Will idle at high rpm - JeepForum.com
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post #1 of 8 Old 11-09-2019, 06:51 PM Thread Starter
Benjamoney
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Will idle at high rpm

Hi everyone, I bought a 2000 jeep tj with the I4 engine over the summer and recently have been having a weird problem. When I push in the clutch to shift gears it'll hover at whatever RPM I was at for 1-5 second(s). I recently replaced I IAC but it seems that as the engine warms up it doesn't have this problem as often. I was thinking perhaps the MAP sensor was broken, but figured I should ask to see if anyone has had a similar problem in the past.

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post #2 of 8 Old 11-14-2019, 09:52 PM
mukluk
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High idle speed is often due to vacuum leaks on the intake side of the engine. With the engine running, spray short bursts of sensor safe carb/intake cleaner at all mating surfaces, vacuum lines/caps, the small rubber boot joining the MAP sensor to the throttle body, and other suspect leak areas while listening to the idle speed -- when the cleaner is sprayed at an area with a leak the idle speed will change briefly.

If you didn't already clean the throttle body and its passage for the IAC, this would be a good idea. Also, rapidly operating the throttle through its full range of motion several times (engine and ignition switched off) and cleaning the TPS connector may help if the sensor has been in service for a while now.

Something to keep in mind is one of the functions of the IAC is to act as a dashpot -- it slows the engine's return to idle when the throttle pedal is released suddenly or when the vehicle is decelerating. If your high idle issue only happens between shifts, this may be what you're noticing.
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post #3 of 8 Old 11-29-2019, 03:59 PM Thread Starter
Benjamoney
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Is it true that if you unplug the IAC and it continues to run without stalling their is a leak in a vacuum line somewhere? I unplugged the IAC and it didn't miss a beat. Also where would all of the mating surfaces be? I'm new to the jeep scene and there doesn't seem to be as much info for the I4 as the I6. thank you
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post #4 of 8 Old 11-29-2019, 04:17 PM
Varnish
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My first thought would be vacuum leak as mukluk described but also make sure not a mechanical stick in the throttle cable.
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post #5 of 8 Old 12-01-2019, 11:45 AM
Jerry Bransford
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Quote:
Originally Posted by Benjamoney View Post
Is it true that if you unplug the IAC and it continues to run without stalling their is a leak in a vacuum line somewhere? I unplugged the IAC and it didn't miss a beat. Also where would all of the mating surfaces be? I'm new to the jeep scene and there doesn't seem to be as much info for the I4 as the I6. thank you
There's not much difference that way between the I6 and I4. Vacuum leaks cause that problem and if the engine idles ok without the IAC operating there's a vacuum leak somewhere. The IAC only comes into play at idle rpms, off idle it does nothing.

The below illustration shows the vacuum lines. Not shown is the large vacuum line between the power brake booster and intake manifold. A common source of vacuum leaks is if the intake manifold had been removed to replace the exhaust manifold and it wasn't reinstalled properly, though that normally causes extremely high engine rpms.
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post #6 of 8 Old 12-02-2019, 03:24 AM
Jonny Jeep
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Quote:
Originally Posted by Benjamoney View Post
... When I push in the clutch to shift gears it'll hover at whatever RPM I was at for 1-5 second(s). I recently replaced I IAC but it seems that as the engine warms up it doesn't have this problem as often...
I don't remember if it was temp related on my TJ as it was many years ago now. When I had an issue like that it was caused by a bad TPS sensor. After a while it got worse, where the revs would actually increase during gear changes. I also developed a surging idle where the revs would alternate between normal and about 2000 rpm.

I eventually got a code for it, but don't remember if I ever had a check engine light. Might be worth getting yours scanned for codes.
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post #7 of 8 Old 12-02-2019, 09:35 AM
Jerry Bransford
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Considering it's job and purpose I can't figure out how the TPS could possibly cause high rpms.

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post #8 of 8 Old 12-02-2019, 04:18 PM
Jonny Jeep
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I don't know how either. Other than mine did, eventually setting a code, and replacing it with a Mopar TPS fixed it. I don't remember getting a CEL either so it may have only been a pending code.
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