I have a 1999 Civic Ex with the D16Y8 manual trans. Did an engine swap in June and ever since I’ve had a bouncing idle. Already checked and fixed any vacuum leaks. I’ve bled the coolant multiple times. Has a new knock sensor. New IACV, new Intake and TB gasket. Ive did the idle relearn that’s in the Honda manual but still can’t get it to idle normal. Also it’ll throw a P0505 code occasionally but not every time. I’ve cleaned and replaced the PCV valve but every time I’ve had it out it’s always caked with oil. Thought about bypassing it but would prefer it to be fixed and not just have bandaid solution. Also has a new O2 sensor in exhaust manifold. Any help is appreciated thanks in advance.

Update: I forgot to put in post it only does it after it’s at operating temp or whenever you touch the throttle. If you just start it up it will idle fine but if you open the throttle it’ll start bouncing.

I have checked the tps multiple times and even on different days and it always tests good and when I unplug the IACV the idle definitely drops and the car falls on it’s face zero power. I’ve read and been told that the paint on the back side of the throttle plate can get damaged and will cause it to not seal but I’ve also been told by a Honda tech that it really shouldn’t matter whether it’s even there or not. I’ve adjusted the throttle cable but did not seem to help. However when driving it if you let off the throttle then get back on it it’ll hesitate slightly then take off. I’m think the throttle is sticking but really dont want to spend couple hundred on a new one and it not work.

I’ve always heard it was Teflon it’s two gray stripes around the edges. Tried to wipe it off but won’t come off. Had the whole tb soaking in carb cleaner till it looked new not a trace of dirt but those gray stripes never came off.