D.I.Y. MAINTENANCE: Idler Pulley Replacement

Do-It-Yourself car maintenance on,

Engine Idler Pulley Replacement

Replacing the idler pulley in a poly V-belt or serpentine belt system is a typically and simple DIY maintenance project. Replacement idler pulleys are typically inexpensive and the consequences of an idler pulley failing while driving on the road make this maintenance project a no-brainer.

Idler pulleys do not typically fail critically without warning. Signs like excessive dust and grinding or squeaking noises are normal indicators that an idler pulley has broken and requires attention. Since these pulleys are usually belted to other systems, such as alternators and power steering, having the idler pulley inspected and replaced at the early stages of failure is worthwhile in most instances.


Example of a broken idler pulley below:
Vehicle: Mercedes-Benz, 2001 SLK320

The driver noticed a squeaky noise coming from the engine in the Fall of 2012, so the owner opened the hood and pinpointed the source of the noise. Without a mechanic's stethoscope available to precisely pinpoint the problem without taking anything off the car, the owner narrowed down the area of the noise and made a best guess it was the idler pulley. Since the owner was able to drive the car around still and wasn't sure how difficult it would be to take off the serpentine belt, the owner didn't worry about it and left the broken pulley alone. It was not until the end of 2013 Summer that the noise got louder and fear of a failure escalated enough for the owner to take action.

The instructions from the website, 'Pelican Technical Article: Mercedes Idler pulley Replacement', explained and illustrated how simple replacing the idler pulley was. As the owner of this vehicle followed the instructions, here is what the owner found.

After the owner got the serpentine belt unloaded from the engine, the idler pulley immediately broke off into two pieces. Having the idler pulley break in this manner was fortunate because it allows a driver to drive to safety (or a handful more months in this case).

Figure 1: Broken idler pulley after the serpentine belt is unloaded.

Even though the idler pulley split into halves along one of the serpentine belt grooves, the bearing surprisingly remained intact and was able to spin freely. Additional pictures of the broken idler pulley taken out from the engine bay.

Figure 2: Close up, front half of the broken idler pulley showing black dust. This is an indicator of grinding between the two halves of the broken idler pulley.

Figure 3: Close up, front half of the broken idler pulley showing grind marks
(with flash on the right and without flash on the left).

Driving with the broken idler pulley in the engine led to a ground down, smooth surface shown above.

Figure 4: Close up, rear half of the broken idler pulley, showing the inner surfaces on the left and outer surfaces on the right.

The following picture compares the broken idler pulley to the ACDelco aftermarket pulley. Since the broken idler pulley bearing was still intact, the owner was able to perform a simple bearing test to compare the bearings. A free-spin test of the bearings proved that the OEM idler pulley was lighter and able to spin more freely than the ACDelco idler pulley. The ACDelco idler pulley was more robust and symmetrical in construction all around but the bearing has a larger rotational inertia, causing the bearing to come to a stop sooner.

Figure 5: Comparison between the broken idler pulley with the replacement ACDelco idler pulley.

Figure 6: Close up of the ACDelco replacement idler pulley. This replacement idler pulley is made symmetrically about the line of symmetry from front-to-rear and side-to-side.

Replacement Part Information
ACDelco 38082 Belt Idler Pulley
Total Cost: $27.83 (purchased from Amazon.com)