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Old 01-15-2020, 01:57 PM
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99 Wrangler idle/throttle issues

Hi,

I have a 1999 Jeep Wrangler. I have been the only owner.
It is an 4.0L I6.
Lately, I have been having issues.
I start it and the rpms are all over the place - leading me to look at IAC.
When I put it in reverse, if I do not gas it hard, it wants to stall out.
Then when I am going, regardless of the gear, the vehicle jerks around - leading me to look at the TPS.
However, it does seem to calm down a bit as the engine warms up.
In any case, I replaced both the IAC and TPS, and the problems continue.
When I am sitting at a traffic light, the rpms will be anywhere between 750 and 1300.

I am not a pro by any means.
Is there anything else that I am missing?
I have also ran fuel injector cleaner through the system.

I appreciate your help.

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Old 01-15-2020, 02:19 PM   #2
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Does that problem happen both when the engine is cold in the morning and after it has started to warm up? Or just after the engine has warmed up?

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Old 01-16-2020, 02:39 PM
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Hi. It is worse when I first use it.
I also smell more gas than usual - but I cannot see how that is related.
Thanks.
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Old 01-16-2020, 02:52 PM   #4
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Hi. It is worse when I first use it.
I also smell more gas than usual - but I cannot see how that is related.
Thanks.
So it is worse immediately after a cold start. That eliminates the easy solution I was hoping for, that it could have been a bad O2 sensor but it doesn't seem to be that now.

Does your temperature gauge seem to work normally? Indicating cold when the engine is first started in the morning and then up to around 210f after it is fully warmed up?
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Old 01-16-2020, 03:19 PM
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Yes, the temperature gauge seems fine, and it does get over that 200 degree mark after a bit.
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Old 01-16-2020, 03:31 PM   #6
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Did you by any chance install aftermarket TPS and IAC products like from Autozone, O'Reilly's,. etc.? For sensors and anything critical like they are it's always best to stay with Mopar or at least OEM manufacturers. Where sensors are concerned I'll never use a store brand or off-brand.
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Old 01-16-2020, 03:42 PM
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Yes, I got them off Amazon. I did not get any codes.
But you think that may be the issue?
Where do you get your parts? Rockauto? Dealer?
Thanks again. This has been frustrating.
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Old 01-16-2020, 04:02 PM   #8
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Do us both a favor... since you did not clean the IAC and instead replaced it, there could be dirt/combustion chamber byproducts still inside the orifice in the throttle body that the IAC's plunger fits into... try spraying some aerosol throttle body cleaner into the throttle body while the engine is idling. The engine pulls all its air through the IAC only at idle rpms so cleaner sprayed into the TB at idle rpms would clean the orifice in the TB the IAC plunger fits into.

The engine will die when the spray floods the IAC but that's good, that gives the cleaner a little bit of extra time to "cook" on any deposits inside there. Just restart the engine and do it again, I try to get a good 1/4 to 1/3 of the can through the throttle body while the engine is at idle rpms.

Once you have that much of the can through the TB at idle rpms, empty the can into the TB while holding the rpms above idle.

This can't hurt and it'll at least get rid of my nagging doubt there could be dirt inside the IAC area. The IAC seldom actually goes bad, it normally just gets dirty so I always just clean them. I've never needed to actually replace an IAC.

In fact if it was me I'd reinstall the Mopar IAC you removed and have it installed while you perform the above procedure.
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Old 01-16-2020, 04:55 PM   #9
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Take a close look at the wiring harness all the way from the IAC/TPS area, around & behind the engine & over to the PCM. Look for any sign of damage &/or exposed wires especially behind the engine. It is tight back there & hard to see to inspect but that is a known place where the harness can rub against the engine or a bolt that sticks through from inside the cab. If the harness is damaged it can cause strange & unpredictable problems.
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Old 01-16-2020, 06:47 PM
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Ok. I will try cleaning it out. If that doesn’t work, where do you like to buy your parts (dealer...)?
I did inspect the wires, and did not see any damage. It’s just a confusing situation.
I appreciate all the input.
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Old 01-23-2020, 09:06 AM
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Jerry,

You were 100% correct!
The Jeep ONLY likes Mopar parts.
I cleaned out the throttle body, and put a Mopar IAC and TPS on, and it’s running smooth.
I wouldn’t have thought it HAD to be. Mopar part.
In any case, a million thanks to you.

Regards.
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Old 01-25-2020, 05:55 PM   #12
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Just an added thought, if you remove your throttle body and IAC clean the underside of throttle body and orifice chamber. also if you have access to a good OBD scan tool check your TPS and make sure it is not exceeding 5 volts. One other thing is checking for is vacuum leaks, they really only present themselves at idle and manifest as rough idling, as rpms increase your motor smooths out. If your fuel trim is high it is an indication of a vacuum leak as more fuel is being dumped into fuel ejectors to compensate for extra air.

Glad you have it running better
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Old 01-25-2020, 06:52 PM   #13
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... also if you have access to a good OBD scan tool check your TPS and make sure it is not exceeding 5 volts.
Not arguing here but how could it ever exceed 5 volts? Its signal voltage is coming from the PCM whose signal/logic voltage is 5 volts. The TPS gets its 5 volts directly from the PCM.
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Old 01-26-2020, 11:12 AM   #14
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I usually graph the throttle plate position with rpms at same time, closed throttle plate around .45 volts as rpms and throttle opens up volts will increase to around 5 volts if there is a dip in curve as rpms increase I would suspect tps at fault. If over 5 volts start suspecting a bad throttle position sensor. In his case the rough idle been rectified with cleaning and replacing with original IAC.

Vacuum leak could still be an issue with older TJs with old cracked vacuum lines and gaskets.

Yes you are absolutely right the PCM does send 5 volts to the TPS however the voltage going back to PCM will vary with position of throttle plate.
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Old 01-26-2020, 12:43 PM   #15
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Yes you are absolutely right the PCM does send 5 volts to the TPS however the voltage going back to PCM will vary with position of throttle plate.
So how could it ever be more than 5 volts?
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Old 01-26-2020, 01:23 PM   #16
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That is the point should not exceed 5 volts which would indicate other problems.
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Old 01-26-2020, 02:22 PM   #17
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That is the point should not exceed 5 volts which would indicate other problems.
That's the question I keep asking... its supply voltage is only 5v so how could it ever exceed 5v???
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Old 01-26-2020, 03:14 PM   #18
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You know I don’t see why you have to keep asking that question repeatedly, without the specific vehicle in my shop I it would be hard to answer your question. I was just trying to give another member so gained advice.

This is the reason I do not like to contribute to the forum, there always has to be someone who likes to show that. “He knows a couple of things”

I’m glad the gentleman resolved his issue
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Old 01-26-2020, 03:24 PM   #19
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You know I don’t see why you have to keep asking that question repeatedly, without the specific vehicle in my shop I it would be hard to answer your question. I was just trying to give another member so gained advice.

This is the reason I do not like to contribute to the forum, there always has to be someone who likes to show that. “He knows a couple of things”
I'm sorry you see it that way but troubleshooting advice, even when well intentioned, needs to be questioned or clarified when it is misleading. You advised him to make sure his TPS voltage wasn't exceeding 5 volts. Since it can never exceed 5 volts he would be led to believe it was ok, even if it has a problem, since it would always read 5v or less. A TPS is strictly a passive variable resistor/potentiometer so it cannot raise voltage above its supply voltage if it is defective.

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