Engine pulleys or idler pulleys perform the task of guiding and tensioning the engine drive belts. They are also well-connected with the alternator, water pump, AC compressor, power steering pump, etc. One can know if they have gone bad from the following signs:

• Visibly worn-out pulleys
• Squealing noises from the belt 
• Coming apart of seizing of pulleys
• Overheated or frequently stalling engine

The average lifespan of pulleys is about 1,00,000 kilometers if you are a careful driver. But when the pulley wears out, it needs to be replaced instead of repaired. Another sign to identify the need for the replacement of pulleys can be the engine warning lights.