Understanding Your Car's Idler Pulley

March 20, 2025

4 min read

March 20, 2025

4 min read

Your vehicle’s idler pulley plays a basic but essential role. It keeps the serpentine belt properly aligned and moving correctly, while reducing wear and tear on the belt. If the idler pulley fails, the serpentine belt can break or slip off, bringing your vehicle to a halt.

See our detailed step-by-step instructions and video about how to replace an idler pulley - opens in new window or tab. . Also, read our idler pulley buying guide - opens in new window or tab. .

What an Idler Pulley Does

A close-up view of a weathered engine, with the idler pulley in the center, the water pump pulley on the left, and the alternator pulley on the right. — The idler pulley on this engine is located between the water pump pulley on the left and the alternator pulley on the right.

Many essential vehicle systems run off of the engine. There’s the AC pump, alternator, power steering pump, and water pump. They all operate via belts that snake around the front of your engine. As the engine turns, the belt turns. As the belt turns, these pumps turn.

The belt must stay in tension. A belt tensioner does that job. It is either fixed or has a spring to push it into place.

The belt also has to follow the correct path. That’s the idler pulley’s job. It maintains and adjusts the tension on that belt so all the different components get power efficiently and smoothly. It also guides the belt around any sharp turns or changes in direction around the engine compartment to reach other components.

When to Replace an Idler Pulley

A mechanic wearing a black nitrile glove removing the serpentine belt from the idler pulley. — After the belt tensioner is loosened, the serpentine belt can be slipped off the idler pulley.

An idler pulley lasts between 50,000 and 100,000 miles. If you live in a hot climate that requires frequent AC, expect less life from the idler pulley. Most of the time, the serpentine belt wears out before the pulley, so when you replace the belt, also check the pulley’s condition.

We do not recommend driving with a bad idler pulley. If your serpentine belt is too loose, it can wear out and snap. When that happens, essential components like the water pump stop working. Problems with the engine failure could soon follow. It’s best to replace the pulley as soon as you notice an issue.

Symptoms of a Bad Idler Pulley

A driver hears something odd coming from the engine compartment. — Chirping or squealing noises from beneath the hood can indicate a worn-out idler pulley.

One of the most common signs that your idler pulley has started to wear out is a chirping or squealing noise coming from the engine compartment. If it’s not at the right tension, the belt won’t turn your various pulleys. Instead, it will slide over them and make these sounds.

You may notice that squealing starts or gets louder when you turn the wheel or turn on the air conditioner. If the serpentine belt runs the power steering pump or AC pump, check the idler pulley.

If your idler pulley goes bad, you may also notice excessive belt wear. Check the belt periodically for fraying or cracking. Poor tension or bad alignment wears the belt prematurely. You’ll see the damage right away.

If you notice belt damage, rotate the idler pulley by hand and check for any rough or loose movement. That will indicate that it needs to be replaced.

What to Expect When Replacing an Idler Pulley

A mechanic wearing a black nitrile glove loosens the idler pulley bolt with a socket wrench. — Removing an idler pulley doesn’t require special tools. But you should use a torque wrench to tighten things up.

For a complete step-by-step article and video, check our guide to replacing an idler pulley - opens in new window or tab. .

It’s a simple DIY job that takes between 30 and 60 minutes. It requires disconnecting the battery for safety, relieving the tension on the belt, unbolting the idler arm, and replacing the pulley. What takes the most time is getting to the parts.

How much should you spend on replacing an idler pulley?

Depending on the quality of the part and its availability for your car’s make and model, you can find a new pulley for somewhere between $20 and $50. If you are also replacing the serpentine belt, expect to spend between $25 and $75 extra. For more information on how to save some time and money with a kit, check out our idler pulley buyer’s guide - opens in new window or tab. .

There more info on the eBay Motros Blog:

About the author

Michael O'Connor

Michael O’Connor is a Bay Area writer who grew up restoring classic cars. These days, he works on his 1979 Fiat Spider. Michael is an alum of Sonoma State’s Creative Writing program. He began professional auto writing in 2015, contributing to AutoList, CarGurus, Endurance Warranty, Safe-Guard, and other car sites.

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This article is meant to provide general guidance only. Automotive maintenance, repair, upgrade, and installation may depend on vehicle-specifics such as make and model. Always consult your owner's manual, repair guide for specific information for your particular vehicle and consider a licensed auto-care professional's help as well, particularly for advance repairs.