To answer your other questions properly it's important to realise that several completely different issues can result in similar 'steering' symptoms with these cars i.e. wayward feeling steering, jittery tugging of the wheel, darting over ruts and cambers or tight spots when turning and erratic feedback. I had all the above when I got my car 10 years ago and by a process of elimination worked through various fixes and it was sorted never to return, BUT it wasn't the steering column issue which is the pre-facelift issue you are referring to.
Most cars with iffy feeling steering behaviour will be down to the below common issues, a relatively small number will have required an electric power steering (EPS) column fix.
1. TYRES: The original spec Bridgstone Run flat tyres cause most tramlining issues where the car darts and steering pulls, sometimes dramatically over white line, ruts and cambers. Made worse if tyres and suspension are worn or you have mixed brand/types of tyre.
Cure. Switch to non-runflats, the transformation is night and day!
2. SUSPENSION: Front end, lower rear control 'lollypops' go soft and the originals can split releasing the hydraulic damping fluid inside giving a generally wayward feel at the front as it causes constant changes in geometry. Cure is a simple swap of the lollypops.
Other bushes including drop links and front anti-roll bar bushes also get soppy and are worth changing.
3. SPRINGS: Springs on the Z4 are made of sugar glass! The rears rarely lasted more than 5 years (mine went at 3 years) the fronts fare better but are getting prone. The thin 'pig tails' break off upsetting the cars behaviour.
4. WHEEL ALIGNMENT: Getting the 4 wheel alignment right on the Z4 seems to require witchcraft! it took three different places 4 goes with mine, but in fairness sorting the suspension components first is a must. The results can be dramatic in terms of steering stability.
5. Steering LOWER UNIVERSAL JOINTS (Not EPS) A shaft and two UJ's are located under the bonnet next to the exhaust and just above the road, they get filthy and baked so its not surprising they get tight. The result is they 'confuse' the EPS torque sensors when you steer or there is feedback in a turn resulting in jerky steering, which feels like the 'sticky steering' people refer to when talking about EPS issues. The cure is easy, lubricating the UJ's works instantly or change the shaft and two UJ's if it's shot which is rare.
6. STEERING RACK: A few people have reported tight or worn racks causing steering issues but is rare. Fix is a replacement.
7. EPS COLUMN: This is where it gets confusing, firstly it only affects pre-facelift M54 engine cars. There are two separate issues, firstly the control unit and motor overheats, when it does it reduces power assistance in 20% steps to reduce load and temperature until it's normal. The result is edgy heavy and jerky steering (same as sticky UJ's mentioned at 5) the only fix is a replacement column which is a £2k job with new parts or a used part from a post-facelift car which will need coding. Available at about £350-450 plus fitting. THIS SHOULD BE A LAST OPTION ONCE ALL THE ABOVE ARE ELIMINATED and is very unusual. It's my guess that some have in fact been changed unnecessarily.
The other column fault is tight or dry internal gearing, there are posts about drilling the column and lubricating the gears which worked for some people, or the fix linked to in the previous post whereby the gears are adjusted seems to be working for most people who have exhausted points 1-6 above.
In my case wayward sticky steering plagued me for the first two of my 10 years of ownership but was cured after doing fixes 1 - 5 so I know they work.
