Cylinder 4 misfire.....AGAIN!!!
 

Hello all,

Yesterday I noticed the wife's JKU had a minor and occasional hesitation, mostly at low rpm. Today she tells me that the CEL began to flash while she was out but it never stayed on. I checked the gas cap since she fueled up yesterday and it was ok. I checked for codes and there were none. I disconnected the battery and then reconnected and went for a ride. Now it was noticeably misfiring and the CEL was flashing. I immediately stopped and checked for codes. I got the P0304 for a cylinder 4 misfire. I got back home and pulled the plug and it was as bad as any I've ever seen. Oil gunk all over it and what appeared to be corrosion on the tip. I also pulled the PCV and it seems good, but will replace anyway.Attachment 557024

As for back ground, it is a 2011 sahara JKU with 35000 miles. It gave a cylinder 4 misfire with a couple thousand miles on it and the dealer changed out the plug and pcv. They said the pcv was clogged and the plug was covered in oil.

This is an oil using 3.8 L. It was going through a Qt every 1000 miles or so until I changed to 5w-30, now its less.

My question is.......what would cause this repeated cylinder 4 misfire?

maybe a valve guide seal? letting oil get into the cylinder... causing both the fouled plug and the oil usage?

-Christopher

Take it back to the dealer, it's a repeat condition and the dealership needs to readdress this concern. It's hard to say what's wrong with it without the vehicle in front of a competent mechanic. There are plenty of issues that could be causing this problem, but with 35k (even though that's low mileage) it's very possible there's a internal problem. If the pcv was just changed it possible it was a faulty part, improperly installed (probably not, but still possible), or it could be a problem with the piston rings letting oil enter the combustion chamber. There's a lot of scenarios...

It seems like "misfire" has the longest list of possible causes of all the JK troubleshooting charts.

Thanks guys....I did a little diagnostic stuff yesterday. I'm not the most competent mechanic, but I can do enough to be considered dangerous. Anyway, I did a compression check and it appeared to be good. I also did a leak down that showed nothing. so I am assuming the rings, valves and gaskets are ok. So my thought now is that it's probably the valve stem seals???? What do you all think? I know one of you already suggested this, but am I missing anything?

It may not help too much since mine is a 2014 JKU but I was getting a cylinder 5 misfire code at 18,500 miles. Brought to the dealer and they ended up replacing the right side head.

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Mine has had a cylinder four misfire code for quite a while. My mechanic said he sees this quite often. Told me to buy a bottle of Ethanol cleaner by Sta-Bil. Just bought it. Gonna try it soon.

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