Hi,
I didn't realize these were the same type of symptoms you had with the old motor. With that in mind, certainly, it could be what you're thinking. Or a problem with the wiring.
However, this is where the manuals are conflicting. If the code is present, according to the manual, it's related to a faulty IAC valve, vacuum leak, coolant temperature sensor, or clogged port in the throttle body.
The only way I find anything on a possible PCM/ECM issue, is if P0505 is not present. When the code is present, the manual identifies specific possible causes.
Take a look at the two pics I attached and let me know your thoughts. Also, let me know if you cleaned the ports on the TB, confirmed the ECT signal is within spec, and if you are using the original throttle body.
See pics 1 and 2 below.
Joe
Images (Click to make bigger)
Tuesday, October 5th, 2021 AT 6:39 PM