My GTA turbo was idling at 500 rpm, or 1000, or 1200, or... whatever, at different times and different temperature states. I had checked out the air idle control valve and looked at vacuum pipes.
At Origine RS, I asked for ideas, and several people suggested look at the vacuum pipes some more. I found one silicone tube to the pressure sensor on the engine bay right hand side was in good condition, but a sloppy push-fit. After adding a new small Jubilee clip to squeeze the tube there was a miraculous healing. Idle steady 700 rpm and no hunting. Good result. I guess the variation in relative thermal expansion at different temperatures between pipe and fitting would account for varying degrees of air leaks; thus ranging from nothing to a gap.
So thank you to all, especially Paul Sage.
Nigel
