Idle hang

[Hen10]

My mx5 has picked up a weird idle hang that I've been trying to figure out for a while now.
the car revs up fine but the idle will hold at 1500rpm and then will slowly return down to normal when the car is put in neutral or when it comes to a stop.
I had codes for my AFM and O2 sensor so those have both been replaced.
I replaced my IACV with a second hand one that was known to be working.
I can't find any vacuum leaks.(not ruling this out yet, still searching)
I'm pretty sure the idle started going weird after my alternator failed, could this have anything to do with it?
what else should I be checking to try sort this out? The car is still drivable and other than it being awkward it doesn't seem to affect normal driving.
thanks for any help

[StanTheMan]

good chance its not the altinator.....but you never know. yes youve checked it all.....check it again.

check to see what you need to remove to get to the alternator think of what would have been taken apart by the mechanic & missed on re assembly....and put all those hoses & bits back on.

also check some of the vacuum hoses on your car....there are a few hidden ones. It could also be that those have a hole in them.there are also some blocked off hoses on the lower side of your intake plenum check those out

generally i would work my way back from the air intake towards the engine. check the air filter, clamps switches, connectors.....do the real basic stuff.

Once ive done that 5 times without success.....I usually then feel its not an air leak.or something else real simple

[93_Clubman]

Some ideas & a possible fix in following: viewtopic.php?t=56504