Idle hesitation when throttle is released

[whiplash willy]

I know there are alot of idle threads, I have read through them all. I can't find a good answer for my problem. I have a stock S13 Blacktop SR, stock boost, SMIC, everything but exhaust.

Here is the problem: 50% of the time when I completely let off the throttle, my rpms will drop down to 500 for 1-2sec then go back up to 800rpm. It runs great other then that. It happens more often if I am stopped and rev it to 2000rpm then let off. I have also noticed the idle barley drops (can’t see the drop from the gauge but can feel it) when my windshield wiper wipes. I have a new ECU Coolant Temp Sensor, Spark Plugs, and O2 Sensor. Any ideas on what this could be?

[FSUDrifter]

boost/vacuum leak

[IanS]

if your wipers are causeing an idle drop, you may have an alternator that is starting to fail, any good shop can output test your alternator

[FSUDrifter]

autozone has good testing equipment for free

[whiplash willy]

I tested my alt and it is good. My boost and vac levels are at what they should be at, so I don't think it is a boost/vac leak either. The problem seems to be worse when my headlights are on, even though I get 14+ volts from my battery when my car and lights are on. Any ideas?

[FSUDrifter]

you might just have some bad grounds. grouding kits have been known to work wonders for some people...

[drftsx]

alternator belt??

[S13FASTBACKSR]

alternator belt???.....anyways..possibly its a coincidence that it does it more or less when the wipers and headlights are operating..are you sure it does it worse everytime when the headlights are on? does it do it everytime when the headlights are on? I would look away from the electrical ideas for awhile and check into your bov and IACV. You may need to tighten your bov spring or just recirculate it..also try cleaning your IACV..what you do is take out the idle adjustment screw and spray lots of carb cleaner in there..get an air hose and spray air into the hole too..also put a long piece of wood on the valve and tap the other end with a hammer..this knocks carbon loose..let us know what happens and let us know if it always does it when the headlights are on too im curious about that

[whiplash willy]

It is very consistent when the lights are on, 9/10 times I would say. When the lights are off, it rarely happens. Also the idle jumps around about 50rpm when the lights are on. It is most noticeable when I am cruising (lights on) in gear at about 3-4000rpms. I then put the clutch in, and break to a stop. When I put the clutch in and start breaking, the idle goes down to 500 and starts stuttering untill I let go of the break, then it will go back up to 500. If I keep the break pressed down when I am stopped it will stay at 500rpm for about 5 sec, or until I let go of the break

I don't think it is any problem with the alternator because when the lights are on and my car is idling the battery is reading 14+ volts. My BOV is the stock BOV and it is recirculating. My setup is completely stock except for the Walbro 255 fuel pump, a Nismo FPR and exhaust.

[whiplash willy]

Any more ideas? This is driving me crazy

[S13FASTBACKSR]

hmmm..I'm thinking the AAC unit on your IACV may not be functioning properly..the sensor on your IACV may be to hold the steady idle when these accessories are on such as your headlights and windshield wipers..possibly that part of your IACV has gone bad and is not doing its job..I would hate to tell you to buy another one and go through all of the work..its just a thought that you may want to research more..another thing i can say is check your ecu for codes..do you know how to do that?..also did we go over your tps and maf yet? are you running rich at all or any misfiring ot hesitation while driving/ accelerating? check the voltage to those yet?