Aha. New observation and new valuable clue. I did not bring that up before because a weak power steering pump normally affects power assist both ways. When power assist comes back when increasing engine speed a little, it can be due to a weak pump. I am still concerned, though, with the hard steering just one way. You may still have excessive internal leakage in the steering gear box, but not a broken control valve lever. With a broken lever, you'd never get the power assist back, even when raising engine speed. With a little excessive internal leakage, raising engine speed could help the pump's higher volume overcome the loss of fluid pressure and give you back the power assist.
The point of my sad story is this one is going to be hard to know which unit is causing the problem, unless we can find someone who is aware of a common "pattern" failure. We never approve of throwing random parts at a problem in hopes one will fix it, and that is even more important here when either suspect is expensive. Since replacing either part is rather involved, and not one anyone would want to go through the work of putting the old part back on, a mechanic would perform a pressure test first. That involves installing a test gauge inline with the high-pressure hose between the pump and the steering gear. The unit also has a valve to block fluid flow.
Power steering pumps have an unusual valve that I do not fully understand, but it causes fluid to internally bypass the steering gear when you are not turning the steering wheel, and it allows fluid pressure to suddenly go very high when you do turn the steering wheel. For that reason, the readings on the gauge have to be taken quickly before pressure drops, so the mechanic has just one or two seconds to do so. The valve is tuned off to block fluid flow, then the highest pressure is read on the gauge. If it meets specs, which is typically around 1100 psi, it is assumed the pump can develop the correct pressure, but that does not say anything about supplying volume while maintaining that pressure. This is where you might get good power assist only while turning the steering wheel slowly, but not during rapid steering maneuvers.
If the specified pressure can never be achieved on the test gauge, the pump is suspect, but that does not totally rule out the steering gear box. A strong, new pump can overcome the leakage in a steering gear, making it look like the problem is solved, and it may be for many years. Likewise, installing a rebuilt steering gear with no internal leakage could make it unnecessary to replace a weak pump. Now you see the dilemma your mechanic goes through when he is trying to give you an accurate repair estimate.
Wednesday, November 30th, 2016 AT 8:22 PM