QUESTION: During the past few months I've noticed a chirping noise from the engine of my 1999 GMC Yukon. Also, if I start the engine with either the air conditioner or defroster on, a much louder squealing noise occurs for about 30 seconds. Turning off the switch makes the louder noise go away. I've been told the problem is my belt. Would replacing it fix the problem?
--Tu Do
ANSWER: I agree the noise is likely originating from the serpentine drive belt used to turn the engine's accessories. That includes the water pump, alternator, power-steering pump and air conditioning compressor, if applicable.
Rather than using multiple slender V-belts as in the past, modern engines typically use a single, wide multiple-rib serpentine belt to spin these important parts. The belt gets this name because it snakes back and forth with the ribbed side driving some pulleys and the smooth back side contacting the others. Serpentine belts require less space and do a better job than multiple belts. The only downside: Should the belt ever break, you'd lose the function of all accessories, disabling the vehicle.
Serpentine belts can become noisy due to several possible conditions. Incorrect belt tension caused by a faulty tensioner can lead to squealing, particularly during air-conditioning use, sharp turns or acceleration. This is most common just after a damp or cold engine start.
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Chirping could also be the result of loose belt tension or pulley misalignment. Each of the engine's belt-driven accessories must attach to the engine perfectly parallel to each other, in order for the belt's ribs to glide in and out of the corresponding pulley grooves. Sometimes a replacement accessories' pulley may not be pressed on to its shaft to exactly the right depth, or an accessory's mounting surface differs slightly from the original. Another cause of chirping can be glazing on the belt's rubber surface. An older belt's friction surface often becomes hardened and shiny, and this can cause the belt ribs to vibrate noisily as they engage the pulley grooves.
Here's how to check for the cause of serpentine belt noise: Using a spray bottle of water and remaining at a safe distance, lightly wet the belt as the engine idles. If the noise diminishes, the cause is pulley misalignment or belt glazing. Should the noise increase, belt tension is insufficient. Removing the belt and reinstalling it in the opposite direction is another test. If the offending noise goes away, for a while, pulley misalignment is indicated. In many cases, replacement of the belt will cure noise, but it may come back, perhaps in a month, if the root cause isn't repaired.
It sounds like your Yukon's belt tensioner has lost its spunk. This is an easy bolt-on device -- Gates #38103, about $30-$50, depending on source. I'd also renew the serpentine belt; the part number depends on the air-conditioning system and the amp alternator. If any accessories have been replaced, a careful check of pulley parallelism should also be made. My favorite serpentine belts are the Gates Micro-V and Goodyear Gatorback.
If renewing the belt yourself, look for a belt-routing decal or sketch the routing prior to removal. Re-establishing the proper zigs and zags of a serpentine belt can be quite a puzzle without a roadmap. Keep the old belt as an emergency spare.
