Cylinder # 3 misfire

J

joseph.bacon4

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I have a 2204 Jeep Wrangler TJ Sport 4.0 liter never had any major issues with it in the past 8 years I've owned the jeep. We had a cold front come through and I had to replace my water pump, heater core, and radiator. Ever since the engine has been idling and running rough and I have been getting a P0303 code for a misfire on cylinder #3. So far I have checked the coil rail changed the spark plugs, cleanded the throttle body, seafoamed the engine, cleaned / tested all the fuel injectors "#3 injector was swapped with #1", and insulated the fuel rail and the fuel injectors.
After all that nothing has changed. I am thinking it might be the head gasket but not sure. Any other ideas?


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The plugs are new and I checked the spark on coil rail. Is there a better way to test the coil rail. Thanks for replying.


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joseph.bacon4 said:
The plugs are new and I checked the spark on coil rail. Is there a better way to test the coil rail. Thanks for replying.


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To test the coil in your TJ you need to use ohm meter to test the coil for continuity. Make sure you have the approximate ohm values for the coil. You can find this information, in a Chilton's or Haynes manual to find out what the mega ohms reading on the coil should be. You want to check the #3 plug now that it has been kick the code again.
 
joseph.bacon4 said:
The plugs are new and I checked the spark on coil rail. Is there a better way to test the coil rail. Thanks for replying.


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Use this method to test the coil rail:


I'm kind of leaning towards the head gasket as well, but I'd test the coil rail first and make sure it's 100%.
 
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When my '04 had intermittent misfires on 1-2 cylinders, it turned out one of the pre-cats had gone bad. So bad its internals suddenly broke up several years ago and blew downstream into the main catalytic converter, clogging it so badly the engine would barely run. Replacing the cats cured the misfires, the original O2 sensors were reinstalled. It has been running perfectly ever since.
 
That's a good point @Jerry Bransford brings up, pre-cats could also be another potential issue as well.

Is there any way to actually determine if it's the pre-cats Jerry? You know, aside from them just blowing up.
 
Chris said:
That's a good point @Jerry Bransford brings up, pre-cats could also be another potential issue as well.

Is there any way to actually determine if it's the pre-cats Jerry? You know, aside from them just blowing up.
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I'm not a good enough mechanic to know the answer to that Chris. I only know that the guy I bought it from (Gerald Lee/Savvy) had already replaced the O2 sensors trying to fix the same misfire and I had already replaced the usual suspects without anything helping by the time the pre-cat puked its guts out. There was no CEL code indicating anything was wrong with a catalytic converter, the CEL only said a cylinder was misfiring.
 
JP98 said:
@Jerry Bransford & @Chris with the head gasket or the Cat would it keep popping the misfire on the same cylinder or move around?
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It'd depend on where the head gasket was blown, and each pre-cat covers three cylinders but I don't recall all three of its cylinders misfiring... like just bad enough to cause problems for the nearest cylinder or something like that. Like I said above, I'm no engine mechanic.
 
Jerry Bransford said:
It'd depend on where the head gasket was blown, and each pre-cat covers three cylinders but I don't recall all three of its cylinders misfiring... like just bad enough to cause problems for the nearest cylinder or something like that. Like I said above, I'm no engine mechanic.
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I'm with you. With a blown head gasket I've been able to see sign on the plug if it kicking a code and usually would not see it stick to just one cylinder. Haven't experienced the pre-cat issue before. Thank you for bringing me up to speed.
 
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Thanks everyone for the info. I have a bit more info to add I just checked the ohms on the coil rail and they all matched. I also pulled the plugs back out and the number 3 piston has more carbon build up than the others.
 
@joseph.bacon4 Let's back up a little. You said that you swapped #3 & #1 injector but still got the problem. Did you change the plugs, get the code again and then swap the injectors? If so, did you check plug condition in between? You may have a bad injector but put a good plug in and then it fouled.
 
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Ok. So new plug and different injector but still building carbon on plug. I would start to think maybe you are burning some oil in that cylinder. If you are getting proper fuel and still smoking up just that one cylinder, I would lean to oil because it burns at a higher heat and puts off more carbon.
 
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