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Need help on Idle issue Maybe IAC problom

P

pma4x4

NAXJA Forum User
Location
Apache Junction, Arizona
Hey, new to the forum. Longtime viewer first time posting.

I am looking at picking up a 96 Xj 4.0 auto A/C But one thing that has got me worried is its idle. Let me explain.

Starts and runs just fine. Has plenty of power and no noises. BUT, about ever 15 seconds almost clock work the idle jumps about 500rpms then goes back down. I thought maybe IAC so i pulled the IAC plug while runninga and the problem seemd to pretty much go away. It was running just a hint rough with the IAC unplugged but i plugged it back in and the problem came back.

There is no CEL but i am wondering if there is one in memory. Is there a paperclip way like some to jumper the connector or do i do the on/off/on/off/on trick like my 93 ZJ.

AM trying to diagnose this before the purchase.

I have also searched thru the forum and found many threads to the IAC but nothing with my symptoms

Any help would be great
THX
 
Last edited:
it might be the relay that sets the idle has one of its wires loose, or the IAC just needs cleaning. My money is on a very small vacumn leak somewhere though, just because of the small amount of rpm increase and it doesn't happen imediatley on start-up. The relay and the IAC would be affected if it happened on start-up since the rpm shift is later, check some of the vacumn plumbing. sorry so long winded. I can type faster than i can think sometimes.
Good Luck
 
but would a vacum leak cause it to jump like that like clock work? Like i said if i unplug the IAC the idle jumping goes away.

It does this 500 or so RPM jump right from a cold start and keep sdoing it even after warmed up.
 
It does it with the AC on or off. The AC needs a charge but the compressor still kicks on and off. Either way no matter what accessory is on or off it still does that idle jump.

I unplugged the IAC and the problem was almost gone. It would still change in idle but not as noticable or as much in rpms. plug the IAC in and it becomes more again.
 
Clean out the IAC, and be careful when you pull it out, I put a small clean rag under it so when pulling it out if anything falls out it doesn't get lost. Sounds like it just needs a good cleaning.( the little pintle gets dirty) Its not getting "set" to be your idle so its messing with the relay and thats why the RPM shift feels like clock work, the relay is trying to do its on off thing and the IAC cant put itself where it needs to be in its slot. So if you clean the IAC and it isn't your problem, go after the relay next.
 
When you remove the IAC, check the o-ring that goes between the IAC and the throttle body. I had an idle problem which was caused by a boogered up o-ring. Got a new one from the hardware store and all was good. Be very careful when installing the IAC back on. Make sure the o-ring seats properly in the throttle body.
 
ramjet308 said:
Will check these later this week.

Where is the "relay for the IAC?

96 XJ 4.0 A/C
Click to expand...

there is no relay for the IAC, but this is an obvious IAC issue, if you can unplug the IAC, and the jeep continues to run, its obviously stuck open. shut the jeep off, unplug it, and start it, it should NOT stay running, give it a good cleaning
 
I really thought the IAC is hooked to a relay. I will have to check the FSM. Well the concensous is to clean your IAC, the other thing with it being so slight in RPM's and it never hurts... is to retighten intake/exhaust manifold bolts. That can have an effect on small to large vacumn leak. Good Luck.
 
So do i just take off the IAC or do i take off that square housing it goes into as well. The square housing attaches to the main bod of the TB

If i do this do i need a new gasket for that housing to TB surface?
 
I swear I did not clean this up at all, it came outlooking like this.

AND YES I unplugged it then started it up and it stayed running. with the IAC IN but unplugged at the time. so do i take that square housin gthe IAC bolted up to off and clean that or what? should i try and see if it runs with the IAC completely removed??

iac1.jpg


iac2.jpg
 
ok.. Closer look has revealed that when the IAC is unplugged the engine dips slightly in RPMs every 15 seconds.

When IAC is plugged in and working it jumps UP about 500 RPMs every 15 seconds.

My guess is if its doing something either way every 15 seconds thne its not the IAC. I think the IAC is compinsating the idle to keep it from stalling.

I did the key onoffonoff deal and got no CEL codes.

Am thinking of just taking it to a dealer paying them the one hour shop time to diagnose the thing.
 
this is kind of a shot in the dark here but when the a/c is on do the idle fluctuations coincide with the compressor clutch engagement? I have seen a/c systems with low charge have fast cycle times and the engine management adjust the idle speed accordingly. even stranger was that it carried over to when the a/c was off, recharged system and cleared the KAM (keep alive memory) and idle returned to normal.
 
Your shot in the dark was a Bullseye!!!!!!!!

The temporary rev coinsides with the AC compressor shutting off. So the IAC cant adjust fast enough Hence the quick rev.

When the IAC is disconnected there was a small engine lag like ac coming on but it wasnt. when i plugged it back in it went the other way. yeah to the exact second its the Compressor.

OK now!! why or how does it carry over ot when the AC is not even being used or better yet how do i fix that.

You mentioned something about Kill Alive memory??? explain. I unplugged the compressor and left the battery unplugged for about 5 minutes and it still does it

Help please would like to take it to work tomorrow.
 
Ramjet, I'd erase the KAM as I described in the other thread (leave the door open), and here's another shot in the dark: Look in the relay center for the "latch relay", and swap it out with the A/C relay. If the trouble goes away, then replace the faulty relay.

The latch relay stays energized for a few seconds after engine shutdown, to set things up for the next restart. Including the IAC. This time delay stuff on the rolling idle makes me suspect the relay.....
 
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