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99 xj Extremely high idle

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Old 09-16-2018, 07:21 AM
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Default 99 xj Extremely high idle

Hey guys, I've got an extremely rough idle sitting anywhere from about 2400 to 3000 in park and neutral, while in drive it sits at about 1400. Having problems going into gear until it hits about 3000 rpms. I was wondering what y'all think it could be? I've cleaned the throttle body, spark plugs and wires, new cap and rotor, tps, and IAC. I don't know what else to hit .I'm goin to try to reseal the fuel injectors, is there anything else I should try?
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Old 09-16-2018, 07:34 AM
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Any codes?

The varying RPMs in different gear tells you a lot about what it may be - more likely sensor/transmission than a vacuum leak. If your issue was caused by a vacuum leak, then the RPMs would be high all the time. Start by pulling the codes.
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Old 09-16-2018, 08:08 AM
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Originally Posted by OldTires View Post
Any codes?

The varying RPMs in different gear tells you a lot about what it may be - more likely sensor/transmission than a vacuum leak. If your issue was caused by a vacuum leak, then the RPMs would be high all the time. Start by pulling the codes.
I have a few 02 sensor ones and the torque converter clutch had one too. No rpm drop at lockup.
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Old 09-16-2018, 10:25 PM
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google high idle on jeep. mine was a slide piston on the throttle body. took it out and sanded, cleaned and lubed it, the reinstalled. works fine now.
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Old 09-17-2018, 01:48 AM
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Originally Posted by OldTires View Post
Any codes?

The varying RPMs in different gear tells you a lot about what it may be - more likely sensor/transmission than a vacuum leak. If your issue was caused by a vacuum leak, then the RPMs would be high all the time. Start by pulling the codes.
Originally Posted by smooth move View Post
google high idle on jeep. mine was a slide piston on the throttle body. took it out and sanded, cleaned and lubed it, the reinstalled. works fine now.
ill check it out, thanks
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Old 09-17-2018, 10:46 AM
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The only thing that increases RPM on an XJ is the introduction of more air (actual air, not to be confused with computer/sensor values). There is only one component that can change the amount of air based on what the computer thinks, and that is the Idle Air Controller (IAC). And of course a failing IAC can stick open or closed (most common is failing and being completely closed, causing stalling when foot is off the pedal). The other two means of introducing additional air are not related to the computer, or sensors, etc. That is a throttle body where the throttle plate is not closing all the way when the pedal is released (because it is dirty, or has some other physical mechanical problem). The last possibility being vacuum leaks, where air is being sucked into the combustion process through some route other than the pedal controlled portion of the throttle body.

The IAC depends on the Throttle Position Sensor (TPS) to know when the pedal has been released by the driver. So a failing TPS that is falsely indicating that throttle position is at zero (foot off pedal) can cause the computer to open the IAC even when the vehicle is not at idle. Which can lead to more air entering the equation than the driver intends (leading to high idle and other weirdness).

But stay focused on the fact that high idle = too much air (not what a sensor is reporting, but actual physical air). That is why the most likely cause of high idle is air entering the system in an uncontrolled way (a leak), and it doesn't take much. Vacuum is highest when the vehicle is idling, so high idle at lower RPMs (or at least where you would expect lower RPMs) that get better at higher RPMs can be indicative of a vacuum leak at the intake manifold or throttle body. For example, foot off the pedal = 2500 rpm (too high), but holding the pedal down a little allows you to hold the RPMs steady at around 3000 (just depends on how much pedal you give it). That is a tell tale sign of vacuum leak.

Watch youtube videos on the various techniques for locating the vacuum leak. If there is no indication of air being sucked in somewhere, then turn your attention back to the IAC and TPS. TPS can be tested pretty easily with a multimeter, but is even easier if you have a cheap obd2 scanner with data (can be had for $20 on amazon). The computer adds the throttle position value to the OBD2 data items. At idle (foot off pedal) mine shows throttle position at 16.2%

Last edited by jordan96xj; 09-17-2018 at 10:49 AM.
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Old 09-17-2018, 02:09 PM
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vacuume leak?
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