Long story short, I put a newly rebuild short block and rebuilt head in my '00 about 1300 miles ago. From day one it has seemed a bit low idling and loud. Not engine noise loud, but exhaust noise loud. to the point where I was considering getting a different muffler put on vs the brand new one I had installed right after the build.(old muffler was shot)

When I installed the new motor, I put in brand new Champion RC12PEC5 plugs. I read on one of the cherokee boards, here or NAXJA that you should only run NGK V-Power ZFR5N 3459 plugs as that is what the Mfgr recommends.

I got sick of the loudness and today as a last ditch effort (read-only $25 vs $125 effort) I gapped to 35 and installed the NGK plugs.

All I can say is WOW... what a difference. My XJ idles smoother, isn't near as loud and has way more power. And after looking at the champion plugs it looks like they were not firing correctly, the white insulator on all of them is a pink color and there is a large amount of carbon build up around the bottom of the threads and the top of the side electrode is covered in white carbon. here are some pics.





Am I crazy or do they look terrible. Is the discoloration and carbon buildup due to an over-rich mixture?

 

Where did you get the champion number? Is that the number from under the hood?

 

Just my impressions;
I've run Bosch platinum for the last 18k in an '01, 0 issues. I don't buy all the hype about a piece wire rod with a gap creating problems.

 

The Champion i just put in my 99 are RC12ECC. It sounded a lot better. The op hadn't changed them in a while!

 

No, The emissions sticker was/is unreadable so I got the number off the plugs that were in the motor I replaced and then I got the replacements from Advance Auto.

I am still in awe at how much better it runs.

 

RC12PEC5 is a single-platinum plug, which can have problems in a waste spark system where half the plugs fire reverse polarity. The problem usually show up as short service life due to gap erosion though, 20K-30K miles- I've never heard of one causing increased engine noise. Not impossible that one or more were bad out of the box in some way.

 

+1 to this. Double platinum (where both electrodes are plated) is what is required. I still think it's silly though since a 4.0 will probably oil foul them before they get to the legendary 100k lifespan anyway

The original plug specified for 2000 and 2001 was Champion RC12ECC and was later superceded by NGK ZFR5N. Both are just plain copper resistor plugs.

 

Well, I am a believer. I will run these for 10k and see how they do. After that I might try the BOSCH Platinum as Turbo recommends.

Well... me either but let me just say that when driving down the road at 70mph the loud and heavy drone that was being emitted by the exhaust was loud enough that even with the windows down you still heard it above the wind. Now it sounds normal and quiet like my 95XJ. (Well, my old 95, I sold it this weekend to a neighbor.)

 

This is my spark plug strategy, based on personal experience.

87-99: Champion copper plugs

00-01: NGK copper plugs

Change em every 30k.

 

My engine runs smoother on the double platinum than the single platinum. Before the dble platI never thought it would ever run as smooth as a normal vehicle.