The belt that drives the accessories on the front of your engine is routed around a number of pulleys. Some of these are attached to things like the alternator and power steering pump, and there is also a pulley on the belt tensioner. You may find that your car has a separate pulley, called the idler pulley, that solely functions to route the belt a particular way.
These pulleys all have bearings in them, and like all bearings, they wear out over time. Checking for wear is simple. A visual inspection is a good first step – if you can see the pulley making any side-to-side movement while the engine is running, the bearings are wearing. You can also hear it when they are worn, as they can produce a rattle, which is particularly noticeable at idle. If your car regularly throws its belt, the bearings may be worn to such a degree that you need to replace the pulleys. Often, however, simply cleaning and repacking them with new grease is enough to give them a second lease on life.
Begin by removing the pulley. This is usually quite a simple task. On my Subaru, for instance, both the tensioner and idler pulleys are held in place with 14mm bolts. If I remember correctly, my SEAT uses a bolt with a torx head. On both cars, I could get at them without having to remove anything else.
A protective cover sits over the bearings, and can be removed using a small flathead screwdriver or a pick. Carefully and gently pry up along the inside edge (indicated by the arrow) and it will come up. Don’t be aggressive, or you will ruin the cover – it keeps the grease contained.
Once you’ve removed the cover, use a liberal amount of brake cleaner while rotating the pulley to remove the old grease and dirt.
Then simply repack it with new grease. I like to put a dab on my finger and press it into the bearings all the way around, and then rotate the pulley to work the new grease in. Do this a few times and you will have used a sufficient amount. Once you’re done, snap the cover back into place. If you have used enough grease, you should feel a lot more drag, and it will be more difficult to rotate the pulley by hand.
The whole process takes five minutes, and once you have reassembled everything, you should notice that any rattles or wobbles you might have had will have disappeared.
It is true that a new pulley is not terribly expensive – a new one for my Subaru would have cost £25. But at the same time, the slight play in the bearings is only minor wear, and regreasing the bearings virtually eliminates this. It certainly will not fail, and will provide many more miles of service. Sure, if this were a performance vehicle, I would be more inclined to spend the money for a new part, but as it is a daily driver that I am running on a budget, every £25 I can save is worth it.