Some positive results finally
After fitting a restrictor plate with two 6 mm holes there were some improvements but not good enough. I then cleaned the ISCV connector and the 'throttle position sensor' connector with a de-oxidizer (and then a protecting electronics spray). Finally I made a ECU reset as described here:
http://forums.focaljet.com/zetec-tuning/459693-ecu-reset.html - see 'iTrentz' post.
After fitting the restrictor plate the engine revs was pleasantly low when starting cold but it still kept revving after acceleration when warm. Sometimes it worked better though and that made me think of bad connections. After cleaning the connectors I did the ECU reset and after 10 minutes of idling the revs were down to 900 rpm.
I have done a few rather long stints to teach the ECU and it actually feels a lot better now. The revs falls in a reasonably normal fashion and it's getting better and better.
Hope this info can be helpful.
Regards
P.S. I learned from a Ford technician that the 'idle speed control valve' operates with a frequence of current 'on and offs'. The valve is kept in a 'neutral' position with a certain number of 'on/off's and is sensitive to voltage changes - bad connections will probably upset the whole system, including mixture, ignition and everything related to the engine management.
