Honda: idle fluctuate, vacuum leak, ground signal

Question
QUESTION: I have a 98 civic ex vtec that when accelerated from 1500 rpms to 2000 the idle fluctuates. This problem only happens when the gas pedal is pressed, by itself the car idles perfect. Also the code for the IACV keeps coming on after i erase it with my scan tool. Problem started when i replaced the engine cause the old one seized up.

ANSWER: If you have a code stored you'll need to troubleshoot starting there. Obviously clearing the code is not going to repair the problem. If I'm going to be any help here, let's start with the code. Is it a P0505?

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QUESTION: The code is p1509 and i've replaced the iacv four time and still the code comes up, i was thinking maybe it has to relearn the idle but im kinda stumped. And like i said it just started after i put the other engine, both engines are the same d16y8 engines

Answer
I think it is going to require a good scan tool with data list capabilities. You'll need to have a decent understanding of what the various voltage readings, percentages, and other data readings indicate. If the car has a vacuum leak it should either idle too high or fluctuate 500 rpms or so. I'd check for proper pin fit,power, and ground signal at the IACV connector.