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Check engine light P0302 (#2 cyl. misfire) after changing spark plugs...rough idle

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akxj01

NAXJA Forum User
Location
anchorage
Anything obvious come to anyone's mind? All I did was change the plugs. Its a 2001 with the coil rail pack (not individual plug wires). Now it idles rough and the P0302 fault code is present. Thanks.
 
Saudade said:
Did you gap them properly?
Click to expand...

Good suggestion. I'd pull the #2 plug, check gap. If gap is off, re-gap. If gap is good, swap the #2 plug to a different cylinder. If the misfire follows to the cylinder with the original #2 plug, you've found your problem.

BTW, what brand plugs did you install??
 
birchlakeXJ said:
Good suggestion. I'd pull the #2 plug, check gap. If gap is off, re-gap. If gap is good, swap the #2 plug to a different cylinder. If the misfire follows to the cylinder with the original #2 plug, you've found your problem.

BTW, what brand plugs did you install??
Click to expand...

I'll try switching the #2 plug to a different cylinder, and also check the gap. They are Champion plugs that are already gapped at .035 (I checked all the gaps). Thanks.
 
akxj01 said:
Anything obvious come to anyone's mind? All I did was change the plugs. Its a 2001 with the coil rail pack (not individual plug wires). Now it idles rough and the P0302 fault code is present. Thanks.
Click to expand...

I also used the spark plug boot grease. I put that on the porcelin end of the plug as well as inside the plug boot. Did i maybe put too much on? I put on what I thought was a reasonable amount.

I torqued plugs and coil rail to spec as well.
 
Last edited:
akxj01 said:
I also used the spark plug boot grease. I put that on the porcelin end of the plug as well as inside the plug boot. Did i maybe put too much on? I put on what I thought was a reasonable amount.
Click to expand...

Don't think that is your problem as that grease is dielectric but for future applications, you really don't need to use very much to do the job......
 
I am having the same issue on mine as well on my 2000 Cherokee.

I changed the plugs, old ones were obviously in need of replacement, but it didn't fix the problem. I still get the check engine light after a day or so, and its always either Cylinder 1, Cylinder 4, or random misfire.

I hate the way the coils are all built into one rail... makes troubleshooting problems with a specific coil much harder to diagnose. I've checked the wiring harness connections to the rail and to the fuel injectors and everything seems to be in great shape. I don't know what my next step should be.

Could vacuum leaks cause the engine to misfire occasionally?
 
jeep uses ngk plugs on the coil on plug egnines and there is a pcm update for a erroneus misfire condition. if it ran fine before you replaced the plugs, i would think you have a bad plug
 
i'm having the same problem with mine currently. I also had this problem a few months back, took it to the dealership (no other option for the town i was in) and they replaced all the plugs and said everything was good. Recently it all went to crap again. I'm going to replace the coil pack and see if that helps.
 
The last time I had a P0302 error is was the coil pack, and it paid a 50% savings to shop around for one. How I determined it was the coil pack was by switching the plug wires around so the number 2 cylinder was being fired by a different coil but in the same timing which made the error "move" to a different cylinder. Or one could swap in a known good plug and wire from a different cylinder to the #2 coil and cylinder, and see if there is still an error on the #2 cylinder.
 
jneary said:
jeep uses ngk plugs on the coil on plug egnines and there is a pcm update for a erroneus misfire condition. if it ran fine before you replaced the plugs, i would think you have a bad plug
Click to expand...

In my particular case, the plugs were swapped because of the engine misfire codes. The plugs I pulled out were definitely in need of replacement, and the jeep runs much better now with new plugs... but still get random or cylinder misfire codes for no apparent reason.

What do yo u mean there is a PCM "Update"? How do you go about updating the PCM?
 
OP- any chance it was raining when you changed the plugs or has it been raining? Check to see if there is any moisture getting to the spark plug.

FYI, I had a misfire cyl 2 a while back. Come to find out there was a pinhole leak in the heater hose that was dripping down onto the spark plug and shorting it out. New heater hose, problem solved. $8.
 
maco98xj said:
OP- any chance it was raining when you changed the plugs or has it been raining? Check to see if there is any moisture getting to the spark plug.

FYI, I had a misfire cyl 2 a while back. Come to find out there was a pinhole leak in the heater hose that was dripping down onto the spark plug and shorting it out. New heater hose, problem solved. $8.
Click to expand...
It wasn't raining while I did it, and it was done in the garage. I put dielectric grease in the boots and on the proper part of the plug.
 
as stated earlier, swap it out with a known good plug. i keep (now an extra set) some extra plugs arround for that type of reason. i ran those plugs for a month and know that they are good.

if you dont have an extra spark plug, then you could try as others have stated, and swap plugs between cylenders. if the problem moves, its a faulty plug. if the problem remains in cyl 2, then the problem is something other than the plug- like the wire, coil etc.

its an easy and cheap test to help determine cause. beats thowing parts at it right off the bat.
 
I had the same problem in my 2001, only I would get P303 and the random misfire code...i put new plugs in it and the problem went away for about six months...then about a year ago it started to happen again...now it's starting to happen more frequently...

This time i think I'm gonna just buy a new coil strip...
 
Update.

Well it appears to have been a bad plug (cracked porcelin maybe). I couldn't see anything wrong with the plug, so I am assuming the porcelin was cracked near the base of the porcelin (towards the middle of the plug), or just a bad plug in general. I didn't drop it or anything, so I am not sure what happened or how. I was told that it could have cracked during shipping. I replaced the plug and now it runs well.
 
good to know it worked out
 
I had misc.misfire codes that only happened on start up after it had run & warmed up already, like going out to the store,shut it off, run in, start it up again and it runs like crap for about 10 to 20 seconds. It turned out to be that in 2000 jeep added 2 mini cats under the intake manifold for emissions purposes and it worked but it also caused any left over fuel in the injectors to evaporate from the excessive heat. It felt like (and the computer mistook it for) a misfire but it was really fuel starvation at least until the fuel pressure stabilized. I fixed it by putting nomex boots on the injectors and larger heat shield under the intake.
My other idea would be to clean off the grease from the plugs/boots/head if it's make contact with anything other than the tip of the plug and the clip, it may be causing arcing. I have seen this before too usually with the (if a little is good a lot is better) technique. I've also seen pinholes or a small tear in the plug boot cause arcing too.
 
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