Replace The Noisy Ac Idler Pulley on a 2005 Impreza WRX STI

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How to diagnose and replace a noisy AC idler pulley on a 2005 Impreza WRX STI. This article will generally apply to many Subaru engines as they don't alter the basic design of this pulley much over time.

My car started making a terrible whining noise at startup, combined with an intermittent janky rattle sound. This was very noticeable in drive throughs with the windows down. Initially I was worried this noise was a bigger, more costly issue (AC compressor?).

Fortunately, I was able to easily diagnose it (which put my worries at ease). If this is happening to you, follow my diagnostic steps - perhaps your noise is as simple as replacing this pulley!

This looks like there's a lot of steps, but I promise it's a quick job: It should easily take less than an hour.

Where to Start?

I'm not sure where the sound is coming from.

Start at step 1

I know it's the AC Idler pulley, I WILL replace the belt

Start at step 4

I know it's the AC Idler pulley, I I WILL NOT replace the belt

Start at step 6

Required Tools

10mm & 12mm sockets.
A socket wrench.
A couple of short extensions to make things easier.

Required Parts

Combined AC Idler pulley, Adjuster, and bolt kit (includes several parts listed below)
Optional: Plastic Idler Pulley Adjuster ring Subaru 73134AC001
Optional: AC Idler Pulley - Subaru 73131FC000
Optional: AC Idler Pulley Adjuster Bolt - Subaru 73790AC100
Optional: AC Idler Pulley Shaft Bolt - Subaru 73132AC000
Optional: AC Idler Pulley Hold Down Nut - Subaru 902380006 (flanged M8x1.25 nut)
Optional: AC belt - Gates K050345

Step by Step Instructions

Diagnosing A Bad AC Idler Pulley

Find a friend.

Although not entirely necessary, it can help to have someone listening while you start the car to help pinpoint the source and timing of the noise.

Open the hood, turn off your accessories (stereo, AC, etc.)

This will give you the best chance to pinpoint and isolate the source of the sound.

With your helper standing in front of the open engine bay, start the car normally.

Have them listen for when, where, and what kind of noise it is.

Mine sounded particularly bad at cold startup.

If the source of the sound is the AC Idler pulley, it should occur regardless of wether the AC is on or off.

Try turning your AC on to see if it changes the sound at all. If it's only noisy when the AC is on, the compressor or compressor pulley may be the issue.

Remove the Alternator & Power Steering Belt

If you know the source of your noise is the ac idler pulley, you can skip removing this belt.

We only need to remove it to:

Rule out the alternator and power steering as the source of the noise
Replace the AC belt (if you have a new one)

The AC idler pulley itself can be replaced without completely removing either belt.

Remove the alternator and power steering belt.

See my article on Replacing the power steering pump steps 1-6 for help removing that belt.

The car will start just fine with this belt removed. It won't turn or charge the battery, so only use this for testing.

Start the car with the Alternator belt removed.

You should only have to run the car for a few seconds.

If the noise doesn't change, continue to removing the AC belt.

If it's gone, the source of the noise is either your alternator or the power steering pump.

Remove the Air Conditioning Belt

Loosen the 12mm AC Idler pulley hold-down nut.

It's the nut that screws directly into the center of the pulley. Leave it threaded on to prevent you from losing parts in the engine bay.

Image 8736 from Replace the Noisy AC Idler Pulley on a 2005 Impreza WRX STI

If the hold-down nut is completely rounded off or rusted in place, it is still possible to remove the belt by removing the 2 bolts at the top of the bracket to remove the whole assembly.

Loosen the tension adjuster bolt.

Loosen the adjuster till the belt is loose enough to remove.

Image 8737 from Replace the Noisy AC Idler Pulley on a 2005 Impreza WRX STI

Don't adjust the tensioner unless the hold-down nut is loose. It's easy to put too much tension on the plastic adjuster ring at the bottom. Which will only result causing it to crack or break.

Remove the belt.

Start the car with the Alternator AND the AC belts removed.

Again, you should only have to run the car for a few seconds.

If the noise is the AC Idler pulley, it will be very clear that the noise is gone.

If your noise persists at this point, you may need to consider checking the timing belt and pulleys.

Replace the AC Idler Pulley

Remove the 2 x 12mm bolts holding the ac idler pulley.

The AC idler pulley bracket will come right out at this point.

Image 8738 from Replace the Noisy AC Idler Pulley on a 2005 Impreza WRX STI

Disassemble the bracket and inspect all the parts.

Check the plastic adjuster ring for cracks.
Check the idler pulley bearings for noise or any movement that isn't smooth.
Check the idler pulley shaft bolt for wear or play (it should fit snug in the pulley)
Check the idler pulley adjuster bolt threads for wear
Check the idler pulley hold-down nut threads for wear