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Old 02-16-2014, 10:40 PM   #1
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Low Idle when T-stat opens

Hello everyone.
So this is my first post, although you all have helped me out many times in the past.
To start I did an engine swap with a rebuilt 98 4.0 1200km ago, so a little over a month. I didn't notice the problem until I installed a light bar a couple weeks ago. after the install, I went out to test the light, and the jeep was almost stalling between shifts.
So here is my problem.
I have a 95 with a 98 4.0 with an idle issue. What seems to happen is when the jeep is just at the point of the tstat opening the idle drops to the first line on the tach after 0. 300Rpm maybe?
Just for a few seconds, then Ill give it some gas and shes good till she is shut off and warms up again.
Ive done a few tests and part changes. They are as follows.
Battery Cables and terminals, problem remains, not as severe.
IAC, cleaned, problem persisted, swapped with new, problem remains.
Throttle body cleaned, problem remains.
Coolant temp sensor, swapped, problem remains.
No vac leaks found.

I tested the ohms on my coolant sensor, but I don't know how that stuff works, so the numbers I got don't make any sense to me. So I had the meter set to the 2000K Ohms. Cold I got 032 and hot I got 001.
I have no clue what this means. Either way, my old sensor and new sensor have the same reading.

I don't know what is left.

I have a video on youtube of the low idle happening. notice as I release the throttle the rpms will drop, then ill give some gas and it comes back up.


Any help or suggestions would be greatly appreciated.

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Old 02-17-2014, 11:29 AM   #2
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Bump, anybody have an idea?

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Old 02-17-2014, 02:15 PM   #3
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Are you getting any codes? And possibly o2 sensor I'm having a similar idle issue and now finally my computer is showing a o2 sensor code.
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Old 02-17-2014, 02:50 PM   #4
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Sounds to me like you have a major vacuum leak. Most likely on the brake booster.

You cant hear the whistling sound?

Try taking the big vac line off the brake booster and plugging the line, then try your test again.
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Old 02-19-2014, 11:56 PM   #5
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No codes. Not yet anyhow.
And no I don't hear any vacuum leaks. I sprayed all the hoses with throttle body cleaner and the idle didn't change at all. And unless I'm mistaken, or it's different with the brake booster, but don't vacuum leaks lead to a raise in idle? I discovered today that my Tstat opens multiple times on my drive home from work.
And every time it drops my gauge to the mark between 40 and 100.
Then when I start driving again, it will raise to 100 and open again and drop. My rpms drop every time my temp does.
Could a bad thermostat cause this, and confuse the computer when it is in open loop?
What should I expect when I plug my booster? And should I plug the hose and the hole in the booster? And if my problem is gone, what would the next step be with that?
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Old 02-20-2014, 09:33 AM   #6
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The whistle sound on your video. You either have a vacuum leak or a really big turbo that is spooling up when you rev it.

You plug the hose coming from the motor. If it cures the problem, it has a bad brake booster. (the big round thing that the hose plugs into). Time to replace it.
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Old 02-20-2014, 05:19 PM   #7
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The whistle sound on your video. You either have a vacuum leak or a really big turbo that is spooling up when you rev it. You plug the hose coming from the motor. If it cures the problem, it has a bad brake booster. (the big round thing that the hose plugs into). Time to replace it.
That's my throw out bearing. Sound goes away when I press the clutch in. I'll test the rake booster anyhow. But all those systems work fine.
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Old 02-21-2014, 08:01 PM   #8
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So plugging the booster didnt do anything.
I sprayed every vac line I could see with throttle body cleaner. No leaks.
I am going to try the Tstat tonight. And I'm pulling the throttle body and hitting it with a tooth brush and cleaner.
Also going to test my o2 sensor. After this, I'm all out of ideas. Thanks for the suggestions. Keep em coming!
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Old 02-23-2014, 04:27 PM   #9
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So plugging the booster didnt do anything. I sprayed every vac line I could see with throttle body cleaner. No leaks. I am going to try the Tstat tonight. And I'm pulling the throttle body and hitting it with a tooth brush and cleaner. Also going to test my o2 sensor. After this, I'm all out of ideas. Thanks for the suggestions. Keep em coming!
None of this worked. I didn't get a chance to do the Tstat. So that's next.
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Old 02-23-2014, 06:00 PM   #10
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None of this worked. I didn't get a chance to do the Tstat. So that's next.
Is this the same jeep that has 6 pages worth of stuff somewhere else?!? If so, anything they've said, will likely be said here too. Ive got a low idle once warmed, and have never figured it out. Ill be following this post too. If I find something, you'll be the 2nd one to know.
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Old 02-23-2014, 10:19 PM   #11
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Is this the same jeep that has 6 pages worth of stuff somewhere else?!? If so, anything they've said, will likely be said here too. Ive got a low idle once warmed, and have never figured it out. Ill be following this post too. If I find something, you'll be the 2nd one to know.
It is. I'm trying to get as many brains working at this as I can. I'm swapping my Tstat soon here. And if that doesn't work. I'm thinking a JY PCM. See if that changes anything.
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Old 02-24-2014, 10:20 AM   #12
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It is. I'm trying to get as many brains working at this as I can. I'm swapping my Tstat soon here. And if that doesn't work. I'm thinking a JY PCM. See if that changes anything.
swap it out ASAP. they're cheap, and easy to do. Not sure why a T-Stat could cause your issue, but hell, its worth a shot!
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Old 02-24-2014, 07:34 PM   #13
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swap it out ASAP. they're cheap, and easy to do. Not sure why a T-Stat could cause your issue, but hell, its worth a shot!
I've got two Tstats on hand actually. Brand new. So I'll be swapping that Friday. Ran out of time this weekend. I think I'll be throwing on my winter front cover as well. Hopefully that will help with the drastic drop. I'm not convinced it's the Tstat, but I've seen weirder things. If that fixes it, this has to be one for the books. Thanks for your suggestions.
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Old 02-27-2014, 03:45 PM   #14
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So you have a 95 ecm running a 98 engine? Surprised thats all the problems you have.
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Old 02-27-2014, 04:25 PM   #15
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Originally Posted by LittleGreenYJ View Post
Hello everyone.
So this is my first post, although you all have helped me out many times in the past.
To start I did an engine swap with a rebuilt 98 4.0 1200km ago, so a little over a month. I didn't notice the problem until I installed a light bar a couple weeks ago. after the install, I went out to test the light, and the jeep was almost stalling between shifts.
So here is my problem.
I have a 95 with a 98 4.0 with an idle issue. What seems to happen is when the jeep is just at the point of the tstat opening the idle drops to the first line on the tach after 0. 300Rpm maybe?
Just for a few seconds, then Ill give it some gas and shes good till she is shut off and warms up again.
Ive done a few tests and part changes. They are as follows.
Battery Cables and terminals, problem remains, not as severe.
IAC, cleaned, problem persisted, swapped with new, problem remains.
Throttle body cleaned, problem remains.
Coolant temp sensor, swapped, problem remains.
No vac leaks found.

I tested the ohms on my coolant sensor, but I don't know how that stuff works, so the numbers I got don't make any sense to me. So I had the meter set to the 2000K Ohms. Cold I got 032 and hot I got 001.
I have no clue what this means. Either way, my old sensor and new sensor have the same reading.

I don't know what is left.

Any help or suggestions would be greatly appreciated.
Which temp sensor? the one on the T-Stat housing, or the one on the driver side rear, next to valve cover?
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Old 02-27-2014, 08:18 PM   #16
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So you have a 95 ecm running a 98 engine? Surprised thats all the problems you have.
Really? The engine is the same. The only thing that is 98 is the block and head. Everything else is 95. That shouldn't be a problem. All the sensors and components are 95.
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Old 02-27-2014, 08:21 PM   #17
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Which temp sensor? the one on the T-Stat housing, or the one on the driver side rear, next to valve cover?
just the stat housing. Does the rear one affect idle?
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Old 02-27-2014, 09:03 PM   #18
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just the stat housing. Does the rear one affect idle?
No sir, that one goes straight to your gauge. Just making sure you replaced the right one.
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Old 02-27-2014, 09:55 PM   #19
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No sir, that one goes straight to your gauge. Just making sure you replaced the right one.
Oh okay. Thought missed an obvious one lol. I'm still planning the stat change tomorrow. So I'll post what I find. If nothing changes I'm doing a compression test next. It's a new (rebuilt) engine. So I can't see that being a problem. But it isn't out of the realm of possibilities. Then I'll be heading to the JY for a PCM. And on that note. We don't get wranglers at the JY around here. I've seen two in the 4 years of owning mine.
So will a Cherokee PCM work with my jeep. I read elsewhere of someone using a 93 grand cherokee with the I6 in it and it was a direct plug and play. Do I just need to find a pre OBD2 with the I6 and a AX-15 trans? Thanks for the help.
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Old 03-01-2014, 12:09 PM   #20
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So swapping the Tstat appears to have fixed my idle surge. It still sits at around 500-550 ish. Plugs look good and cylinders look new still. So I'm still working on it.
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Old 03-01-2014, 01:30 PM   #21
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So swapping the Tstat appears to have fixed my idle surge. It still sits at around 500-550 ish. Plugs look good and cylinders look new still. So I'm still working on it.
But its not stalling now?
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Old 03-03-2014, 09:56 PM   #22
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But its not stalling now?
No, never did stall out completely. I kept it alive every time. But It would have for sure. So it's just a low idle now. I tried swapping to my old air box but I didn't notice a change. I have to rig something up so I can drive it like that though. I just tried it at idle as my air box doesn't fit anymore due to the XJ steering pump. So I want to see if it changes while driving.
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Old 03-04-2014, 07:40 PM   #23
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No, never did stall out completely. I kept it alive every time. But It would have for sure. So it's just a low idle now. I tried swapping to my old air box but I didn't notice a change. I have to rig something up so I can drive it like that though. I just tried it at idle as my air box doesn't fit anymore due to the XJ steering pump. So I want to see if it changes while driving.
look on your TBI. You know where you can rev it up from inside the engine bay? There's a little set screw that it rest's on when your not on the gas. Maybe you can give it half a turn? Its dark here, or else i'd take a picture.
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Old 03-04-2014, 07:47 PM   #24
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look on your TBI. You know where you can rev it up from inside the engine bay? There's a little set screw that it rest's on when your not on the gas. Maybe you can give it half a turn? Its dark here, or else i'd take a picture.
I actually just thought of that on the way home. I noticed it when I cleaned everything. Might e my last option.
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Old 03-06-2014, 08:00 PM   #25
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Adjusted screw. Brought idle up, smoothed it out as well. So I'm going to leave it where it is now and see how it goes.
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Old 03-06-2014, 08:19 PM   #26
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Adjusted screw. Brought idle up, smoothed it out as well. So I'm going to leave it where it is now and see how it goes.
Awesome!

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