99 xj 4.0l IAC issues help please

Hey so I have a 1999 xj 4.0l auto and 2 days ago started experiencing rough idle and at a stop light my idle dipped down to about 500 rpm and my fuel gauge went all the way past (E)to the needle stopper but drove fine. I know a few things about cars so I figured to clean the IAC(very carefully, did not turn the pintle) and that made no differences after I reinstalled it. I then bought 4 different IAC’s one from Autozone, Napa, and orileys having a different one every time and it still did not fix the issue. I was able to manually adjust the brand new one I have in the throttle body right now (very very carefully) and yes I know everyone advises against it or says they’re not adjustable but every time I installed a new one the idle stayed at 2k for over 5 min until I killed the motor.The idle is where I want it around 900 rpm but my fuel gauge still isn’t working and check engine light is still on. Now I believe there is a short in the circuit but I’ve resistance tested all 4 pins in the harness from the plug to the PCM and all seem fine. If anyone has tips or pointers on how I should go about fixing this they’re greatly appreciated. Thank you.


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If the Check Engine Light is ON, your Jeep is trying to tell you what is wrong. Buy, borrow, or rent an OBD-II trouble code reader and post the trouble code numbers.

After you installed the new IAC, did you re-boot the engine computer ? The computer stores data and will try to use the faulty data to run the new IAC, thus you will have a high idle until it is re-booted or learns new parameters for the new part.

Tim_MN said:

If the Check Engine Light is ON, your Jeep is trying to tell you what is wrong. Buy, borrow, or rent an OBD-II trouble code reader and post the trouble code numbers.



After you installed the new IAC, did you re-boot the engine computer ? The computer stores data and will try to use the faulty data to run the new IAC, thus you will have a high idle until it is re-booted or learns new parameters for the new part.

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Hey so I’m getting code p0505 and yes every time I installed a new IAC I disconnected the terminals and touched the positive lead to the negative for about 1 minute then did the headlights before I started it, it reset and the light was off for about 1 minute then came back on and as soon as the light comes on my gas gauge goes to all the way full then all the way empty.


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Genuine Jeep IAC or Chinese ? Most auto parts stores sell cheap crappy Chinese made parts, some that even come with a " Lifetime Warranty". These parts are poorly manufactured and/or made from inferior materials. They are often out of specification, or even failed, right out of the box. The ones that are not faulty many times will have a short service life before they fail. Always buy top quality replacement parts and genuine Jeep engine sensors. Numerous threads detail long and frustrating searches for a "problem" that ended up being cured simply with genuine Jeep repair parts.

Cheap parts are cheap for a reason.



P0505 JEEP - Idle Control System Malfunction

Possible causes

•Faulty Idle Control System motor
•Idle Control System motor harness is open or shorted
•Idle Control System motor circuit poor electrical connection
•Vacuum leaks or damaged hoses or improper routing