hi.
i have civic 98 ej9 d14a4 M/T with idle problem.
4 months ago i do IM swap to y8 and all was good, 1 months ago i start the car and the idle started acting weird.

the problem:
1) cold start 2000-2200 and drop to 1500
2) when the car is warm the idle is 900-1000
3) when i hit the gas and leaves the RPM drop to 900 up to 1200 and back to 900-1000 and sometime drop to 750 and back to 900-1000
4) when i turn the headlight/ac/open or close the windows/radiator fan come on, the RPM is up and humming sound coming from the starter area.......
5) when i fully close the idle screw the humming sound start too

things i've done:
1) remove and clean the TB and the IAC very good!
2) new oem TB and IAC gasket
3) TPS calibration 0.45v - 4.5v
4) check all vacuum line and all was ok
5) check one more IAC
6) new oem distributor rotor and cap
7) check the IAC function with battery, full open is ok but when i make him full close its seams its close to mutch and leave 10% open.
8) bleed air from the cooling system

all that not fix my problem...

Things I noticed today:
some little pops come from the exhaus and little psssss sound come from back of the IM (cylinder 1 area).
when i do the swap i cleane the old gasket very good and install new one but no oem.
gasket can be destroyed after 3 month?

what more i can do or need to check?