here's an idle puzzle for you
 

My s5 turbo idles poorly (super low rpm's) and dies when in nutural but stays running with little pedal action. happens when the engine is cold or at temp.

but get this, while in gear and clutch pushed in it idles fine but its at 1200 rpm..:ponder:


I have a Rtek ecu tuned for 720cc secondaries but im running 750cc secondaries and stock but flow tested / cleaned this past winter primaries. no piggy back fuel controller

high flow exhaust and tial q bov

fmic, piping is 2" from turbo compressor to fmic, from the fmic its enlarged to 2.5" all the way to my tb neck which has a 3" opening <-- this is making me wonder

oil pressure is above 30psi at 700rpm
wide band is reading 17-19 at low rpm poor idle (under 700rpm) so it looks like i'm not getting enough fuel. but why!!??

feel free to ask me questions and give me pointers on what to test/or look at

Sounds like a vac leak to me man. It's not getting enough fuel while spinning the trans, take the load of spinning the trans and the extra air speeds up the RPM's. A nipple cap could have rotted off, or something could have popped off. Did you use all new gaskets last time you closed it up?
Do you have a wideband O2 sensor? Can you read the AFR when it tries to idle without the clutch pressed in, VS with the clutch pressed in.