Why does the belt make squealing noise at startup?

This blog will explain the causes of the belt squealing noise on start-up and answer the following questions: how to replace the engine belt? Is it good to use a spray to get rid of belt squealing on start-up?

Why does the belt make squealing noise on start-up?

The belt makes squealing noise on startup because of the following reasons:

  • Worn out belt
  • Belt tensioner
  • Jammed idler pulleys
  • Alternator overrun pulley
  • Blown front crankshaft oil seal

Let’s take a closer look at each of the causes and their remedies:

Worn out belt

The engine belt or more commonly known as the serpentine belt is used to drive the ancillary on the engine. The belt is connected to the crankshaft which then drives components like alternator, power steering pump, and air conditioning compressors.

The belt is made up of rubber with grooves on the inner side, it gets worn out as the mileage of the vehicle increases. The grooves can develop cuts and the overall thickness of the belt reduces over time.

As the thickness of the belt reduces, the tensioner is not able to maintain the required tension in the belt. It starts to sleep over the pulleys which results in the squealing noise. Also, the belt is used to drive the coolant water pump.

If the water pump leaks coolant on the belt, it can make the material of the belt hard. Cracks can easily form and cause the belt squealing noise when the engine is started. Therefore, a visual inspection of the belt is performed to diagnose belt squealing noise on start-up.

The worn-out belt can overheat if it is not replaced. It can break down into strands and jam the pulleys of the water pumps and alternate. Hence if the noise is coming due to a worn-out belt, it must be replaced as early as possible.

Belt tensioner

A belt tensioner is used to keep the belt running tightly over the pulleys and absorb the vibrations in the belt drive. It consists of a pulley attached to a strong spring on one side. The tension in the spring is reduced as the mileage increases.

Similarly, the spring can also get jammed due to dirt build-up and it is unable to absorb small vibrations in the belt drive. The belt tensioner can be checked with a 17 mm flat ring spanner, the spring should not be too tight or loose.

Incorrect tension in the belt will lead to belt slipping or excessive belt wear. Both conditions can cause the belt to squeal noise. Many manufacturers recommend replacing the belt and the belt tensioner simultaneously, to prevent damage to the new belt.

It is important to lock the belt tensioner using a sliding pin, otherwise, the spanner can slip and damage nearby coolant lines.

Jammed idler pulley

There can be an idler pulley in the belt drive of the engine, idler pulley as the name suggests is not connected to any component. It is just a pulley with bearings and its function is to provide additional tension to the belt. 

Depending on the number of components on the belt drive, it is also used to guide the serpentine belt over the pulleys of other ancillary components.

The squealing noise can come if the bearing of the idler pulley is jammed. If the pulley is not replaced it can also make a loud noise. When the engine is started and the squealing noise increases with the engine speed, it is a good indication that the idler pulley is bad.

The idler pulley can be checked by removing the belt and rotating the pulley by hand, if the pulley does not turn freely; it must be replaced. There should not be excess play in the pulley, it can increase the vibrations in the belt and damage the bearings.

Alternator overrun pulley.

If the belt squealing noise is coming on start-up, the alternator overrun pulley is the most common cause, the overrun pulley turns freely in one direction when the engine is overrun. It protects the alternator from rotating too fast and overcharging the main battery.

When the overrun pulley is jammed it cannot free the wheel which creates more tension in the belt. In some cases, if the pulley is not replaced, it will break the drive belt. This leads to vehicle breakdown because the battery is not getting changed from the alternator.

All the pulleys cannot turn at the same speed as the crankshaft. Hence, if the overrun pulley is bad; the squealing noise on start-up can also come when the hydraulic power steering is turned to the end position. 

Blown front crankshaft oil seal.

This is applicable to only gasoline engines, a squealing noise that is very similar to the belt squealing noise on start-up comes when the front crankshaft oil seal is blown due to excessive pressure in the crankcase.

The pressure in the crankcase can increase if the positive crankcase ventilation system is not working correctly. This excessive pressure can escape from either the front or the rear crankshaft oil seal.

The engine oil can also leak from the damaged crankshaft oil seal. Hence before replacing the belt and the belt tensioner, it is a good practice to check the crankcase ventilation system. If the squealing noise goes away after opening the engine oil cap, it is a good indication that the oil separators are not working currently.

How to replace the engine belt?

The procedure to replace the engine belt is given below:

  • Open the engine hood and check the orientation of the engine.
  • Remove the front right wheel if the engine is in a transverse position.
  • Remove the engine cover if the engine is in a longitudinal position.
  • Draw the pattern of the belt on paper or take a photograph of the old belt.
  • Use an appropriate socket to pull lock the belt tensioner and remove the belt
  • Install the new belt and release the tensioner, rotate the crankshaft a couple of times to check if the belt is seated correctly.

Some manufacturers also use special tools to lock the belt tensioner. In older cars there is no spring in the belt tensioner, it is adjusted mechanically by tightening the adjustment bolt. Therefore, marking the position of the bolt before loosening it can help to maintain the correct tension.

Is it good to use a spray to get rid of belt squealing on start-up?

No, using a sticky lubricant spray on the belt can reduce the noise temporarily but it will not remedy the problem. In some cases, it can even cause the belt to break leaving the vehicle stranded.


This blog explained the causes of belt squealing noise on start-up. It is not expensive to fix the belt squealing but if the noise is ignored it can lead to expensive repairs. The engine drive belt is also not covered under the factory warranty.