WHat is the best way to cure a squealing fan belt? My fan belt doesn't squeal when idling or accelerating while driving, but squeals loudly if I rev the engine while idling in park. The belt is fairly new and the pulleys are aligned properly. No power steering so it's only driving the water pump and alternator. The belt feels like it has the right amount of tension on it as well but could it maybe need a little more tension/slack?

You can media blast the pulley grooves to restore some texture to them, it will help dramatically. It's an often overlooked but effective step in set-up.

I've also used some 120 to 150 grit Emory cloth to cut the shine from the pulley grooves, which gives the belts some bite.

If you have just changed the belt, lightly scuff any shine off it with the same paper, then clean well and reinstall.

Use quality belts and be sure to tension properly.

I've made special belt tensioning tools out of turnbuckles with welded "T"'s on the ends of the bolts to fit securely in the "V" grooves to aid in getting the belts tensioned properly.

Are the pulleys powdercoated? is the belt oversized in width?, meaning it's not sitting down in the groove far enough?, these things can contribute to a squealing belt

Napa has belt dressing, you cant see it and it leaves no residue.

Just gives the belt a bit more grip and lasts a long time.

The worst thing on our old junk is over tightening.

https://www.napaonline.com/p/MCR1397

Overtightening can ruin the alternator. Been there, done that.

Make sure you have the correct width fan belt. Too narrow or too wide can cause problems. Is it a Quanta Belt?

Cogged belt?

Ed

Dry bar of soap rubbed on the sides of the belt should cure it. It's not a cure-all for being too loose, or the wrong belt.

Soap does sometimes work, but I have had better luck by rubbing a candle on the belt. If there are no overriding problems, the wax will quiet the squeal.

Pulleys wear out over time. If the pulley is worn the belt wont sit in the groove correctly. There are pulley gauges available to verify the pulleys are good.

Sounds like a responsive 440 with a engine driven fan. It is basically a challenging thing to overcome.