99 xj with high idle, slow return to idle and false codes - JeepForum.com
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Unread 05-11-2012, 04:37 PM   #1
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Join Date: May 2012
Location: Indianapolis, Indiana
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99 xj with high idle, slow return to idle and false codes

I have been working on a friends XJ recently and it has me at my wits end. to start, she had to jump it about two weeks ago and crossed the cables to her battery, she did not crank it that way, but removed them cables when she realized the mistake. a couple days later she called saying it would start then die immediately. I initially advised a new alternator, however, after reviewing this forum, I decided to check the IAC and battery. the battery checked out and the IAC failed, so I replaced it with a Borg-Warner part. That fixed the initial problem of dying immediately, however, it began to idle high, around 2000 RPM. Also it would not return to a lower level of idle if you gave it any throttle. (if you brought the RPM up to 3000, when you let off the throttle it would stay at 3000, unless you put it in gear at which point it would drop to 1100, and start the process over) This was obviously a bad TPS, so I changed it, again, with a Borg-Warner. I also took this opportunity to give the throttle body and IAC housing a good cleaning. i put it all back together and fired it up, no change. I flashed the PCM and it ran normally for about 5 minutes. it then bumped back up to high idle. Flashed the PCM again and it began running normally, I let it run like this for a while, checking different parts and listening with my stethoscope until i though it was going to be ok, packed up my tools and put got in the drivers seat for a test drive. idle jumps back up, this time w/CEL indicating bad TPS, i have bought bad sensors before so I got ahold of a new one and installed it, flashed the PCM, now its throwing a bad IAC code as well as a Bad Speed Sensor code(?) clear it, start it, same IAC code. Check TPS and IAC voltage, both are good, removed battery and cleaned and Di-electric greased all connections, it's still acting up. I finally decided to join up on here to bring you my specific issue to see if you guys have any thoughts?

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Unread 05-11-2012, 05:13 PM   #2
It's the crank sensor!
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1999 XJ Cherokee 
Join Date: Nov 2007
Location: Central Minnesota, MN
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Please post the EXACT code numbers here. There are a number of codes for engine management and with OBD, the devil is definitely in the details.

And it's always a good idea to Google the code + Jeep Cherokee to see if anyone else has experienced this. For instance, if you have a P0505, Google "P0550 Jeep Cherokee". If you have multiple codes, Google them all.

The OBD system isn't perfect, but it is pretty darn accurate at pointing you to the suspect component or "circuit".
99 Cherokee, 4.0 AW4, NP242
Past Jeeps: 49 Willys, 81 Scrambler, 88 Comanche
Without "data", all you have is an opinion!
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Unread 05-11-2012, 06:27 PM   #3
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I got a P0505 (IAC), P0500(speed sensor) and P0122 (TPS low voltage) I also got a no bus code a couple days ago, but i can't recall what the code number was on that. it only threw once and cleared itself out
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Unread 03-03-2013, 11:30 AM   #4
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I have almost exactly the same problem. 99 Cherokee Classic 4.0L 4WD. Idles high and when I give it a little throttle it will not return to normal even though the butterfly valve will return back to closed position. I threw a code for my IAC and replaced it only to have the problem get worse. I can put my palm over the butterfly valve on the intake and stall the engine so I think that means my intake manifold is sealed without vacuum leaks. I tested the TPS and the voltage seemed ok. Slightly high... I Don't have the money to replace it just yet. Could a bad TPS even make the engine idle at 3000 rpm with the butterfly valve completely closed? Seems crazy. I guess I don't have many answers for you but at least you know your not alone.
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