Help needed with odd low RPM issue

[Beherrschen]

I have searched and searched and found no one with my particular problem.

2004 Unlimited with 4.0L and auto transmission with no error codes.

So the problem is intermittent low idle at a complete stop. The idle jumps up and down about 100 rpms around the 400rpm range at a period of about 1 second. There are no troubles while driving and it starts the moment the wheels actually fully stop. Twice in the last 6 months it has actually cut off while coming to a sudden stop but started right back up with no issue. I have cleaned the throttle body, replaced/cleaned IAC, replaced TPS, checked for vacuum leaks, replaced spark plugs, fuel injector cleaner, higher octane/different gas, and replaced ignition coils assembly (it had gone bad). Here is the part that makes it odd compared to everything else I can find. While rough idle is occuring (and it occurs during cold and hot engine times) I can stop the low 400rpm idle by two methods: 1- lightly tapping the gas. Just enough to jump it 200 or so rpm and then the idles just fine until I drive and then come to a stop again. 2- shifting into neutral (or just out of drive) for even a short moment. I can then shift back to drive and it will idle fine.

I spoke with a couple of mechanics and they have both insisted it was in need of an IAC cleaning despite my insistence that I had taken it apart and cleaned it thoroughly and that it wasn't very dirty to begin with.

Any help with this would be greatly appreciated as I am out of ideas and can't find anyone with this exact problem (only similar problems).

 

[twistedlv]

That almost sounds like a torque converter not wanting to unlock. But you would think that would throw a code.

 

[mots]

Sounds like it could be a torque converter issue to me too, since neutral alleviates the low idle. I think you may be able to disconnect a connector on the transmission to disable. I don't have an auto, but I recall reading this somewhere. Would be worth checking into to troubleshoot this theory further.

P.S. if that doesn't pan out, also check into a bad brake booster. There should be ways to troubleshoot that as well if you think braking is involved in the stalling (plug booster hose and see if you can repeat the stalling).

 

[Beherrschen]

Thank you for your replies. I thought about the torque converter but I have no other symptoms and it has been going on for months now. My understanding is that when torque converters go bad it happens fast. That true?

I didn't think about the brake booster. Would that only cause the stalling low idle when first stopping or anytime the brakes are pressed? Maybe to test I am thinking that after stopping and stopping the rough idle (putting it in neutral and back to drive) I could let go of the brakes and then press them real fast and see if anything happens with the rpm??

 

[mots]

Not sure about the torque converter. There is a factory service manual (FSM) posted somewhere on this site that you can look at for troubleshooting. Perhaps someone else here knows for sure if you can disconnect the connector from the transmission in order to troubleshoot if the problem goes away (like I mentioned before).

For the brake booster, you should be able to tell pretty easily if using the brakes is playing a role in the low idle situation. I believe you can unplug the vacuum line from the booster and then put a plug in it (proper size bolt with hose clamp). Then, try idling again to see if it still drops. I'd also check the FSM for more about troubleshooting.

My first guess was also IAC, but you had already mentioned looking into that. I completely removed mine and thoroughly cleaned that and the throttle body when I got the Jeep. It did make it idle a little better, but I didn't have the problem you have. Perhaps yours is actually beyond cleaning and needs replaced.

 

[Beherrschen]

Thanks and I'll take a look at the brake booster as you mentioned. I tried to see if the engine reacted at all to sudden presses of the brake pedal but got nothing.

I have replaced the IAC.

 

[Beherrschen]

So, as soon as I unplug the vacuum line to brake booster the idle goes up and stays steady. Probably up from normal rpm of 500- 600 at idle to 700 (this is a guess as I wasn't smart enough to look). The rpm when it is idling low is closer to 400.

Also, it never seems to happen in the AC or heat is on.

 

[pagrey]

Some people don't realize the first small dot on the tach is 500rpm, there is no 250rpm mark, it's important when you are talking rpm so we are all on the same page. Mine did that for awhile about 50k ago. IAC and spark plugs fixed it. What exact plugs are you running? Seems silly to swap the IAC again but it really fits your description. Also I'd check the throttle cable and cruise control cable and make sure nothing is binding. I have a friend that had some strange throttle problems on an 06, the gas pedal was caught in the firewall insulation.

 

[Beherrschen]

Thanks for your reply and help. Thanks for pointing out the odd rpm markings.

There are two marks between 0 and 1000 rpms. I never thought the first one was 250. I took the first mark as one third of 1000 and so it would about 333. Looking at it now I see that there are 3 marks between the other 1000 rpm marks. So let me put it this way... when it is running smooth and fine it is up near (and sometimes at) the second mark (so the 650 to 700 range). When it is struggling (slightly bouncing up and down) it is just above the first mark (500 to 550 range).

I used Autolite XP985 Iridium XP Spark Plugs.

You think that my replaced IAC would be defective in the exact same way as the previous one OR are you saying that all bad IACs will act the same?

 

[pagrey]

I only mention IAC because what you describe is exactly the symptom of a sticky or defective IAC and sometimes new doesn't mean working, sorry to add to the list of people suggesting that. Maybe the connector is dirty or the wires are broken? Mine idles low as you describe sometimes, 500-550, that is normal for me at least. Stalling is not. If tapping the gas fixes the problem then maybe the throttle position sensor is dirty, sticky or giving erratic readings. If you have a volt meter as I remember it's easy to test to see if it jumps around. The idle should be low until the A/C start up, it's possible your idle is just rough. When I was trying to figure my issue out I also pulled the injectors to make sure they looked even and none were leaking. I also replaced the o-rings when I had them out. Something I did but I don't know if I'd call it part of the solution.

 

[pagrey]

Thanks for your reply and help. Thanks for pointing out the odd rpm markings.

They are odd, most people don't know how to read the gas gauge either...

 

[Beherrschen]

Thanks again.

I have replaced the TPS and it was no help. I have also replaced the fuel injectors. Feel like I've been guessing and checking a lot to fix this. It has been a few months since it cut off on me (it was only twice). I have heard that the RPM range is not really that low but when it is down there it is running a little rough. This is my 6th TJ and I've never had one run rough like this.

I will take a look at what you suggested again.

 

[pagrey]

Sound like you've done allot, some of these things take forever to get ironed out.

If you have a scanner like a cheap ELM327 that can graph data you might check some things from the computer. I'd look at the voltage curves for O2 sensor 1 on bank 1 and 2. They should be very similar. I'd also see how they looked around 1500rpm and drop back to idle looking for anything unusual when you transition back to idle. Next maybe the long and short term fuel trims, the same tests but look for big jumps in the short term at idle or when you transition to idle.

Normally these things throw codes but it can help you find a slow O2 sensor or catch a small vacuum or fuel leak. It is a little difficult, for me at least, to "read" fuel trims and understand what's really going on. Probably better information floating around on that.

 

[Beherrschen]

Thanks for the idea but I do not have a scanner that graphs. I'll take a look at the one you suggested.

 

[Beherrschen]

pagrey,

I got a scanner that graphs everything out. The graphs of the two O2 sensors you suggested are different. Different spikes at different times (different large and small spikes). I'm not sure what would count as unusual behavior though. I took a screenshot of them.

I haven't done the fuel trims. I will check them out the next time it acts up (likely on my drive home from work).

Thanks again for your help.

 

[Btaylor9311]

You ever get this figured out? My jeep does the same thing.

 

[chotomobile]

You ever get this figured out? My jeep does the same thing.

Same question to you brother, could you fix the problem?

 

[chotomobile]

pagrey,

I got a scanner that graphs everything out. The graphs of the two O2 sensors you suggested are different. Different spikes at different times (different large and small spikes). I'm not sure what would count as unusual behavior though. I took a screenshot of them.

View attachment 78693
View attachment 78694


I haven't done the fuel trims. I will check them out the next time it acts up (likely on my drive home from work).

Thanks again for your help.

Were you able to fix this? My Jeep is having the same issue.