Power Steering shudder

[trevorno]

I get a shuddering sound and vibration when turning the steering wheel. It happens when I'm stationary or moving. I replaced the return hose o-ring already, which helped with the ps whining sound, but I still get this shudder/slight vibration. Plenty of fluid and I've done a couple of locked turns in either direction.  I've just ordered a new return hose.

The car is an 09 Legacy AT with 140K miles. I bought it 1.5 years ago. 

What should I look at?

[KZJonny]

If no other leaks are obvious from visual inspection (hoses, rack, ps pump etc...) then maybe find a parking lot and do some more figure of eight turns? Having a flashlight to look at the ps fluid reservoir for bubbles might give you some indication of if/when you've bled any air out of the system...

PS pump could be suspect, but best to rule out the simple stuff first.

Have you bled out the old fluid as much as possible as replaced with fresh ATF? Maybe old/deteriorated fluid is the culprit....

Edited February 2, 2023 by KZJonny

[Febreze Mee]

+1 on checking for leaks. It's a closed system, so the aeration of fluid shouldn't be happening really. Certainly not for casual driving anyway. The joints on the high pressure hoses can weep a bit, especially at this age. Otherwise, if all areas @KZJonny mentioned are dry, it's possible you have a bad o-ring in the pump housing, and/or have dirt in the pump housing. I promise rebuilding the pump isn't so hard. 45mins uninstalling the pump from the car, 1hr disassembly/reassembly, 20mins reinstallation time into the car, and $25 in parts for a next-to-new PS pump. Ask me how I know. 
 

EDIT: Adding to this. 
 

I'm trying my hardest to not freak you out, but you could also have sheared something internal of the rack. Unlikely at your mileage, but still possible. When I rebuilt my STi rack the first time, I ended up crushing a crucial washer. Everything went back together fine, but only with some encouragement (shouldn't have needed it). When it came time to drive, I experienced something very similar to what you've described. The steering wheel also seemed to not reset itself as I later found the rack was getting caught up on the deformed washer.

A little blacksmithing put everything back to factory and it rides new again. 

Edited February 3, 2023 by Febreze Mee

[trevorno]

Well now I am freaked out! 

[VespaGTS]

I’m about to do a deep dive on my PS pump as well. It’s a bit more obvious in my case. See pics.