Stubborn idle surge 325i... HELP!!!
My '01 325iT has an stubborn idle surge. This happens primarily when slowing or coming to a stop after driving (like when sitting at a stop sign or light), and is especially noticeable when the AC is on. Engine idle oscillates between 400-1000, eventually settling into the nominal 700rpm.
I recently replaced the intake adjuster unit (DISA) + idle control valve (ICV) + oil separator. Moreover, intake boot and vacuum lines were replaced last year. EGR valve + vacuum acuator valve + air pump have also been replaced in the past two years.
I've been looking and listening for intake or vacuum leaks but haven't identified any. The only other components I can think are the brake vacuum one-way valve (BMW jargon = "sucking jet pump") or the hose to the brake booster, but I'm sick of playing the shotgun replacement game.
I've taking it to the dealership and they couldn't replicate the issue or find any vacuum leaks... but still charged me the $200 diagnostic fee!
I know idle surge is a common topic on this forum and have done my research before posting. Just wondering if anyone has any solid advice considering the specifics of this problem. Could it be related to the brake booster pressure differential when coming holding the brake at idle?
HELP!!! Ideas? Experience?