harmonic balancer, necessitating the replacement of that pulley?

 

My wife drove the truck to work because I needed to use her car at the time. It's a roughly 8-mile commute and the belt had already started coming off the PS pulley by the time she got home. It hadn't started fraying yet, so I think it's still good. When I installed this belt I checked to make sure there wasn't any debris from the old belt stuck in the pulleys, so I don't think it was because it wasn't seated properly. At any rate, it always comes off at the same place: the power steering pulley. And that wouldn't be the case if the crankshaft pulley was the source of the problem, would it? The PS pulley sits between the idler pulley on the tensioner and the AC pulley. So perhaps there is an alignment issue between the PS and AC pulleys that I'm not seeing. I'll try the water spray test, even though I haven't noticed any belt noise.