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Old 07-27-2013, 04:30 AM   #1
03Ruth's Avatar
Join Date: Dec 2010
Location: Missouri
Posts: 694
High erratic idle

So I posted this in JF also but figured I might reach more this way. So I have been trying to find the source of my high idle with no luck. After I start it up the rpm's will run around 1500 for about a minute and then drop down to normal range. Once I start driving I will get the rpm's up to around 2500 and once I push in the clutch to shift the rpm's will stay there unless I leave the clutch in for a bit they will come down. It's really annoying and just seems to be getting worse. The weird thing is there is no check engine light and no codes but there is definitely a problem. Around 5-6 months ago I took off the throttle body, disassembled it and cleaned it shiny new. I have cleaned the IAC and replaced the map sensor about a year ago. The throttle assembly isn't sticking and I have checked for vacuum leaks using the propane method and brake cleaner with no luck. Any ideas on what else to do?

03' Freedom edition, 4.0, 5 speed, 4" zone lift, D30, Superior D35 with detroit locker, 4:56 gears, 33x12.5 bigfoot xt, 15x8 eagle alloys

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Old 07-27-2013, 05:07 AM   #2
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Join Date: Feb 2011
Location: pennsylvania
Posts: 1,588
Mine only does that very occasionally on very hot humid days, with the a/c running.. it has been doing it since it was brand new.. The humidity goes away and everything goes back to normal.. I've had a bit of success removing the + cable from the battery for a while, then hooking back up and going on my merry way.. The stealer ship was no help at all, and my mechanic buddy just shrugs it off..

04' Rubi, with all the gizmos we need for long range excursions, / home modified excursion trailer;

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Old 07-27-2013, 06:25 AM   #3
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Join Date: Jun 2013
Posts: 89
Try the tps. My 98 4.0 was doing that mostly at red lights, but sometimes while driving. After I replaced the sensor it hasnt happed again.
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Old 07-27-2013, 06:43 AM   #4
Join Date: Mar 2011
Posts: 426
Read up on the "idle air controller" in the service manual. This is a "stepper motor" on the side of the throttle body and under the control of the PCM. It's purpose is to hold the idle speed constant under a wide variety of conditions.

Typically, when the IAC port is plugged up, the idle is too low and the engine dies without your foot on the throttle. When the IAC is stuck open, the idle is too fast.
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Old 07-27-2013, 03:59 PM   #5
Join Date: Jun 2013
Location: RVA/Tri-Cities
Posts: 64
I have the same issue. It was taking about 2500-3k RPMS...just hit the IAC with a blunt object until you can replace it. Well, it was a quick fix for me anyways
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