what can cause lean idle ??
 

first of all i will introduce with my specs.
miata 95 1.8
garrett t28 turbo from s14a silvia (starts to build boost from 2000rpm)
diy exhaust manifold
2.5 straight pipe
walbro 200lph fuel pump with stock injectors
vortech FMU with diy o2 clamp
running 0.5 bar boost
ignition retarded to about 6 degress
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in the beginning everything was running really nice
but now strange things started to happen
on cold start car starts only if i hold a little bit gas pedal
idle rpm is the same when car is cold or hot ~ 700 - 800rpm
AFR is about 17
when the car is warmed up
sometimes afr on idle is even leaner 17-19
but if i press quick on gas afr stabilises to 14-15 and car runs normally for about 1 minute then there is something like click and afr goes lean
there is no problems when driving
14.1 - 14.8 when cruising without boost
12-11 under boost.
what can cause there problems ??
sometimes like today after driving harder and stopped car idles perfectly afr 14.7 and hold still on 850rpm
i have ngk iridium spark plugs and one nissan guy told that iridium spark plugs suck if car sometimes run rich
or maybe i need to clean idle control valve ??
btw before car was stock i always need to hold a little bit gas pedal when cold starting it. (it started but running on ~300rpm for 3 seconds and then rpm goes up like normally)
but i was thinking that might be it was dying fuel pump so when turbocharging car i changed it.
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and what can you guys say about AEM F/IC Fuel / Ignition Controller ??
i can get it used for about 220$
after getting ecu my plan is to get bigger injectors and run about 0.8-1 bar boost