I have a JDM SpecB Legacy with similar idle problems as jacksie66 is experiencing.   I'd like to know if jacksie66 you ever were able to figure out the root cause and solve the issue?   I have btssm monitor on the ECU and the AVCS solenoids are equally controlling the intake for both left and right, but still not certain if the AVCS gear(s) are worn out.   Checking the grounding next.   I have replaced spart plugs and the ignition coils.   FYI: My car is also converted to run as CNG and the same problem is with gas as petrol, so this is not related to fuel injectors.   Any idea what I'd need to check next?

Aftermarket top mount isn't necessary. I am making 270kw with a Stock VAB STI top mount intercooler. Tuner said IAT's dropped 15 degrees when I swapped to this from a process west one.    If you are out flowing a Stock STI top mount, you are better off going to a front mount, before considering any aftermarket top mount.

that'll be next step hah : I also wanna do the Push in version and the ROH one I have that's unobtainable   happy to run 5c (or so) PLA replaceable's once I get going hah (I was just thinking I should just order a set to get print & Posted today hah snap)

I found this https://xcceleration.com/pinouts/Subaru pinouts/ECU Pinouts BC5 BG5a WRX (4 plug) - Single Swap.pdf and looks like adding wires for TPMs and it’s ground is needed. Possible the pins or wire is damaged or a joint cracked that won’t show up when the car isn’t moving.   I was thinking alternatively you may not have an electrical issue. Maybe you have a split in a vacuum or boost line that opens up and stuffs up the idle.You could get the same with worn spark leads when they get hot. Anything that drops revs outside where the ecu target is and throws an error. So a leak when the ecu is trying to idle down would throw an error for idle control as that’s what it is doing. It can’t predict air leaks.   Have you checked and replaced the throttle butterfly closes fully?   I’d also remove the ecu case and check the capacitors for leaks.

Yup, was thinking similar - this Bc5 was originally Auto - the Ecu is from a Manual WRX Ver1 (Z4 ECU) The Idle switch is built into the TPS and is functioning fine and not throwing a code (it did throw a idle switch code until i adjusted the TPS with a multimeter to spec volages with the car on, that fixed the idle switch code)   I got right under the dash with flashlight to have a look and there is no corresponding plug that the blue plug on the accelerator pedal to plug into - really scratching my head on this one    Im guessing the ECU does have an input for the Accelerator Pedal Switch, otherwise it wouldnt know to throw the code for it   Bit odd though because I owned and drove the car for ages without it throwing a code for the Accelerator Pedal Switch    As for Vacuum lines - what vacuum lines are you referring to?    Z4 ECU Wiring Diagram doesnt show any additional input for any unknown sensor except (Self Shutoff Control)    Let me know if you spot anything