Car: X300 4.0 N/A 250^000km
Symptom: From one day to another the idle RPM set at 1400 revs steady.
Daalership with one of the last orig. WDS did a reset which cured it instantly. But: after 2 weeks the 1400revs came back.
Changed the ECM to a secondhand one from ebay. I have now normal idle again and driven 2000km.
As an intermittend fix until I could corner in the ECM i did this: After starting the cold car and engine running at 1400revs I waited until the rising coolant temp made the ECM signalling the idle air control valve to close and gradualy lowering the revs to 900 rpm. At that moment I unplugged the IACV. So I could drive the car without hassle except when starting at cold I had to apply some pedal to keep it alive as idle when cold was about 500RPM.

Hope that helpes anybody pleaged with this famous X300 fault.