Tools and Supplies

Item Notes
Replacement idler pulley ~$37
Replacement bolt and plate You can re-use your old bolt and plate but it was cheap to get new
15mm wrench / socket For releasing the tensioner
14mm wrench / socket For idler pulley bolt
Torque wrench
2013–2019 Subaru Idler Pulley 23770AA07A
2013–2019 Subaru Idler Pulley Bolt 23771AA04A
2011–2024 Subaru Idler Pulley Cover 23772AA050

Specs
Idler pulley bolt torque 30 ft-lb

Supplies

Parts in their bags

Replacement

When I changed the serpentine belt previously I noticed my idler pulley was making a bit of a noise when I spun it. A slight grinding/not-smoothly-turning. Since it has almost 90K on it and its a $30 part I just ordered a new one. I also ordered a new bolt and plate that comes with it just to replace the whole set.

It’s pretty straightforward. You release the serpentine belt using a 15mm then use a 14mm to take off the idler pulley. Its only on at ~30 ft-lb so it broke free pretty easily. Then its a matter of putting the other one on.

Here is a picture of the crusty old pulley

Old idler pulley

I took note of the plate because it looks like its directional. The “nipples” or indents are pointed out. So, when I put my other one back on I made sure they were out as well.

Cover plate orientation on old bolt

Cover plate orientation on old bolt

I followed the simple advice from my dad that I believe his dad told him: when you take a part out to replace it, hold it up to the new part to make sure its the same. Everything looked good — the pulleys were the same, the bolts were the same length, and the plate was the same.

Comparison of old and new pulley

So I screwed it in, torqued it to spec (30 ft-lb), and put the serpentine belt back on

New pulley installed — which annoyingly I found trickier this time around than just replacing it. The order of operations matters and you have to start with the water pump and ultimately end with the alternator/tensioner pulleys. A good write up on the serpentine belt and a routing diagram are here.

The first start-up I thought I heard a noise (like a whining) which had me worried I got a bad pulley or I did it wrong, so I turned it off, triple checked everything, and then tried again. When I started it back up I didn’t hear it again — I think I was just being super noise-sensitive after the replacement. There is an outside shot my old pulley was making a noise that covered up another sound that I can now hear. Either way, I’m not sure I hear it now and I definitely don’t hear it from inside the car. I’ll just call it good and monitor for any changes.