CattyXJ

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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tjwalker

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.

CattyXJ

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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CattyXJ

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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CattyXJ

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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northline76

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.

CattyXJ

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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BuckB91XJ

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.

CattyXJ

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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CattyXJ

Still running good?

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northline76

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.

CattyXJ

Is it missing too, or just idling rough?

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tommyxj2000

crank sensor....had the same issues

CattyXJ

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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94JeepCherokeeMan

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.