i just but a 98 y7 in my 95 hatch and thats what it came out of and it was running fine but now in my car it is idleing up and down will your at a stand still but runs fine i know most of the time this is bc of the iac valve but that doesnt make since seeing that it was fine in the other car i know i have the wrong ecu in it would that make a diffrence bc i have a p28 vtec ecu and the y7 is a non vtec 98 model any help would be great

 

check for any other vacuum leaks. even a small slit in one hose can cause a funny idle. i dont think the p28 is the issue though with idle. though i could be wrong.

 

im not sure about the ecu, but a vaccum leak would cause this. also what temps does it do it at... Is it at idle no matter what temp or is it only when the car is cold

 

its at what ever temp

 

I'd still go back to the iac valve, it may have gotten dirty during the swap. Mine started screwing up after i got my car painted. I think they sucked a bunch of dust up in the intake after they sanded the car.

 

IACV gets my vote as well.

Vacuum leaks usually mean a more constant higher than normal idle. Meaning, it will stay at a high idle rather than fluctuate.