There's three things to look at for a squealing belt. Newer cars use thin, flat serpentine belts that drive some things on their backside. There will be a squeal if any pulley is turned or tipped or if any pulley is forward of the others by as little as 1/16". Any of those conditions makes the belt slide across the pulley as it goes around it, and that's where the squeal comes from. Most engines use a spring-loaded tensioner pulley to set the correct tension. Sometimes they get rusted tight. With the engine not running, grab the belt and tug on it. You should be able to make the pulley move, then it should spring back freely and put the belt back under tension.
V-belts can tolerate some misalignment, but when they squeal no matter how tight they're set to, it's almost certain the belt is too thin. That causes the bottom of the belt to ride on the flat center of the pulleys. There's no way it can grip enough, especially for the generator, power steering pump, and AC compressor which take a lot of power to turn. V-belts need to be wide enough that they get wedged into the sides of the pulleys.
There is never a good reason to use any kind of belt dressing. People use those products when they don't know how to correct the problem. No car ever came from the factory with belt dressing, and they never came with a belt squeal either. Belt dressing isn't needed once the underlying problem is corrected with a V-belt, but it actually causes problems with serpentine belts. Debris and road dust will get embedded in the belt when it sticks to it due to that dressing. That debris will cause more of a squeal. The only fix for a serpentine belt that has been attacked with dressing is to throw it away, scrub all the pulleys with an engine degreaser or soap and water, then install a new belt.
Sunday, April 19th, 2015 AT 9:17 PM