Hi I have a 1992 d250 with cummins engine. I have a belt squeal when I first start it up. It goes away after about 15 min. I have replaced the belt it was fine for a couple of weeks ,then it started to do that again. All the pulleys look in line. I have a new water pump. Tensioner is tight. Please let me know what you think.

 

Check all the accessories, make sure that they spin freely or with minimal resistance. Usually belt squeal after startup until it's warmed up can be caused by the alternator, fan, or tensioner. Make sure all these spin freely, no tight spots, no burned spots on the pulleys, etc. If it starts hard, then it might be the batteries are getting old, and the alternator is working hard to charge them after it starts.
Also, with an auto tensioner, "tight" is a relative term. I think the spec testing with an actual belt tension tester is in the 15-20lb range.

 

try some belt dressing. it comes in an aerosol can. just spray a little on while the engine is running.it may be just a temporary fix. had an alternator going out a few years ago which caused mine to squeal under acceleration. you may wanna look into that or any other accessory

 

I wouldn't us belt dressing evan if it was just to sell the truck! JMO
If you can get hold of a heat gun,the one that shows heat not make heat!
after the cold start belt squeal has stopped,shut he eng off and start checking pulleys for heat.....you will find the problem

 

I had the same problem when I bought a belt at autozone. Within a couple of days it startd squeeking and I must have checked everything and couldn't find the problem. I can't remember why but for some reason while working on the AC I put the old belt back on and it hasn't squeeked since. Our trucks don't like cheap belts.