I've owned my Cooper S for about 18 months and love it. Put on about 45,000 miles in that time but recently encountered a problem that I though would be related to the (infamous) power steering pump.
The car would 'oversteer' through the corners (would not come out of a turn unless you physically steered it out). Steering was noticably stiffer also.
I believed it was probably the pump soI ordered and installed a new (reconditioned) one.
The problem is still there.
Now I've never heard of ball-joints getting stiffer as they wear but that sure seems to be the issue.
Any ideas?
Alex

 

It oversteers in corners?.. Like, drifts? Maybe check your front sway bar and end links, I've never heard of anything like this.

 

Not drifts, exactly but simply doesn't center the wheel as you exit the corner.
It used to be that as you came out of the corner, you simply applied drag to the steering wheel to control how quickly it sraightened out but now you have to physically steer the car to get it to go straight.
This resistance can be felt while driving on the highway also. While it handles normaly most of the time, if the turn in the road is more than, say, the amount of turn you might apply if changing lanes, you can fee extra resistance. If that turn is held in, you can fee that you have a few degrees of "normal" steering to tighten and loosen the turn but feel resistance again when road straightening means that you must steer the car more than those few degrees in order to 'come out' of the turn.
Asside from taking the whole front end appart, I don't know how to isolate this problem.
Appreciate ANY input.
Alex

 

Can you hear the whine of the power steering pump?.. Check the fuse, it will have popped when the old one burned up and it's an easy thing to overlook.