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Old 12-31-2011, 12:39 AM   #1
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Need help! Tj idle issues! Video of issue posted!

Hey guys, I'm having an issue with my tj. It's a 1998 sahara edition, 4.0 6 cylinder with 166k miles. Ok, so when it's at adle, it sometimes has weird fluctuations in rpm's. On a bad day, this is as bad as the crazyness gets.



This doesn't only happen when I first startup the jeep. It happens randomly, warm engine or not, anytime i'm in neutral and stopped. Also, I have been having some other issues. The oil pressure seems to be pretty low according to the gauge. It usually stays below the 20 mark, and idles at about 10 when warmed up. Sometimes I can get it to the halfway mark, but that doesn't happen often. I just changed the oil with 6 quarts of 10w30 full synthetic, but the problem still exists. Also, the check engine light is on for a small evaporation leak.

Any help would be greatly appreciated, thanks

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Old 01-05-2012, 06:40 PM   #2
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Anybody have any ideas? I really don't have to take it to the dealership.

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Old 01-05-2012, 06:50 PM   #3
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Check the vaccum lines and cannister///look for split hoses?
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Old 01-05-2012, 06:53 PM   #4
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your oil pressure seems low to me, mine generally runs above 40. I would check the sending unit and possibly the oil pump? but I wouldn't think that would do anything to the idle... very strange
Ill think about this and let you know if I come up with anything
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Old 01-05-2012, 07:00 PM   #5
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Quote:
Originally Posted by Jeep tin man View Post
Check the vaccum lines and cannister///look for split hoses?
x2

vacuum hoses should be stiff but not brittle, if they are hard and brittle or floppy and loose replace them. specifically check the vacuum hoses near the throttle body and near the idle control valve.
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Old 01-05-2012, 07:02 PM   #6
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My check engine was on and i plugged it in to the machine and it said it was a vac leak so i replaced the gas cap($10) and the light turned off.. try replacing the throttle position sensor for the idle problem.. you can get them for like 35 or 40 bucks... oil pressure.. you got me there bud.. mine normally runs right over the middle marker...
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Old 01-07-2012, 12:01 AM   #7
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Well, I cleaned up the IAC and that seemed to have helped. I drove it around a lot today and I had no idle issues at all. I still have the evap problem and the low oil pressure reading, but the weird revving issue might have been fixed. I probably will replace the oil sending unit soon because it's relatively inexpensive. Hopefully that will solve it.

As for the evap leak, I'm not finding any hose issues anywhere. I'm not really positive what a vacuum hose is and where they're located, but I couldn't see any problems with what I did see. Thanks for the help guys
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Old 01-07-2012, 12:03 AM   #8
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Oh yeah and I forgot to mention that I already replaced the gas cap, so that's not the problem. Any other ideas?
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Old 01-07-2012, 10:53 AM   #9
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The trick my old man taught me on vacuum leaks is to get a can of WD40 and spray each line individually. If the idle steadies out, then you found your offending line. If it didn't, then check sensors, etc.. I usually just replace every line if one has gone bad.
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Old 01-07-2012, 11:34 AM   #10
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First... older TJs have oil pressure gauges that indicate the true oil pressure which rises and falls in step with the engine rpms. Newer, after 2000 or 2001 or so, have oil pressure gauges that pretty much stay put mid-scale, they do NOT indicate the true oil pressure.

The real oil pressure fluctuates and a good rule of thumb for acceptable oil pressure is approximately 10 psi for every thousand engine rpms. At ldle, it will be under 20 psi and 10 is technically acceptable. Since the oil pressure pump is mechanically driven by the engine, oil pressure will rise and fall in step with the engine rpms. 30-40 is what it will climb to when accelerating or crusing.

Since few of today's young adults buying Jeeps know much about engines, such fluctuating oil pressures scared them enough that Jeep was forced to modify its oil pressure gauge so it stopped showing the real oil pressure so they would stop complaing to the dealerships. Instead, Jeeps afer 2000-2001 or so now just show a mid-scale reading to keep them happy (and dumb). Only if there is a real oil pressure problem will the gauge show anything other than a mid-scale reading... so no, 40 psi at idle as mentioned above is not the real oil pressure.

So far as the rough idle is concerned, I'm going for either a dirty IAC (idle air controller) which is mounted on the throttle body, or perhaps a bad upstream 02 sensor. Since it costs $6 or less to buy a can of throttle body cleaner so you can clean the IAC and restore it to good operating condition, I would clean the IAC first before buying an $80 02 sensor.

The IAC is easily removed and cleaned, torx screws hold it to the throttle body. Once it is off, spray the IAC's plunger and the oriface in the throttle body it fits into with throttle body cleaner and gently scrub the plunger with a toothbrush. Use a baby bottle nipple brush to clean the oriface in the throttle body. The below photos show where the IAC and an older version of the IAC.

The IAC is what provides all of the engine's air at idle rpms when your foot is not on the gas pedal so when it gets dirty, it can cause a rough idle. A pretty good indication a rough idle is caused by a dirty IAC is when the engine idle smooths out by giving it just a little gas with the gas pedal which opens up the butterfly valve on top of the throttle body which then provides the engine with its main source of air.

The IAC in the below photo works by solenoid action to move the plunger in or out of the throttle body to provide more or less air as required to maintain correct idle rpms. Off idle, all air is provided by the throttle body itself and the IAC no longer does anything.
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Old 01-08-2012, 08:07 PM   #11
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Hey Jerry, thanks for the very descriptive response. I cleaned up the IAC a few days back while I was figuring out how to take apart the throttle body. My weird idle issues have mostly disappeared! I didn't use throttle body cleaner, so I'll probably redo the cleaning with cleaner and a new gasket so as to finish up the job. I have nothin going on tomorrow so I'll do that and check out the vacuum hoses. Thanks for the help guys.
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Old 01-10-2012, 01:20 PM   #12
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My check engine light just went off on its own. This is the first time it's gone off on its own in months. The only thing I did was take off the throttle body and clean up the IAC. Does that have anything to do with the evap system? I don't get how that would fix a small evap leak.
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Old 01-10-2012, 01:31 PM   #13
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An evap code can be set by even forgetting to securely tighten the gas cap or starting the engine with the gas cap off. Maybe that's what set yours and simply driving it cleared the code.
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Old 01-11-2012, 07:22 AM   #14
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I don't think that was it. The light had been on for a few months and I has replaced the gas cap a month or so ago. In that month, the battery was replaced and i have driven several hundred miles. Oh well, it's off so I'm not gonna complain! Thanks for the help guys.
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Old 01-11-2012, 10:04 AM   #15
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I would still think about your oil pressure, that seems a bit low to me.
One thing that may help keep oil pressure stay up (oh god I'm going to mention it.. please forgive me..) a K&N oil filter. they have a valve to help maintain the oil pressure even when the engine is off. I used to have to use one for that specific purpose in my '83 Porsche 944. It was a bear at getting up to pressure.
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Old 01-11-2012, 10:05 AM   #16
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also...
glad to hear things are starting to work out and its running better.
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Old 01-11-2012, 12:59 PM   #17
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Quote:
Originally Posted by 5249matt View Post
Hey guys, I'm having an issue with my tj. It's a 1998 sahara edition, 4.0 6 cylinder with 166k miles. Ok, so when it's at adle, it sometimes has weird fluctuations in rpm's. On a bad day, this is as bad as the crazyness gets.



This doesn't only happen when I first startup the jeep. It happens randomly, warm engine or not, anytime i'm in neutral and stopped. Also, I have been having some other issues. The oil pressure seems to be pretty low according to the gauge. It usually stays below the 20 mark, and idles at about 10 when warmed up. Sometimes I can get it to the halfway mark, but that doesn't happen often. I just changed the oil with 6 quarts of 10w30 full synthetic, but the problem still exists. Also, the check engine light is on for a small evaporation leak.

Any help would be greatly appreciated, thanks
When I'm at an idle my oil pressure is about the same, while drive'n the oil pressure is anywhere between 40 and 60psi, as for the idle issue my 4cyl jeep does the same thing except it gets down to 250rpm, b4 it brings it self back up. I've always been told its a JEEP thing.
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Old 01-11-2012, 02:48 PM   #18
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Quote:
Originally Posted by Jerry Bransford View Post
The IAC in the below photo works by solenoid action to move the plunger in or out of the throttle body...
No. It's motor action, not solenoid action.


Quote:
Originally Posted by Jerry Bransford View Post
Off idle, all air is provided by the throttle body itself and the IAC no longer does anything.
Not true. At higher engine loads, the IAC opens fully to provide maximum air flow. That feature is precisely what causes high idle conditions in conjunction with TPS failures.
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Old 03-07-2012, 08:10 PM   #19
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Late

I know this is a late response, but for anyone visiting this page in the future I thought I would throw in my .02. I have a 2000 TJ, automatic 6, and had this exact same problem (random spikes/drops in idle - just as is depicted in the video). I pulled all spark plugs and replaced (ensuring a proper gap) and the issue disappeared - for the past 6 months I have a very constant idle.

Recently I've had issues with idle right after a cold start. The TJ starts fine, but after it's put in gear it stumbles and sometimes stalls out. Once I drive it for 4-5 minutes there is no issue as the Jeep idles at a consistent 600 or so. Today I pulled apart the throttle body and the IAC and cleaned with throttle body cleaner. I'm hoping this solves the issue!

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