this is incorrect, the tacho will always be correct if the signal is the same, and from what i gather about his swap, then thats not his problem. or do u mean that its out abit in terms of ecu/engine rpm v display? if so, this has nothing to do with the problem.
Judging by what u wrote:
The fact that you unplug the iacv and the idle drops right away, that is ur problem, u must have a faulty IACV, change the iacv to a known good one to test it, and my money is that you will have a better idle.
there is an even easier way to test the IACV, there is a small hole at the bottom of the throttle body in front of the butterfly plate, when the car is running, just block that with ur finger and if the idle drops to a normal rpm then theres ur problem.
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