Riddle me this, my idling problem. - Jeep Wrangler Forum
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Old 05-15-2013, 05:35 PM   #1
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Riddle me this, my idling problem.

First, let me say hi to everyone. Super long time lurker and thourougly enjoyed
and appreciated all the posts I've read. In the past, should I be experiencing a problem, I could do a quick search and find answers. However now I have a problem and can't figure it out.

93 YJ 2.5 5Spd. Driving down the road at any given time, everything runs fine, no problems, come to a stop and my idle wants to sit at about 500 RPM, a little too low, kinda shaky. Now here's the riddle/problem, lets say I'm driving at any speed, lets say 40 mph like I did today, drop the clutch and slip it out of gear, release the clutch and coast. While coasting down from 40 mph I'm watching the tack, it's sitting at or a little above 1k RPM and stays there until about I'm nearly stopped, then drops back around 500 rpm. I can't figure it out, I have no load on the engine at all during coasting, not applying brake, nothing but it's own momentum. Why is my idle staying where I would like it, but then dropping low, just a few rolling feet from being stopped?

Thanks again everyone. Hopefully I can be more of a contributor than a lurker from now on since the ice is broken.

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Old 05-15-2013, 05:51 PM   #2
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Are you throwing any CEL codes, my guess would be while coasting you have more airflow (kinda like the ram air concept).

If you don't know how to check for codes let us know. It could be a number of things causing your idle to be low such as TPS, O2 sensors, IAC sensors etc. Best bet would be to check the codes first.

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Old 05-15-2013, 05:53 PM   #3
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Have not checked codes, don't have a scanner. If it matters, just went through
emissions a week ago. But then again, this is an older OBD system and they didn't check, at least I didn't see them plug into the connector on the driver side firewall.
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Old 05-15-2013, 05:55 PM   #4
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I forgot .. Welcome to the forum, don't forget to introduce yourself when you get some free time. Post some pics too, they are worth a thousand words you know. ;0)
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Old 05-15-2013, 05:59 PM   #5
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Don't need a scanner. Turn the ignition on, off, on, off, on, off then on. The check engine light will flash. Codes are two digit numbers code 12 would look like flash ...... Flash, flash. 33 would look like flash, flash, flash .........flash, flash, flash. At the end of the codes you'll get a 55. That means end of code sequence.

Once you write down all the codes and double check them post up and we'll tell you what they mean!
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Old 05-15-2013, 06:07 PM   #6
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Another note, it is very common for emission techs to retard the timing to factory settings. This could cause low idle, check your timing to see if it needs to be advanced now that you passed.
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Old 05-15-2013, 06:07 PM   #7
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Okay, just did it. After doing the on/off routine, the CEL lit for about 1.5 seconds and then started doing it's flashing pattern. I get a 12 and a 33.
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Old 05-15-2013, 06:08 PM   #8
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And thank you for the welcome. I'm Robert in Surprise, AZ. PIC's will be in the future.
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Old 05-15-2013, 06:14 PM   #9
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Ok 12 means your battery was unhooked in the last 50 start ups and 33 is A/C. If you don't have A/C don't worry about that one.

Both of those codes are normal, so I would do some research on idle air control IAC, they get clogged with carbon but they can be cleaned plenty of threads on that.

Next would be throttle position sensor TPS, they usually throw a code when going out.

I am typing this all off memory, as a new jeep guy one of you Jeep lifers giggle check my advise.
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Old 05-15-2013, 06:17 PM   #10
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No A/C, so that makes sense. I had already planned to clean the T/B and IAC, I was just dumbfounded why those two items would have anything to do with just coasting, except for maybe airflow through the T/B. Thank you for the help.
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Old 05-15-2013, 06:23 PM   #11
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You are welcome.

Remember for everyone that helps you, you should give back to the forum by helping someone else. That is what makes this forum great, people willing to help.

Even if I am wrong it might give the next poster something to piggy back on, a memory jogger of sort.

Good luck getting your jeep fixed.
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Old 05-16-2013, 08:48 PM   #12
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A little more information for my problem. Today coming home from work I intentionally and patiently messed around to see if I could get any more thought as to why this is happening.

I found a piece of road that would allow me to coast for quite a distance without any help from me, via gas pedal. What I found out was, I could literally coast along around 3 mph, (I had my GPS on to verify speed), and the idle would stay around 1000 RPM. 3 MPH is about what the average person walks, pretty slow in comparing to vehicle speed. But as soon as I get just about to a staop, RPM needle drops to around 500 RPM, rougher than I would like idle.

I'm wondering if the VSS-Vehicle Speed Sensor is the culprit. Anyone care to comment please?

Thanks again!
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Old 05-16-2013, 09:45 PM   #13
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My 95 2.5L did the same thing last week but would idle too high for way too long, got some carb and linkage cleaner and scrubbed the TB intake and just cleaned er up nice and now it settles around 750 rpm,

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95 2.5L, 5sp, 4" RC Lift, Lock Right front diff. 8.8 LS 4.10 gears, M/T 31" Baha Claw
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