1992 Buick Century Serpentine Belt Issue

Either 1) something is out of alignment, one of the pulleys 2) it is not tensioned and simply comes off ..I don't know what kind of tensioner that Buick has..spring loaded or what,....you are using the wrong belt. (I learned this trick from Tom) with the belt off, take a string and route it around all the pulleys where the belt goes and measure it, compare it to the belt and it should be the same. Also put the belt on one pulley and make sure it fits in, does not hang over the edge and fits perfectly in the pulley has the same number of ribs and grooves. When installed push on the belt (engine off) and it should give just a little bit, feel tight.. Look at all the pulleys and try to see if they are aligned, the belt needs to be straight, no sideways offsets. And you are putting it on with flat side on flat tensioner, and ribs on pulleys, not flat side of belt on ribbed pulley...make sure the used pulley you bought is the right one, a wrong one one may not align .... or...there is a bad bearing in one of the components and a pulley is wobbling

Sounds like the wrong belt- where did you buy it? You could always buy one from the dealer to make sure- these answers above are the best- but here's another one- tow the car to the dealer and spend a little more money- guessin' and replacing parts and gettin' aggravated is expensive, too