We have a 1996 Subura Legacy with 157,000 miles whose power steering has just begun to shake. The steering wheel will turn back and forth (probably from 11 o’clock to 1 o’clock) but the rest of the car doesn’t seem to shake or vibrate. Every once and a while the steering wheel will jerk to one side and then it will be okay. It doesn’t matter how fast you are driving and this shaking comes and goes. It may happen once a day or once every week. The car is aligned and the tire where replaced last year.
The rack & pinion assembly spool valve O-rings are probably worn out. The spool valve is controlled by the steering wheel. When you turn in one direction, the spool valve diverts the hydraulic pressure from the power steering pump to a piston in the rack assembly so it can provide power assist. If the O-rings leak, the hydraulic pressure is no longer being properly diverted. This then allows uncontrolled hydraulic pressure to be applied to the piston in the rack assembly.
Thanks for the comments. Now if I could spell Subaru correctly, I might be in better shape.