These came out of my '96 XJ 4.0. Plugs from cylinders 2-6 all looked like the plug on he left, barely to somewhat tanned, normal. But the plug from cylinder 1 is on the right and it obviously has something going on. The plug threads are significantly darker than the others and on the plug ceramic the coloration is much darker all the way around except on the side where the electrode extends out of the plug where there's no coloration at all. Whatever this is it's happening in cylinder 1 all the way at the front of the motor.







Motor has 150k on it.

Anyone have an idea of what's going on here? Thanks.

 

No pic.

 

What plugs were installed?

Your year engine calls for Champion RC12LYC plugs, gapped to .035".

 

Enjoy

 

#1 cylinder plug looks like it was "indexed" towards a valve that is pumping oil.

The dark threads may be from the plug not being torqued properly. 27 ft./lbs., without anti-seize.

Try doing a compression check on #1, then do a compression loss check on #1. Apply a steady 70-90 psi air source to #1 while it is at TDC compression.

If you can hear air escaping from the TB there may be an intake valve defect. If you can hear air escaping from the tail pipe there may be an exhaust valve defect. If air is escaping from the oil filler port on the valve cover there may be bad rings.

If the compression check and loss check are okay, there may be a bad valve stem seal on one of the valve stems.

 

I had a #6 plug look like that a while back.
~turned out to be a faulty injector~

 

I believe the correct plugs were used. Plug number is shown upside down in the third pic.

Forgot to mention though, these came out during a complete tuneup using all mopar parts from the coil to the plugs. Motor had a miss before and runs better afterwards but still has somewhat of a miss. I was wondering if it was maybe a bad motor mount (I think a bad motor mount can mimic a miss?).

The darkened area has no kind of gunk build up. It's exactly like the other plug far as surface condition goes, just with the dark coloring. There doesn't appear to be any oil residue.

I've had suggested that the plug was facing a bad valve thus the pattern coloring. I'll have to say the coloring is so symmetrically aligned with the electrode that the chance of the plug's electrode and valve lining up that exactly is smaller than the chance that whatever's causing this to happen, the pattern is more likely to be showing up as a function of it's relationship to the electrode.

 

If it is oil from a valve stem seal depositing on the plug at initial engine start, the residue may quickly burn off when the engine is started. A bad valve stem seal will only leak when there's no combustion pressure present.

Hot Rodders are **** about indexing their plugs. They attempt to get the electrode aligned with the exhaust valve by using shims.

Google Spark Plug Indexing and you'll find articles that talk about indexing.

 

Granted some sort of compression test is in order. I sort of like Clicks thought on the injector. I haven't done it, but can't you listen for a healthy "tick" by putting a long screwdriver on the lifter, and the handle to your ear?

 

With the OP posting this same question on 8 million Jeep forums he's bound to find the easy way out answer he's looking for.

 

Yep, that's the way I post a lot of my questions...on a couple different boards. Historically I've just had better results that way.

 

Where do you park your Jeep? Bikini Bottom?

 

Do the contacts in the distributor cap show any flashing or oxidation more than those of cylinders 2-6? Swap a couple of the wires and drive it for a while, then check the plugs again.


Edit: Swap the entire wire. Don't just swap the connections for cylinders 1 and 2... I'm thinking you may have a cold spark and/or faulty plug wire.