Idler Pulley Noise – Common Reasons & Information

In Engine by Magnus SellénLeave a Comment

bad idler pulleyIf you open your hood, you may notice that you have belts that are attached to the alternator, water pump, AC compressor, and the power steering.

To keep these belts in place there are idler pulley/s fitted. The idler pulley guides these belts so that they do not interfere with other engine parts and give a better grip to the pulley wheels.

Most of the cars have only one idler pulley but in some cases, you will find cars with more than one.

With time, you may start hearing a squealing sound coming from the engine.

Common causes of noise from the pulley

There is not necessarily always the idler pulley itself that is creating noise from the belt. Here are a few common reasons.

Worn out idler pulley

Like most engine components they do wear out with age. The idler pulley rotates the belts at a very high speed and will generate tear. A closer examination of the pulley will reveal cracks that when coming into contact with the belts will produce the squeaking noise. The most common fault with the pulley is worn bearings inside of it. 

Idler pulley slippage

Once the pulley gets worn out it may start rubbing itself against the engine’s belt. This will produce the squeaking noise which gets even worse as you continue driving the car. If not replaced immediately it may cause damage to parts around the belt if it fails. 

Squeaking noise from the belt should be taken seriously and repaired as fast as possible. 

Complete pulley damage

If you do not replace the pulley when the squeaking noises arise then you will start noticing more cracks forming on the pulley. This will spread to your belts and finally, the pulley can no longer be able to guide any of the belts. If the belt jumps off and you have the water pump driven by the serpentine belt, the engine will start overheating and stalling while driving.

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The only solution at this point is to replace the idler pulley together with the belt completely.

How it works

serpentine belt systemThe idler pulley is used to drive the belts in a car’s engine. Most often the car is fitted with one serpentine belt, but sometimes you have 2 or more belts fitted on your car. The serpentine belts do transfer power to the AC, Power steering, water pump and the alternator. This may differ a bit depending on what car engine you have. 

If the idler pulley starts malfunctioning then you are going to have problems with these accessories. The idler puller is consists of sheave with grooves from one end to another. The belts will be placed along the grooves on the pulley. When the crankshaft gets power it rotates the idler pulley and this turns the belts. Power and torque will then be transferred from the engine to the various accessories attached to the idler pulley.

The idler pulley is under enormous pressure and this causes it to wear out faster. When this happens you will experience more belt slips than normal. In extreme cases, the idler pulley will completely stop functioning and this will cause the car not to move. The idler pulley provides the right amount of tension for the driving belts.

Problems with idler pulley

When your idler pulley is worn out and the grooves are not holding on to the belts well you will hear a squealing sound coming from the engine. If the bearings are worn out then they will make a rattling sound. At this point, you should stop your car and diagnose the problem.

Most mechanics have a simple way of identifying a faulty pulley. They will often remove the belt from the pulley and then try to rotate the pulley by hand. If the pulley does not move freely or have a bad noise when turning, then they know that the pulley bearings are faulty and are in need of replacement. You should never attempt to touch the belt when the car’s engine is running. This can result in you losing some of your fingers.

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Here is a good video of how to spot a faulty pulley:

Another way to spot a faulty pulley is to switch on the engine and then observe how the belt moves. A functioning pulley should rotate the belt in an even way. However, if you notice that your belt is wobbling – especially when you rev up the engine – then your pulley is faulty. In this case, you may be forced to purchase a new pulley as it will continue adding unnecessary pressure on the belt.

When your idler pulley starts malfunctioning you will need to replace it fast before your car stalls in the middle of the road.

Modern cars have replaced the typical V-belts with serpentine belts. The tension on the belt has to be just right for the belt to function effectively. The pulleys are either made from plastic or metal with bearings in the middle. These bearings secure the pulley to the belt shaft.

If the bearing is sealed it can serve you for many years without the need for repairs but may it fail anyway after several years.

Replacing the pulley

When you are replacing the idler pulley it is also important that you replace the belt as well. The belt does also get worn over time and it gets longer and longer after some time. If you wait for too long, it will be too long for the pulley to be able to keep it tight.

The first thing the mechanic will do when replacing the idler pulley is to inspect the whole pulley system and check for every worn out part. They will listen to the noises coming from the engine to know what needs replacement. The mechanic will then ensure that the pulley is well aligned before letting you drive off with the car. You should request your mechanic to inspect all components being driven by the idler pulley to ensure that they are in good working condition before driving off.

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The idler pulley is vital for transferring power from the car’s engine to the alternator, air conditioner, or the power steering. However, with time the idler pulley will become defective and this will make it hard for it to rotate the belt. The bearings can also be worn out and in need of replacement.

Always check for other worn components when you are replacing the pulley. For example, I do always recommend to also replace the serpentine belt when you are replacing the pulley.

Hello I'm Magnus, the owner and the writer of this website. I have been working with cars since I was 16 and I'm specialized with in-depth Automotive diagnostics. Also been driving drifting for the last 6 years. I'm here to give you answers to all your automotive questions and I hope that you enjoy our content.

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