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IAC

XJeeper87

NAXJA Forum User
Location
FL
What can make the IAC not function properly? it aint the relay and the IAC seems to be functioning right cus it will kick out but not come back in and i even tried 2 other IAC's what else would cause the IAC not to function properly???


BTW its an 87 XJ 4.0L RENIX
 
Sounds like your IAC is getting the extend signal from the ECU but not the retract signal.

Pin 23 is retract, pin 24 extend on the 1990.
 
joe_peters i think that is backwards the plunger is retracting and allowing more air wich increases idle but it will never go down. It usually goes up to between 2,000 and 2,500 RPM and just sits there
 
Sorry joe_peters i siad it wrong in my first post it was going in but not going back out. Thats what i meant to say
 
Are you sure you don't have a vacuum leak?
 
Sorry joe_peters i siad it wrong in my first post it was going in but not going back out. Thats what i meant to say

No problema. Check those pins and for continuity in the harness.
 
joe_peters i think that is backwards the plunger is retracting and allowing more air wich increases idle but it will never go down. It usually goes up to between 2,000 and 2,500 RPM and just sits there

the iac only makes small adjustments to idle, if it is completely extend/extracted, it will not cause the idle to go that high. that high of an idle could be a vacuum leak or a bad or out of adjustment tps.

there are really long threads about high idle issues with the renix system. lots of causes for this with lots of cures.
 
I need to test this myself, but try something? Unbolt the IAC, remove it from the vehicle but leave it plugged into the harness, start it up... I think the plunger should push all the way out. Now use something (NOT your fingertip, intake vacuum is pretty high and could bruise your fingertip badly by sucking the blood through the skin) to block the hole in the center of the IAC valve plunger seat more and more... don't block it completely. I think the IAC plunger should retract if you do this.

Also, isn't the IAC actually a bipolar wound stepper motor? I'm pretty sure those don't have retract/extend signals, just four terminals for two windings, each is driven by an H-bridge type transistor switching network in a certain pattern to move the plunger in and out one tick at a time. I need to check the schematics in my FSM to make sure but I think that's correct.

EDIT: thinking about this more, this has the potential to damage your IAC by running it up against its stops, and could stall the engine. Be careful, feel free to not do this, I'll be doing it myself at some point as an experiment with a spare IAC.
 
Unbolt the IAC, remove it from the vehicle but leave it plugged into the harness, start it up... I think the plunger should push all the way out.

This is a really bad idea. The FSM even warns about it. If you let the pintle over extend, it may break and you can't get it threaded back on the lead screw. IIRC, the ECU runs the pintle until it stalls, then backs off a predefined number of steps. It adjusts from that point.

You are correct about the IAC being a stepper motor and is driven by a H bridge in the ECU. If the harness or motor shorts, you can blow the corresponding drive transistor in the ECU.
 
OK, glad I added that edit almost as soon as I finished writing the post... I'd really hate to be responsible for someone (else) blowing up their IAC :shocked:
 
It isnt the TPS if i manually adjust the IAC it will idle properly but as soon as i plug it back in it idles up
 
This sounds very similiar to what I am dealing with my 87. It will calm it self down sometimes. Makes it very hard to drive the vehicle and I am worrying about the transmission (have to put it into neutral durring braking). I have cleaned my IAC which helped my idle a little bit but its still ideling very very high (mine will hit 3000rpm sometimes). Next I will be looking at my latch relay and the TPS.
 
I will give that a try, how does this affect starting though? sometimes as old and cranky as she is my XJ won't start in the cold (I think the 23 year old fuel pump wants to give up the ghost, I cycle the key at least 2-3 times to make sure the system is pressurized) and do you think that with the IAC closed it would be easier to stall out at idle?
 
it isnt closed completly.And i have never stalled other than by driving error(5 speed) and mine starts fine but cus the IAC is disabled it doesnt rev up to 2k rpm then drop back down it just goes to 700-750 rpm u have some SMALL hesitation during acceleration but that hesitation is way better than a jeep reving 3,000 RPM constantly
Like i said it is a temporary fix
 
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