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Old 07-09-2010, 04:51 PM   #1
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06 4.0 with engine hesitation at rolling speeds

I have an 06 6 speed rubicon with only 24k miles and seem to be getting some sort of hesitation or stutter when i take off at a rolling speed.

For example: If I am in 1st gear and bring the rpm's to 2,000 and hold it then hit the gas it hesitates for about one second before it takes off. It will do the same thing at 1,000 rpm in first or even 2nd at 2,000 rpm.

I noticed the hesitation because my head swings forward unexpected
instead of keeping level like it would under normal power and throttle response and mainly notice it between 1-2 shifts.

I just changed the plugs yesterday because with the mileage there has never been any maintenance done besides oil and filter changes. When I got the plugs out they were the factory Ngk's and the gap on them was anywhere from .020 all the way to .057 when the gap is supposed to be .035. I bought the Champion 438's and gapped them exact to .035 and I noticed very minimal changes but now it seems like it has a miss at times on idle and one time it started idling rough for about 15 second while i waited to make a left turn then it went away. Ive had about 3 hours on the plugs since yesterday. Just after installed the plugs I also ran stp fuel system cleaner to see if it would help at all but noticed nothing yet. The k&n drop in filter is spotless and I cant think of any of the no brainers to check at the moment.

Ive been hearing that the cam position sensors like to take a crap and sometimes the IAC get dirty but I know for a fact this jeep has never seen the mud and have no reason to believe that dirt would be the cause for any of this. I did put the scan tool on it last night and got no codes.

Im just looking for anything else dumb to check that anybody might know of before I start running through the whole system to completely diagnose it myself. Maybe its just me and this hesitation is normal but im about 90% its not and it just seems like all the power isnt there until I mash it down.

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Old 07-09-2010, 06:30 PM   #2
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I have a 2006 LJ with 24k miles and it is doing this also... Mine does it when shifting from second to third mainly though...

Hope someone has some sort of idea of what this is...

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Old 07-10-2010, 10:30 AM   #3
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I do notice it shifting into 3rd and I think ive narrowed it down to a couple things. Its seems like either the upstream o2 sensor is goin out or the iac might be dirty. Im gonna try cleaning the iac today and try unplugging the o2 sensor to see if i notice a change.
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Old 07-10-2010, 05:25 PM   #4
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Ok so of all the o2 sensors I pulled the very front one on the head which I guess is the upstream o2 sensor. I just drove it around the block and in first gear at 1 or even 2k rpm it doesn't hesitate. it seems like there might be a bit of it still in second but i think that should go away after i put in a new sensor.

When I turned the key on-off-on 3 times to display the codes all it said was "Done" so i guess that means it wasnt reading the error
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Old 07-10-2010, 05:37 PM   #5
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Also just before I messed with the o2 sensor I tried cleaning the battery cables, then pulled the iac off and inside it was spotless. So I bolted it back on and removed the intake tube to see in the throttle body, it was clean so i sprayed off near the iac inlet anyways with some carb cleaner. It seemed to idle worse after and dropped to less than 500 rpm for a second several times. After unplugging the o2 sensor it all went away. Im gonna run and get a o2 sensor and see if it completely clears up
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Old 07-10-2010, 06:16 PM   #6
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ok so what i unplugged was the front downstream o2 sensor. It made a difference but everybody is saying its the front upstream that should make the difference so now im gonna try the front upstream and see what happens.
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Old 07-10-2010, 06:57 PM   #7
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Ok so I unplug the front upstream and it seems to stick when i let off at 2k rpm and not run any better. Everybody is saying upstream causes this problems but im getting the most relief when i unplugged the downstream.
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Old 07-11-2010, 12:53 PM   #8
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When you unplug an O2 sensor you are forcing it to go richer.
So - based on that, I'd first suspect some sort of a vacuum leak - it has to be in a place that affects ALL cylinders, else you'd be complaining about a "thumper". (Only 1 or 2 cylinders would be too lean - thump thump thump.)

Look very closely at the Throttle Body base gasket, - especially around the MAP sensor area. Spray a BURNABLE carb cleaner on the base plate. It should not make any RPM change if it's sealing properly.

The IAC - the "I" means idle - Idle Air Control - no significant effect at 2000 RPM.

It could be the coil rail too - high voltage breaking down. Have someone with excellent hearing listen close to the engine while you snap it from 2000. If they hear the "crack" like lightening, it's breaking down internally.
It should still be under warranty for that, but convincing the dealer that's what the trouble is is tough (yes, been there done that). About the only way to be sure is to swap in another coil rail. Don't buy one from Autozone even though they are cheaper - Chinese junk (been there too.) NAPA or CarQuest has the same mfgr's as the dealer - Standard Ignition - for about $100 less!
Even if that's not it, someday you'll need one anyway - they give more trouble than Jeep will admit. I've probably replaced 9 or 10 so far - 2 on mine, the rest customer's.

For the techies:
Duh -- Why does unplugging the O2 sensor help if it's the coil rail? It's richer, easier to fire the wet mixture, takes less voltage. The voltage stays below the breakdown point.

Rest assured, if it is a coil rail breaking down, it'll get worse.

There are 3 coils in it - if one breaks down it affects 2 cylinders - but they are compliments of each other. The engine remains in balance - doesn't thump, just feels like hesitation or power loss.
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Old 07-11-2010, 04:53 PM   #9
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It does sound like I have a sort of a hissing noise if im at say 2000 or so rpm and let off. when it goes back to idle its quiet. I notice it the most when the truck is sitting still and i have the doors closed. If I have the hood up and listen for it its hard to trace but sound like its coming from the front

When i put the o2 sensor in it seemed to run better and not have near as much hesitation and just about none when i would try in first gear. Maybe something else that was or still is wrong caused the sensor to go out.

Also, is there a way to individually test these coil packs? I've had to do it on other cars by testing the resistance but never done it on a jeep not that it should be any different.

When i pulled the plugs a few days ago, even tho they were gapped way off I did notice that they all looked like they were burning the same.

With the hissing sound im leaning towards a vacuum leak, ill be doing some testing tonight on it
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Old 07-12-2010, 12:09 AM   #10
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I wish there was an easy way to test each coil - but even if you found one bad one, you can't replace it individually.

One thing you might try, but it's probably not bad enough -=- yet. Run it at speed for 5 minutes, then check the cat temperature with an infrared temp gauge. If a coil is breaking down while you run it, pre-cat will be hot, the other quite a bit cooler.
But since it's not misfiring constantly, I doubt it will show anything.

I keep a spare coil rail, when I suspect it's a rail problem I just swap them. Only takes 5 minutes or so
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Old 07-12-2010, 05:00 PM   #11
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Ive been trying to talk with the dealer to see if they can do the "good will" warranty since this jeep is a 3/36 warranty but has such low mileage and of course they are hesitant to it. I will be running by there tonight to see if they wanna get their hands on it and come up with any for sure problem. If they wont warranty it i will just have to finish it myself. I might also try goin back to the ngk plugs it came with from the factory.
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Old 07-13-2010, 01:50 AM   #12
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Alright so I go to the local jeep dealership and discuss the issue with the shop foreman and we go for a drive but he couldnt see the issue I was having when driving the jeep himself.

So we hooked a scan tool up to it and came up with no codes, idle was fine and nothing appeared out of the ordinary until we got to firing order. Every few seconds it was detecting a miss on the number 1 cylinder at idle then after about 4 misses another miss came from the number 4 then the others started joining in. They were very slow to occur but it tells me something is still off. I ordered the factory NGK plugs today so I will be picking them up in the morning and see if they make any difference. If they happened to clear this mess up I would recommend never using anything but the factory NGK's but we'll see.
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Old 07-13-2010, 06:30 AM   #13
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You ordered the plugs the underhood label calls out, not what the kid at the parts store said - I hope.

Hissing noise - aftermarket air intake or stock airbox and filter?

You might try to locate that noise - hold a 2 foot hunk of heater hose to your ear - use it like a stethoscope and probe around. Have a helper run the R's up till he hears it while you search.

If the coil rail was breaking down from high voltage it would sound more like a "tick tick tick," getting faster with R's. I doubt you'd interpret it as a hiss.

Random misfires on different cylinders - sounds like mixture like you suggested.

If it were here I'd try richening it up slightly with propane. You can too if you make a cheap tool for it.
Get a cheapie propane torch, remove the torch tip leaving just the tube. Look at the bottom of the torch - there's a very tiny orfice to limit the amount of propane - remove or drill it out. What you end up with is just a valve.
Slip a hose over the tube long enough to reach the driver's position from the Throttle body or air intake.

Take it for a ride - make it do it's misfire thing - while it's misfiring add some propane - see if it helps or gets worse. Obviously if it helps the system is too lean.

You can also use the propane to check for vacuum leaks - simply spray a little around suspected areas. It's safer than Ether or carb cleaner - it doesn't puddle. Liquids puddle, then if for some reason it ignites you have a bonfire. Propane flashes, scares hell out of you but goes out. Careful around the coil rail and alternator.

Be sure to spray carefully around where the intake manifold attaches to the head. Sometimes the leak is actually on the bottom. Poke the hose through the gaps to get the propane under it - or crawl under.

The propane will burn just like it was gas, but you can control how much and how and where it's applied. It's a very handy diagnostic tool.

Remember the commercial where a kid tests a breakfast cereal on his brother - where he says "Mikey likes it!" If your engine "likes it" - too lean. Use at any speed or load.

A very handy tool when used with a dyno too.
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Old 07-13-2010, 06:57 AM   #14
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BTW - you said """""Ok so of all the o2 sensors I pulled the very front one on the head which I guess is the upstream o2 sensor."""""

There are 4 O2 sensors. The front 3 cylinders #1, 2, 3, is one bank, and it has a sensor on each side of the pre-cat. The one closest to the head is the upstream, the other one after the cat is the downstream.

Same thing for the rear bank - the rear 3 cylinders, 4, 5, 6 are the 2nd bank, with 2 more on each side of the cat.

The 2 closest to the heads (upstreams) have the most influence as to mixture, the 2 downstreams are mainly to check the efficiency of the cats for emissions.

The computer gets inputs from all of them. The 2 upstream sensors get averaged, the the computer sets the mixture for BOTH banks equally based on that average. So a problem in one bank affects the other one.
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Old 07-13-2010, 12:23 PM   #15
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That does make sense, when we watched them on the scan tool yesterday they all stayed in the range that they were supposed to be in. I know sometimes they can be malfunction and not throw codes but even on the scan while running they didnt show anything out of the ordinary.

First things first, yes I did F up and take what the autozone genius said was what the factory called for (Champion 438's) because they were out of the ZFR5N's that are factory for the truck. I just picked the ZFR5N's up a few mins ago and will be installing them now as soon as the jeep cools off. Unless it feels like a whole new truck I will be goin back to the dealer to monitor the firing to see if there was any change.

Based on the problem we were seeing we pretty much eliminated a vacuum leak as being the problem. When unplugging several other lines and causing vacuum leaks that nearly killed the engine, it was able to compensate enough for it that the scan showed it to be perfectly normal even with a line open. However if the plugs dont take care of this I will be doing the procedures you explained to either find or eliminate that possible cause.

If not the plugs or vacuum I believe the only other culprit could be the coil rail. After everything I might go buy a coil rail or swap with a buddies to see. If nothing else brings results then I will test the plug wires.
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Old 07-13-2010, 05:31 PM   #16
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Ok so I installed the NGK ZFR5N's earlier and went for a drive. It seemed to accelerate better and pull harder passed 3k rpm. I dont notice as much of a surge and cant seem to hear a miss anymore. Im gonna try hooking up the scan tool to see if the miss completely went away but im about 90% sure the plugs were the problem.

When I pulled the champions out the gap on them went from the .035 that i had them set at to below .030 on the coin gauge so they were actually more like .025. So before I put the new ones in I checked them on the feeler gauge and tho they say they are pre-gapped they werent very close. I found that .035 on the feeler gauge is about .040 on the coin gauge so I dont think ill use that pos coin trick anymore.

In conclusion I think the problem was a mix between the champions not wanting to comply with the ignition and the gap mysteriously changing after installation.
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Old 07-13-2010, 07:58 PM   #17
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Who says they are pre-gapped - the onion slicer at AutoChina?

Most plugs are used in several applications, so pre gapped for what?

The best gauge is the round wire type made for plug gaps. The flat feeler gauges will throw you off. The coin type are super inaccurate.
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Old 07-13-2010, 08:37 PM   #18
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Hahahahah, its probably the custodial engineer that they blind fold to take his best guess at it.

I think what they mean is that they are pre-gapped for specific motors, just you have to happen to get ones that are all gapped the same lol.

If you get the feeler gauges flush with the tip of the plug it can work but as far as the coin gauge goes i dont think any 2 are the same. Ive never had a problem using the coin before but i guess this has to be much closer.
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Old 09-17-2011, 10:09 PM   #19
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any update on your issue. i guess it has not returned. i have some of the same issues and dont want to chase rabits. thanks
05 TJ Rubicon 54,000 miles

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