right then peeps we have at work a impreza turbo 00my that when starting the engine it revs up to 4000 revs then goes back to idle then back to 4000 revs. select monitor shows nothing abnormal and we can't find an air leak.
another garage has had the engine out to repair bodywork and replace the cam pulleys on the o/s , we have checked the timing and it all matches up and have also tried a new air flow sensor . has anybody had this problem before or any ideas on what it could be.
cheers in advance

 

is the idle speed controlled by a stepper motor or some such device this could be sticking then opening/shutting violently as it unsticks resulting in racing then back to normal.i have had cars do similar in the past and this area is usually first check.
hope this helps richie

 

bang on richie we took it apart today and visually there looked nothing wrong and when plugged in it cycled. but when we looked at a new one you could see the difference swapped them over and it cured it
funny though how the select monitor didn't pick it up though?

 

sound, glad bits of the old brain still work.
the ecu wouldnt see a fault unless the motor was blown i think.happy boosting richie