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Old 08-13-2015, 08:00 PM
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What is causing surging idle in Park/Neutral?

Hello everyone!

I just returned from my leave in the States. I only get to drive my Jeep once a year and this year I noticed a strange idle issue. I have a 2005 Limited with the 5.7L. My Jeep is not driven while I am not in the States and sometimes the battery will go dead (Someone occasionally drives it for me but sometimes it goes long enough between drives where the battery will die).

In 2013 I put on a Fastman throttle body. This is where I first noticed the issue. Driving seemed fine but if I stopped to park or something and put the Jeep in Park, the idle would sometime surge and fall and continue to do this until it either a) stall out and dies, or b) eventually settle at the normal idle. I was playing with it more this vacation and found that it does it most often when the Jeep is in Park (like starting up or parked idling) or when I put the Jeep in Neutral. If it is doing this surge and drop cycle, if I put the Jeep in Drive/Reverse, you can feel the line pressure in the trans take affect (or I assume) and then the idle will almost instantly settle where it should be (around 750 rpm).

My first thought was the Fastman throttle body. I contacted Dan and he suggested calibrating the TB. Well, I got home and calibrated it a dozen times and it never seemed to fix the issue. I ended up taking off the Fastman TB and going back to stock to see if it solved the problem (and made sure it was the Fastman TB). Well, with the stock TB on, the Jeep still does this surge and drop routine but it is more subtle and will not go to the extent of causing the Jeep to die. This points out that the Fastman TB is not the culprit but only makes it more noticeable.

In 2013 I replaced the MAP sensor and the PCV. In my ZJ, I would certainly look into the IAC but this 5.7 does not use it. What should I be looking at to replace or what could be causing this issue?

I appreciate any help!

Derek

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Old 08-13-2015, 08:06 PM
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Re: What is causing surging idle in Park/Neutral?

I heard from one where he solved a similar problem on a 5.7L with a new crank position sensor.
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Old 08-13-2015, 08:28 PM
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Re: What is causing surging idle in Park/Neutral?

Thanks for the insight. Seems both the Camshaft and Crankshaft position sensors are pretty cheap and easy to replace. I will be sure to give those a shot next time I am home.
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Old 08-13-2015, 09:53 PM
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Re: What is causing surging idle in Park/Neutral?

Since this all started when you installed a new TB, I believe you'll need to do a "real" electronic throttle relearn. It'll take a good computerized diagnostic tool to do it correctly though. Just pushing the throttle a few times with the key on and engine off won't do a complete relearn. I had similar problems with my BBK throttle body, and I used my Autel DS708 to do the relearn and that solved the problem. Does your cold start idle surge high also? Once warmed up does everything work OK or do you still have the surge?

Your problem with putting the old one back on is that the PCM was already in the process of trying to relearn the new one. Anytime a new TB is installed a complete ETC Relearn should be done. Sometimes you'll luck out though and everything will work fine, but almost all of the time it'll take a while (even after the ETC Relearn) for the PCM to become completely sync'ed to the TB.
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Old 08-14-2015, 12:46 PM
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Re: What is causing surging idle in Park/Neutral?

Well, I don't really want to buy the Autel to fix this. Must be nice to have such a tool. I see updates are $800/yr? Wow!


How long would it take for the PCM to relearn everything? I was home for 4 weeks and the issue never cleared up. I disconnected the battery while swapping the TB's so that should have reset the PCM right?
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Old 08-14-2015, 01:54 PM
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Re: What is causing surging idle in Park/Neutral?

Some say that ethanol gas has a short shelf life.Since your jeep sits that long i would use non-ethanol gas.
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Old 10-01-2015, 08:26 PM
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Re: What is causing surging idle in Park/Neutral?

I doubt the issue is caused by ethanol in the gas. It's probably the TB
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