UK Car Repair: vauxhall/opel astra high revs upon start up, vauxhall astra, vauxhall opel

Question
hi,
i have a vauxhall astra automatic mk4 2003 1.6 16valves. i have a problem with the choke,
most mornings upon turning on the car, depending on the weather conditions. if its 15C or over then the choke increases the revs to 2000 or even to 2500, which isnt normal. this then decreases slowly to 1200 where it stays for a couple of minutes then while driving the revs decrease to the normal 800revs. if its under 15C then the revs go upto 1500 then decreases slowly to 1000revs then to normal while driving. which i think is the way it should be.
this also happens when i leave the car for 3+ hours this also happens again. i know that this is wrong as ive had a few astras all automatics before and never experienced this problem,
ive visited vauxhall dealer and did a diagnostic test and they said everything is normal.
ive been told by a mechanic that it could be the throttle valve potensioner which is quite dear as im not sure this is the part with the fault.
thanks in advance,
AL

Answer
The idle speed is controlled by a number of sensors acting through the Engine Control Module, primarily the coolant temp sensor -this is different from the coolant gauge transmitter- controls cold idle.  The system needs to be scanned to identify the cause.  That being said there may be some type of adjustment at the throttle, The potentiometer usually does not control cold idle, it responds to to your foot on the acc pedal.