Devolved a miss when cold and would go away when warmed. Now today it was there no matter what. Yanked a plug and the porcelain is gone. Clearly these plugs are not happy. What would be a good plug to replace these with? I have seen nothing but champion coppers on all the threads I've seen but I need something in a colder heat range I think.




I would have the child behind the local parts counter cross reference them but they always screw up.

 

those champions are the OEM and should be fine under normal circumstances, no need to find a different temp range etc. So, how are the other plugs? if more of them look like that it could be bad lot but otherwise it might be need to be chased whats caused that. Were they gapped to the correct distance before they were put in? Always needs to be checked.

but to answer your question otherwise: some forum members are happy with the NGK's, sure someone will chime in with the type/part #.

But once again, those OEM'S should last 30-50k.

edit: sorry, you have a Renix....so not sure if oem plugs were different in those. Cruiser54 will def know

 

Copper plugs normally don't last beyond 30,000 miles.

I detest Champions. I have had too many fail when not all that old, causing me to suspect anything but a new plug. Many frustrating hours spent checking everything else.

I use NGK Iridium plugs. Very happy with them.

 

And I detest anything other then copper plugs in a 4.0. ngk, champion, autolite, Bosch, whatever you get, it depends if youve got an oil leak, rich condition, etc. Go with what you like honestly.

 

Those plugs way too hot for the 4.0. Those are 2.5 plugs, and melted 2.5 plugs, at that. 4.0 calls for RC9YC as OEM plug.

 

The NGK part# is ZFR5N (copper). I've got a set in my XJ and they work great (currently about 12k mi on them), noticed a slightly better idle than the Champions they replaced though it wasn't a massive diference (I just prefer NGK in general which is why I got them). An alternate part number is NGK ZFR5-1, these have an extended "shield" arround the tip that may be beneficial if you are having persistent issues with blowing out insulators but I've heard they hurt idle quality.

Only on Renix, later (91+) "HO" 4.0L I6s used the RC12LYC / RC12ECC. They are a little hotter than stock for a RENIX but I wouldn't expect them to blow apart like that (especially since they were used in later versions of the same engine).

 

Yea I went to napa and got the rc9's. They where all gapped at .035. Only one cylinder was like this and the rest looked alright.

 

Which cyl? I had something similar happen when I first replaced the plugs om my XJ (with champion RC12LYCs). I accidently tapped one on the head/plug tube for the #6 cyl. (this is an especially tricky one to guide in without hitting anything), a couple thousand miles later I noticed a slight miss, upon pulling the plugs I found the insulator on the #6 plug was cracked (not blown apart like yours though), it seems it doesn't take much to do damage. After replacing them with NGK I haven't had this re-occur, though I was much more carefull when installing the plugs as well.

 

Looks like detonation from here. Carbon deposits in that chamber I'd guess.

 

Autolite 3924 works for me, and plug color is about perfect..

 

It is very possible that yoy got a plug that had a crack in the porcelain. I have seen it before and it not that uncommon. I would replace the plugs and take a look at them after driving a bit and go from there. I bet it was a defective plug and has nothing to do with your Jeep.

 

Yea that was my plan, run them for a oil change or two and check them.

 

Once I put my NGKs in, I don't have to check them or worry about them ever.

Even Richard Petty doesn't use Champions.

 

The only reason I told him to recheck the plugs after running it for a few hundred mile it to make sure that it was a faulty plug and it's not another issue with the engine.

 

It was probably caused by either a persistent knock or ping in that cylinder, or extreme heat caused by a lean condition.

My Champion plugs just quit firing. The didn't break like that.