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The engine in our cars is not only responsible for producing the energy needed to turn the wheels. It also generates the power needed to run other components in our vehicles so that we will have a more comfortable, safer, and definitely more enjoyable ride. But what if the system that connects these components to the engine turns bad or starts to fail?

car engine

The Idler Pulley

We said that the engine is the power source of our car, quite literally. On the engine are drive belts that are connected to various components such as the air conditioning compressor, the power steering pump, the alternator, and the water pump. Each of these components requires energy to operate and they have to draw this energy from the power produced by the engine.

Since the energy produced by the engine comes from the linear (typically up and down movements) motion of the pistons, this has to be transmitted to the crankshaft to convert it into rotary motion. The drive belts that are connected to the engine also move around as the crankshaft rotates. As these drive belts run they also power the various components that rely on engine power like your power steering pump, water pump, air con compressor, and alternator. Without the engine drive belts, there will be no mechanism to transmit the rotary motion of the engine crankshaft into power needed by these components.

The engine drive belts follow a specific route in a very specific manner designed by the car manufacturer to optimize the energy transmitted to the various components or accessories. These drive belts have to be guided and routed in such a way that they do not interfere with one another as well as other engine bay components nearby. This is the function of accessory belt pulleys.

Cars have 2 types of belt pulleys, the idler and the tensioner. Some cars have only one of each type, although there are those that may have more than one idler pulley. The idler and tensioner pulleys look the same and function in the same way, too. They remove the often-annoying engine drive belt vibration that generally occurs if there is too much slack in the drive belt system. The difference between the two is that the idler pulley is mounted on a fixed structure while the tensioner pulley is mated to an adjustable device.

Specifically, the idler pulley provides a smoothly and efficiently rotating point for the car’s engine belt drive to move and loop around, preventing unnecessary slack. This also helps in increasing the tension on the belt, allowing the engine to deliver power to the different accessories in a more efficient manner. The idler pulley also functions in the prevention of flat belt drive slippage. As the name implies, a flat belt is one that has a relatively flat surface on the entire length of the belt. The pulley prevents this belt from ever slipping out of its position. The idler pulley may have a smooth outer wall, but the edges typically have retainers while some may have a middle groove to keep the drive belt in place.

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How to Tell If You Have a Bad or Failing Idler Pulley

We know that an idler pulley rotates on its own shaft, moving the engine drive belt along its route. As such a bad or failing idler pulley can give you the following symptoms.

Squealing

How many of us always make it a point to inspect our engine drive belt system every time we roll our cars out of our garages? Chances are very few. While we do check for our brake fluid levels, transmission fluid levels, and radiator fluid levels, very few of us will look at the integrity of our drive belts, much less the pulley. As such, the only time we may be alerted to the presence of a bad or even failing idler pulley is when we start our engine and we get that ear-splitting squeal coming from under the hood. Squealing is one of the very first signs that will alert us to a problem, although the problem may have already existed much earlier.

Remembering what we said about the idler pulley ensuring the proper route for the drive belt, a squealing noise is almost always indicative of the bearings of an idler pulley really going bad. The bearings allow for the smoother rotation of the idler pulley on its base or bolt in pretty much the same way as your wheel bearings allow the wheel to rotate freely, frictionless, and quiet. Having said that, it is also possible that other noises may be heard, such as rumbling and clattering.

Not only are the bearings of the idler pulley at fault if you hear squealing sounds under the hood. It is also possible that the surface of the pulley is already worn. Because the surface is no longer smooth, it can also produce noise. The pulley can also seize or freeze. And since the drive belt is still technically moving, it will be rubbing against the motionless pulley. This rubbing creates the squeal.

Visible Wear

For those who religiously check everything in their vehicles before they get on the driver’s seat and turn on the engine every time, visible wear on the idler pulley itself should already help you isolate the problem. But there’s a trick. You have to know where the idler pulley is located and what it really looked like when new. This is to help you make the conclusion that there is already significant wear on the pulley itself.

What you will be looking for are scuff marks, scoring, or any other signs of wear especially on the surface of the pulley. This is quite expected since the pulley is in constant contact with the drive belt. Over time, the contact surfaces eventually wear out. That being said, it is also likely that the belt itself will also be worn. So when you check the pulley make sure to check the drive belt for signs of wear, too.

Frozen Pulley

We mentioned that the pulley rotates on its shaft courtesy of ball bearings. If something is wrong with these bearings there’s a chance that the pulley will not rotate on its bolt anymore. This is what we call frozen pulley. It gets stuck. It won’t spin. Or, it may still spin, but not as smoothly as before.

To find out if you have frozen idler pulley or a pulley that is hard to spin, you will have to slip or remove the engine drive belt from the idler pulley. Observe caution when removing the drive belt off the idler pulley. Make sure the engine is not running. Remove the car key from the ignition just to be sure. Once you’ve done that, give the pulley a spin using your hands. Ideally it should spin freely. If not, or if you notice that it is quite difficult to spin on its shaft, then you will need to replace the idler pulley.

vehicle engine

Belt Travel

We said that the idler pulley should be able to route the engine drive belt in its correct loop. That is why if you look at the engine belt while the engine is running at idle, it should move smoothly along the system. If you notice that it is somehow wobbling or moving from side to side, then you’ve got a really bad idler pulley. You really need to get a new one.

This typically occurs if there is already significant deterioration in the idler pulley hardware. This can lead to a rather loose pulley, unable to maintain the tension on the drive belt system so that the drive will not slacken. This can also push the drive belt off the idler pulley resulting in belt travel. It is also possible that the middle groove on the idler pulley has grown wider. This can also destabilize the belt’s movement. Whatever it is, it’s something that you need to remedy.

Damaged Pulley Bracket

Sometimes you may have a perfectly good idler pulley complete with fully functional bearings and hardware. Unfortunately, the bracket upon which the pulley is mounted on may have come loose or has gotten warped. This mounting bracket is secured by a bolt onto the engine. Vibrations can loosen the bolt over time, especially if it was tightened when repairs were made. Since the idler pulley relies on a stable platform to help in its function, a loose or warped pulley mounting bracket can also create problems.

Since the idler pulley is no longer stable or fixed onto its mounting, it wobbles and can force the drive belt off the path of the idler pulley. This can also lead to faster wear of the pulley system. The good news is that replacing the idler pulley mounting bracket is easy.

The idler pulley is an important component of the drive belt system of any car. Without it, you might not even be able to start your car, use your AC, and even steer the vehicle. Knowing the symptoms of a failing idler pulley or one that’s gone bad is crucial to keeping the integrity of this component.

Sources:

  1. How to Diagnose a Bad or Failing Idler Pulley? – YourMechanic
  2. How to Replace Your Car’s Idler Pulley -YourMechanic

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