S: Bouncy idle between 1500-2500 (KA24DE-T)

May 06


S: Car would bog down with idle hovering up and down especially after car warmed up.

KA-T bad idle – video #1

C: 1 or 2 issues.


-Clogged IACV (Idle Air Control Valve) sticking on return

-TPS (Throttle Position Sensor) out of spec — was 0.78v @ no throttle — changed to 0.45v

-Another issue was MAF.  Performed diagnostic using SAFC 2 sensor check screen – every time car would bog down – at same time the voltage from maf would drop to 0v and return.  Culprit turned out to be a bad z32 mafs.

After some changes…

Swapped 480 cc injectors, ecu, maf died (0 volts)

ka-t #2 after some changes

After maf and ecu change.. car runs good again.. One thing that keeps coming back since beginning is that I cannot get better than 10 BTDC — it seems to have happened ever since I put on SAFC 2.  It’s not so bad since I am running around 14 psi almost so timing’s safe but it would be nice to get the extra 2 or 3 degrees advance.

Please note that in some cases, I found that the rpm going up and down can be caused by ecu sending incorrect signals.  Also if you replaced IACV with another one but it doesn’t seem to respond or ecu code 25 keeps showing up (IACV mulfunction), check the pins on the IACV connector – if you have swapped the IACV without pulling the manifold, chances are the pins could be bent.  You can check & if they are bent, you can straighten out the pins very easily with a mirror and small screwdriver without removing the iacv off the motor.


For those of you who want to verify that indeed it is IACV / AIV that is affecting your idle…

Here is quick and $free$ way to check if the issue with rpm going up / down is IACV / AIV related.  On KA24DE, there is a 5/8″ hose which leads to intake pipe bypassing throttle body.

If you are running stock setup on KA (non-turbo), you will find this is the biggest hose running into your plastic intake pipe coming from the airbox.

1. Unplug the 5/8″ hose end from the intake pipe.

2. Take an old clean spark plug and plug the thick end of it into the 5/8″ hose to prevent vacuum leak and block up the line going to IACV / AIV.  Clamp the spark plug to prevent air leak (see photo).  Make sure that you also block up the nipple on your intake pipe with piece of 5/8″ hose and spark plug again – otherwise, if your intake has a 5/8″ hole that’s open, your car will run very rough (rich) since it will dump metered air.

3. If you were to turn over your car now, the car will stall because throttle body plate is closed and it has no way to get any air since you blocked up IACV / AIV.  So raise the idle rpm by opening up the throttle body plate a bit, simply by adjusting the cable or the stop point on the throttle body spring – both nuts are 14mm – do 1 or the other only.
I would recommend adjusting the cable tension as that is easiest to return back to factory state after testing.

*** Rule: to open up the throttle plate, you must tension the throttle cable – to do that – move both of the cable tensioning nuts down a bit at a time.  Start car – see if it turns over – if not, hold the gas a tiny bit and when it starts let off.  All you need is to get it started and see whether idle is still moving up and down.  If it doesn’t stay and dies, tension it a bit more so that there is enough air going into the motor but not too much or you could reach about 2500 rpm – if so – turn off the car immediately and loosen up the tension a bit.

Once started – check idle – if it’s not moving up and down anymore, you’ve isolated the issue to either IACV or AIV or ECU.  Afterwards, you can set the cables back to factory state and deal with the issue – could take off iacv/aiv and try to clean them, or try spare iacv/aiv if you find them cheap, etc.

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