Replace The Noisy Ac Idler Pulley on a 2005 Impreza WRX STI
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
- 10mm & 12mm sockets.
- A socket wrench.
- A couple of short extensions to make things easier.
- 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
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.
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.
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.
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
Re-assemble with whatever parts you are replacing.
From back to front:
- Shaft Bolt
- Plastic adjuster ring (line up the ear with the adjuster bolt)
- Hold-Down Nut
Do not tighten the hold-down nut
Install the newly serviced bracket.
Install the belt.
The AC belt goes around the inner crank pulley, AC, and idler.
Now'd be a great time to replace the belt if you need to.
Adjust the tensioner bolt till the belt is at the correct tension.
Tighten the adjuster hold-down nut.
Install the Alternator and Power Steering Belt.
If you removed the alternator & power steering belt, put it back on. Reversing the steps you took to remove it.
Loop the belt round the crank pulley, power steering pump, and alternator.
Adjust belt tension.
Tighten the tension adjuster hold down bolt.
Install the belt cover panel
Install the intake tubing
I'll throw in a few links to videos you may also find helpful here. Subaru engines are all very similar so here's a few fr
Subaru A/C Idler Pulley Replacement
Quick video showing removal and replacement on the same GD year STI as this article.
Idler Pulley Removal and Replacement - (aka tensioner pulley) Subaru Legacy GT and WRX
More complete video with examples of the sound sound diagnosis, removal and replacement, but on a different model and year.
How to Replace Idler Pulley 00-09 Subaru Outback
Another removal and replacement video on an Outback.
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