I have a problem with an erratic idle I can't get to the bottom of. mk2 gti 8v PB digifant engine. When I start the car, it idles on 1000rpm then up to 3000rpm then back down, it continues to go back and forth, never settling. When I remove the ISV plug the idle drops to around 700/800rpm and stays. Strangely when the ISV is connected back up but the blue temp sender plug is removed the idle settles at around 2500rpm. Wiring between ECU and ISV is good. ISV is new and clean. I don't suspect it to be a vacuum leak as the idle is fluctuating so much. Any advice is much appreciated.
No-one played with the throttle stops? Did you check the wiring to the idle / WOT switches? Thought it does sound like an ISV issue. Take it out and clean?
