A little background:
Did a factory rebuild on a d16y8 (new bearings, pistons, rings, oil pump, water pump, timing belt, tensioner, all seals)

Put the engine back in the car and it started up fine and runs ok when the car is cold but then idles like crap when it comes up to temp, and will fall on its face if I try to put any load on it (try to drive it, turn on the AC etc). CEL came on and it's a P1509 (IAC valve circuit problem)

Did the testing per the manual below, I highlighted in RED the part I'm not 100% on. It suggests that I should find battery voltage between B6->Ground and between B15->Ground. I do find 12v with connector B unplugged from the ECU and key on posII but I find like 10v on B6->Ground and around 2V on B15->Ground with the connector plugged in.

Can anyone check their working car and let me know if they see the same?

 

Fixed it!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! !!!!!!!!!!!!!!!!!!!!!!!!!!

As many people have done, going through the test from the manual for the p1509 CEL can indicate that everything is fine but the code still persists.

Here's how I discovered a clue and fixed my issue:

From all the posts I've found by scouring the internet; what I found that most of them had was that they all had gotten water in the bay (car was sitting for a long time or was in a wreck or sprayed down the bay at the car wash)
In my case, I did also spray down the bay and the harness got wet.

So, to fix this I unplugged every single connector from the engine bay that goes from the ECU to the engine and checked each one for moisture and cleaned them out as much as I could, after that I let the whole car air dry out in the sun for a couple hours with the connectors unplugged. Plugged it back in and BAM no more P1509.

I seriously hope this helps someone because I haven't found a single thread regarding this issue that's provided any hope.

 

Great job man. Thanks for posting your solution... maybe it helps someone in the future.