Originally Posted by
skyshack
? only on boost?
Any leak at idle will tend to suck extra air and go lean but most of the inlet pipework at idle is at atmospheric pressure. Only real vac is after the throttle. So most likely a small vac hose or part of idle system attached to manifold. Idle air assy 16mm hose to cold pipe before throttle. Air regulator (cold start extra air). Carbon can purge line (the thicker one), FPR/hot start, SR has other vac hoses, egr, 1st/2nd gear boost reduction? Some SR don't have air regulator like CA but uses a wax capsule to open the throttle a touch when cold.
What breather system do you have? If the PCV valve and breathers are not connected to trunk after AFM it will suck un-metered air, that leans it out at idle.