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IAC work?

Ron4x4

NAXJA Forum User
NAXJA Member
Location
Reno, NV
I hydrolocked my XJ last weekend. cleaned out everything since. and will be changing out the oil again here in a few days.
the only thing left that is driving me nuts is: it is idling high 2000rpm. If I let the IAC close and then unplug it when the engine is off, I can start it up and it runs fine, drives great. As soon as I plug it back in it opens back up and idles high.
Is the IAC supposed to be all the way open when the engine is running? I can see the IAC move back (pull open) when the engine starts. I have checked everywhere for a vacuum leak, by the mainfold, but when we ripped off my bumper it did pull the vacuum ball out and I also followed those lines with carb cleaner and had no affect.
I think it is the IAC valve, but I would like to hear from someone who knows better than me.
Ron
 
doesnt sound good for the IAC. Im not 100% sure but it should stay pretty stationary when the throttle is opened. Otherwise it should cycle back and forth some at idle, (depending on how well it idles it may move alot). A vac leak would cause it to close due to the extra air after the throttle plate.

What year?? It may require some more diag depending on if its simply a IAC or if its is pcm or another sensor.
 
Ron4x4 said:
I hydrolocked my XJ last weekend. cleaned out everything since. and will be changing out the oil again here in a few days.
the only thing left that is driving me nuts is: it is idling high 2000rpm. If I let the IAC close and then unplug it when the engine is off, I can start it up and it runs fine, drives great. As soon as I plug it back in it opens back up and idles high.
Is the IAC supposed to be all the way open when the engine is running? I can see the IAC move back (pull open) when the engine starts. I have checked everywhere for a vacuum leak, by the mainfold, but when we ripped off my bumper it did pull the vacuum ball out and I also followed those lines with carb cleaner and had no affect.
I think it is the IAC valve, but I would like to hear from someone who knows better than me.
Ron

The TPS (throttle position sensor) may be fouling up the IAC circuit. Or, perhaps even more rare/screwy - the ECU is running on latch relay power and thinking the engine has been shutdown. The latch relay provides power to ECU in order to park it in the open position for the next restart. It should always go to the open position at shutdown but it should close as the engine starts and ECU receives sensor information from the MAP/TPS/CPS ect.

I do know the IAC driver circuit is built into the ECU on my Renix model. I've built a bi-polar stepper motor electronic circuit ($15) kit to test my IAC and I also use it when it goes high idle and I can't find anything else wrong. The circuit runs off AA batteries so I plug in my homebuilt excersizer and adjust the idle back to normal or whatever I want. Unplug it and leave it until I get home. 9/10 times it returns to normal when I reconnect the ECU.

I was gonna wire it in fulltime but my wife doesn't understand a manual idle speed control so I/we just use it as an emergency remedy.

That's all I got for you...
 
Thanks guys, It is a 1990 Renix system. It actually idles great with the IAC all the way closed, except when I first start it in the morning, which it actually runs and idles great after a few seconds after I start it. My first guess was a vacuum leak somewhere else and the IAC being open raises the idle. I undid the battery for about 15 minutes thinking it would reset the computer, but it seems that doing that did not reset it.
If the engine is off and I unplug the IAC I can watch the IAC open (with the intake off)
and as it opens I can see the idle speed increase. When I shut the engine off it slowly closes. then I un-plug the IAC sensor and start engine and it idles fine 750ish rpm. Could the TPS or other sensor be telling the ECU that the engine is just starting and needs to be open? or is the IAC increasing the idle because map sensor is sensing too much vacuum? Ideas are welcome.
Ron
 
the renix system uses a actual stepper motor to move the throttle linkage or is it like the new systems where it varries air flow through an oriface?

Im thinking of the newer style.... which changes my diagnosis some....
 
Maybe I am missunderstanding you but I dont see any kind of motor, accept the IAC itself (I think). If the TPS is not operating correctly i.e. making the ECU think the throttle is closed wouldn't it keep the IAC open?
 
Ron4x4 said:
Maybe I am missunderstanding you but I dont see any kind of motor, accept the IAC itself (I think). If the TPS is not operating correctly i.e. making the ECU think the throttle is closed wouldn't it keep the IAC open?

Renix IAC is a bi-polar stepper motor that has nothing to do with the throttle linkage. It regulates air only. Vacuum leak SHOULD cause ECU to close the idle air port off. TPS influences IAC control but I'm not convinced it's the mojo bone that causes high idle speed condition. I've unplugged TPS at normal idle and nothing changed on my rig.

Since ECU regulates IAC in both closed/open loop operation that helps narrow sensor information down to the MAP and/or TPS. Getting either of those sensors wet probably ain't gonna help and I'd also double check that latch relay circuit and ECU power supply wiring.

Do you have a Renix FSM? I can email wiring and troubleshooting sections if you need them? PM me if your email can handle large attatchments.
 
XJXJ:Is there a place to buy a RenixFSM? Where is the latch relay circut located? Yes my e-mail can handle large attachments.
[email protected]
 
AZcrawler said:
XJXJ, how did you build your IAC exerciser?

Price has gone up since I bought the kit but it's still a good deal. I powered mine with 6 AA's. You have many switching options with additional circuitry or computer but I use a simple momentary contact for movement, fixed switch handles direction. The only weirdness was figuring out the motor connection but once you determine the two motor coils it only takes a couple experiments to find the right driver combination. I snagged an IAC for testing and a connector from the junkyard.

http://www.electrokits.com/electronics/motor-control/81.htm
 
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