Misfire
 

I've been having misfire/ rough idle issues for a while that I fix, then they return. Seafoam seems to fix it for a short time, then they'll return in different combinations of cylinders, or a multiple random misfire, according to the OBD scanner. So last night I pulled the fuel rail and injectors and replaced some bad o-rings on the injectors, ran seafoam through the rail, and cleaned the injectors. It cleaned the idle up a bit but now the misfire is rhythmic, as if it's one cylinder, but it's like every 3 seconds...you could time it. I usually have a good idea as what to do next, but I'm at a loss. I have good fuel pressure, and I've done a compression test which was fine. The ignition system is all new. Any ideas? I'm about to the point of taking it to a mechanic, which is painful for me. Any ideas?

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Random misfires are often from one "common" source. Assuming you have a solid 49 psi of pressure at idle, which is the spec for the 96 4.0, I'd start searching for a vacuum leak somewhere, or even an exhaust leak can cause this.

How common is it for the vacuum ball behind the front bumper to go? I think I need to start there

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Well no vacuum leaks that I can find and the vacuum ball looks good. I know there aren't any exhaust leaks, I just replaced the entire system Starting with the manifold. Any other ideas?

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Ok in case anyone else is having the issues I was having above, I read a post on here...I wish I could find it again, by a guy with similar issues to mine who solved it by revving the motor to 3k rpm's while in neutral for about a minute. It sounded pretty unrealistic to me, but I figured WTF anythings worth a try, so this afternoon I went out and did it. For whatever reason is beyond me, but it's PERFECT. No misfires, no roughness. It purrs. If anyone knows the why it worked, I'm curious, but whatever, it's fixed.

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Thanks for posting that, been following your thread because I've been having similar problems. I litterally have tried almost everything to fix the rough idle and nothing has been a good fix. Tried the revving it for a minute and that's seemed to do the trick.

The real question is if this last, I know seafoaming mine fixed it for a day and then it ran rough again. Let's see if this revving trick fixes it for good. Its got to do with the sensors I'm guessing.

I took my kids to my grandmothers today, about a 2 hr round trip on back roads, and it's still running great. And I'm still baffled.

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You probably had a small engine crud deposit on one of your valves or valve seat. Revving it like that got it hot enough and with a long enough rush of air to burn it away.

Just like back in the day when you had a car with a big block V-8 and did a lot of city driving. You would have to go out on a long straight road or the highway and blow all the crud out of it.

That's a possibility I guess. I'll make myself feel better by saying the $60 worth of seafoam I put though it over the last few months loosened it up :)

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Still running good?

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Nope back to running like crap for me. Its gotta to be a sensor of somesort. My ignition is brand new, my injectors are recently rebuilt 784s. Seafoam fixed it for a day, this fixed it for half a day. Put in a new TPS and that didn't help. The rough idle and misfire only happens at idle, normal driving there's no trace of the misfire. Down to O2 sensors or CPS. Can't think of any other sensor that could cause a rough idle like this.

Is it missing too, or just idling rough?

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crank sensor....had the same issues

I'm thinking about putting a new CPS in just because. It seems to be such a common problem I think I just want to eliminate it before it's an issue....

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O2 sensors only last 150000 miles on average, so it's always a good idea to replace those. Running the engine at high rpms like that temporarily burns crap off the O2 sensor, but it builds back up. You can try removing it and cleaning it with brake cleaner. Clean the sensor on the intake with the wire going to it. That one is intake air temperature. Cheaper than a new CKP at any rate... If you decide to replace the O2 sensor, go with NTK ONLY.

when you do,save yourself the headache and get one directly from jeep, heard a lot of people complaining aabout them from autozone, o'reillys, etc

Its got a slight miss at idle that causes it to idle rough. Got a friend who's a jeep tech, he listened to it and he said while it is missing its not doing any damage. There's no miss or hesitation under load. Cps is 3 years old, but O2 sensors are factory. Those are all on my list of things to try. Its upsetting because this is a crate engine with 10k on it.

haha... ghetto fixes... well, my first thought is that there is a partial clog somewhere in the fuel or exhaust system and that running it at high rpm for an extended period of time is enough to break it loose. my other thought is that this might just be gypsy magic. did you recently aquire some gypsy tears in a little crystal vial? cause i needs me some of them! really idk though... there is a high and low idle "switch" that is activated when you get on the skinny pedal hard enough. i have this buck and chug and hesitate issue with my 99 xj and when you first start it up and drive it, it runs rough. but if i floor it for a coula seconds (like enough to get up to 35mph) it clears up and runs like a top. ive replaced everything! and yet it persists... idk... maybe the cat is going...

o2 sensors make a big dif!!! if your gonna do th o2 sensors, get the goor quality 4-wire, pre heated ones. and do both the downstream and upstream sensor!! doing one is only half the equation!! they are like 100+/ea.

BEFORE SPENDING $$$, get it on a code reader! even if the CEL isnt on, there might be a code that has stored in there that will shed light on issue!! i discovered this recently when we plugged my xj in: no CEL, but a couple codes!

Yeah I used up all the gypsy tears on this one. My misfires came back a little but the idle is still rock solid. I had put in Bosch plugs, I'm going to switch them out with champions and see what that does. I just changed my O2 sensors with Denso ones, made a big difference on mileage but that's it. Anyway my sister just started dating a mechanic that drives an XJ, so I'm going to grab a sixer and let him figure it out...

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The downstream only check to see if the mini cats are working. Upstream is the only ones that actually affect how the truck runs. Napa carries ngk oem ones that I'm gona look into. I've had a code reader on it before, I work at a tire and lube place.