A local 90 245 I did an AC system swap on has a surging idle - a constant 300rpm vary (900-1200). If its under throttle or normal driving conditions, its fine. The moment you put it in park, it starts fluctuating again.

Today I picked the car up to do some other things to it and noticed when I applied the brakes, about 1-3 sec later, the surging stops, so - as I suspected from the beginning, I think its a vacuum leak. I have swapped out literally every vac line there is with new (including all the interior climate lines). Intake gaskets are new. The IAC has been thoroughly cleaned (and swapped with 2 others) and the intake hose replaced with new. Can't imagine any other place there could be a leak except for the booster or check valve (although no hissing is heard). And FWIW, this surge has been going on since the guy bought the car 2 yrs ago, but seams to be getting worse.

His ECU is a gold painted 563 pink label with a sticker that its a factory refurb, installed March 2005. I did swap in a 93 951 with no change in the idle.

Any other ideas?