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Old 06-04-2010, 05:56 PM   #1
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Miss Firing

Hi guys/gals, my first post but I've been a lurker for awhile. Recently I purchased my first Jeep. I picked up a 99 sport 4.0L auto with 39k on it. I did a search and couldn't find any post similar so this is basically whats happening. Any time I start and drive away within 1 mile or 2 it starts miss firing and will loose all power but not stall but it will idle rough. I can feather the gas and it will accelerate but it will continue to miss fire depending on the throttle position. If I hammer the throttle to the floor it will accelerate as normal and use a ton of gas. After about a mile or 2 the problem will disappear and the jeep will run fine. I took it to a local garage and they could seem to find a problem and suggested I talk it to the dealer. The dealer told me that since the check engine light was not on, there wasn't anything they could do. They told me to bring it back when the light comes on, charged me an hours labor and sent me on my way(great customer service, but that's another story). Needless to say, the engine light has still not come on and the problem happens 95% of the time. Even if I park it for 20 minutes and head back out the same issue. So far I've replaces the crank sensor and the throttle position sensor and still no resolve. Someone mention that it could be an O2 sensor because under full acceleration the sensor shuts down. I'm wondering if anyone else has heard of this and what the fix was. If I want to avoid the miss firing, have to let it idle for about 5 minutes in the driveway that might be fine in the winter but now that summer is almost here I just want this problem to go away. Thanks in advance for any help/advice.
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Old 06-04-2010, 06:03 PM   #2
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well I have no idea but Welcome to the forum! I know someone here will know!

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Old 06-04-2010, 06:27 PM   #3
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The O2 sensor could be it but I would put my money on the notoriously bad coil packs. Low mileage for those to go bad though.
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Old 06-04-2010, 06:48 PM   #4
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It could just be a bad battery. If the battery is almost shot it will run like you describe. The alternator isn't enought to keep it running smoothly.
. . . . picking the lines with my twenty-nines . . . .

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Old 06-05-2010, 04:16 AM   #5
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I recently bought a 2000 tj 4.0L with 113,000 miles on it. it was running kinda rough during idle and at low speeds. I bought a couple bottles of STP injector cleaner from wally world (about $4.00 a bottle). ran the jeep down to half tank, got a skinny plastic funnel 8 inches long or so (.97cents). inserted funnel into filler neck, then put a bottle of the injector cleaner in & filled the tank with gas. ran that tank through, then repeated with the other bottle. then continued to run medium grade (octane 89) in it. now it cranks right up, idles great & runs smooth! I get 16-17mpg with 30x9.50x15's on it. that should clear up your problem. if it doesn't, then start your trouble-shooting from there. The reason for the long funnel; when I tried to put in the first bottle of injector cleaner in my tank, I found out the bottle neck wasn't narrow or long enough to open the flapper valve in the filler neck of my tank, so most of the contents ended up on the ground. so I bought that funnel and another bottle. just thought I'd save you the hassle.
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Old 06-05-2010, 10:43 AM   #6
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"""""The O2 sensor could be it but I would put my money on the notoriously bad coil packs. Low mileage for those to go bad though. """""

That must be it! The coil rail used on 2000 and up won't work right on the OP's '99.

But it does sound like an ignition problem - weak ignition. Check coil wire and coil, look close at the rotor and cap for carbon traces.
When you let it just idle it runs fine because the plugs don't need much voltage to fire. When you accelerate they need more, but at WOT the mixture is richer and they need less again.
If it's an insulation problem you may find it quickly by spraying water, 409, window cleaner, or most anything wet (but obviously not flammable) from a spray bottle. When you hit the insulation leak it'll misfire.
It's no wonder the country is falling apart - stupidity abounds!
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Old 06-05-2010, 10:48 AM   #7

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I have been going thru the same thing. eliminated all the electrical problems, my mechanic found it pulled a code but when we tried to download software the jeep wouldn't take it.
One of the posts talks about cleaning the throttle body and has a diagram showing what to do.
At this point until I try the cleaning, it missfires until warm.
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Old 06-05-2010, 10:10 PM   #8
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So I replaced the battery today just cause it barely made it through the winter last year and it needed to be done regardless. No resolve so I bought new plugs, wires, cap, rotor, and coil pack cause the cost was really minor and my fuel economy wasn't that great and its 10 years old. Again same issue. Thank you for the ideas, if anyone else has anything I willing to give it a go. Its starting to get frustrating cause its not give a code and everything else has almost been replaced.
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Old 08-23-2011, 07:44 AM   #9
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engine miss

hi, I have a 2005 tj 4.0 motor has miss under load truck has been unused for a couple of weeks any ideas much appreciated thanks......jeff
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Old 08-23-2011, 04:12 PM   #10
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X2. Mine does it too after a heat soak.
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Old 08-23-2011, 04:27 PM   #11
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besides bad plugs, incorrect gap, poor sp wires or Dirty fuel it could be a valve problem and you will need to check valve turning proceedures. Also see my new thread posted a few minutes ago. However, dirty fuel system components are waht I see as the common denominator
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Old 08-23-2011, 09:18 PM   #12
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I realize your post was over a year old. But mine has this exact same problem. Rough loping idle after a short warmup. If I startup and drive away it has a terrible miss and rough running until i feather through it. Then it will run fine until I shut it off and restart. Or if I let it idle for and extended time.

Did you ever find a fix for yours?
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Old 08-24-2011, 03:28 PM   #13
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Had similar issue with my recently bought 97 SE 2.5. After reading all the related posts I did the basics - plugs, air filter, cleaned iac and throttle body. All helped a little but not enough. She finally threw the CEL code 21 for O2 sensor. Guessed it was the upstream and replaced it.

That finally seemed to fix my rough idle when cold. Now idles steady.

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