Mischievous misfire - Jeep Cherokee Forum

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Mischievous misfire

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Old 05-22-2018, 11:16 PM
  #1  
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Join Date: Aug 2015
Location: Sunnyvale Ca
Posts: 6
Year: 1993
Model: Cherokee
Engine: 4.0 straight 6
Default Mischievous misfire

My 93 has a weird stumble and weak misfire that happens every so often. It throws a code 27 and 34. The 27 is a fuel circuit issue. What are all possibilities the 27 code could be. Could a bad injector throw a 27. Could any other parts fail to create a 27? Like say a bad ignition coil? Or something wrong with the air flow? Like what are ALL possibilities and I use google a lot so Iíve heard and tested all the basics like tune ups etc. It drives nice when it doesnít misfire which is about half the time. But last week it felt like it was on 4 cylinders and wouldnít clear up. I let the car sit parked for a week because for two days Straight there was a a heavy misfire that did not clear up like it normally does. And then a week later the double misfire was all gone. On its own. I didnít touch a thing. What would chamge in the motor after a week to fix everything. Iím so confused. Itís usally never a double miss. Itís usually a small miss that comes and goes once I stop at a red light. Then after picking up speed it goes away. And it doesnít happen immediately after turning the motor on. It usually happens 10 minutes after driving in city traffic in any weather and temp. So in summary it starts up fine sometimes a rare weak start. But the misfire usually only shows itself after 10-15 minutes and itís usually not heavy. Once or twice it is. And I can hear all my injectors click when I place a screw driver to them against my ear. They all fire even when I hear the motor miss somewhere. I put in a new distributor and new ignition coil about a year ago. It ran kinda nicely after all that work then a week after the new parts in it hiccuped and slowly over months got worse. What in the actual world is the problem!! Iím so sad..also could it be a bad cks? I out a new one in but at the time didnít know about calibrating it. Iím wondering if itís off by a bit to cause it to misfire in extremely random paterns. Also the problem usually surfaces when I stop and go a lot. I went on a big road trip and did a long drive in the mountains and it only misfired once during the whole weekend. I was blown away how it behaved so nice infront of my mechanic friends. Good it be the altitude, 5,000 feet so not that high up. Or Iím thinking itís because I wasnít stopping and starting like I have to do in the city. The city you go 0-40-0mph every 5 minutes you know. Start stop start stop. And after stopping itíll quietly miss or obviously miss at the light. What controls that scenario to make the motor unhappy starting and idling starting and idling? Iíve looked everywhere online and YouTube and nothing really helps.
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Old 05-23-2018, 11:58 AM
  #2  
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Join Date: Nov 2017
Location: Groton, MA
Posts: 2,196
Year: 1995
Model: Cherokee(XJ)
Engine: I6 4.0L
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I think you got a crappy ignition coil. What brand is it (if you remember)?

Check and tighten all your grounds and battery cables first, then grab the multi-meter and see if you can find something with your coil and/or CPS so you're not just firing the parts cannon. If you can get the misfire to happen when you can inspect stuff in the engine compartment, you're at least in a good spot to check things in the state where it is having a problem.

What usually happens in a week is heat related changes. CPS and Ignition coil are prone to thermal failure, so you might see spotty function when the engine is cool vs. when it has been running for a bit. Also consider that all metals are subject to thermal expansion and contraction - including electrical wires.
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Old 05-23-2018, 03:10 PM
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Join Date: Jul 2011
Location: West Covina, Ca.
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Year: 1988
Model: Cherokee
Engine: 4.0
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Note from DJ
I have a couple of things that you might try.
1. Take a spray bottle set to mist and full of water and go out to your Jeep after dark, raise the hood and start up the engine, look for sparks jumping if no sparks jumping; mist things down especially spark plug wires; distributor and coil area while looking for sparks.
2. Hairdryer or heat-gun can create hot running condition in a hurry while parked at home.
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