You got me thinking about how it looks, as if combustion gasses have gone part way up the spark plug threads. So, I went out and checked a head in the garage and there is about 3/4" from the gasket seat of the plug to the combustion chamber.

OP, what's the PART NUMBER of the plug?

 

I bought the champion copper plus PN RC12LYC. Now, the first plug that did this was a denso Q20R-U 11. When this first happened I thought it was my mistake because the denso plug is for the 90 4.0. But I dropped a 95 4.0 in the 90. So I ordered the Champions for it, which are longer. It ran very well with these plugs. (except for my hesitation problem). Three weeks ago I started getting a miss, pulled the plug and found that....again.

 

Here are some more pics. The one on the left is the first plug that had this problem. The second one is the champion.

 

Just ran out and checked the "spares" in my glovebox, identical to the one you have installed now. length should be ok.

Completely off topic, I have a table that looks just like that, with a very similar red stain from someone else. What the hell is it? lol sorry.

 

my jeep had a hesitation issue with a bad tps, and it would backfire sometime, once it backfired hard enough to blow up the muffler, i wounder if it backfired once

 

I don't know why your reluctance to show us the other side of the broken plugs so we can see WHY it happened.

If all you did was use the '95 motor but used all the sensors, wiring, PCM etc from the 90 - just the heavy iron - that's fine. You effectively have a 90. If you tried to use the 95 distributor, you'll have problems.

The 90 will run slightly richer even though the head is virtually the same - 90's were richer than 95's.
The slightly richer mixture needs a slightly colder plug.

The stock Champ plugs - According to the FSM for a 90 are RC9YC
For the 95 they are RC12LYC - 3 clicks hotter. The Densos are supposedly about the same as the 9's. I don't trust Denso's - they've proven themselves to be junk..

But in your case with the insulation shattered, like you hit it with something, I'd more suspect pre-ignition than anything else. If it was just running hot all the time it would look more melted.

I believe it's hot - or something in the chamber is hot - causing the fuel mixture to ”explode” rather than slowly burn - pre-ignition, detonation, ping etc. That explosion shattered the insulator.
It could be just full of carbon from having the wrong plug in it and misfiring. Do a compression check on that cylinder - if it's choked with carbon compression will be way too high.

But - what I really suspect - you have a vacuum leak close by #1.
Do a smoke test - THE most effective way to find a leak.
Engine off - hook a hose to an intake port - blow cigarette or cigar smoke through the hose.
The ONLY place it can get out is at the throttle body - ANY place else needs to be sealed. Cover the TB with a rag or duct tape.
DON'T BE A SISSY - Blow lots of smoke in it.

If it was a timing problem, ALL the cylinders would be affected.
If it was an injector problem it would not cause detonation, it would just be lean.
Something is glowing red hot and causing the mixture to explode in just that one cylinder.

 

This

X2 exactly my thoughts

 

Thanks for the advice. I will try anything at this point. I am working on it today so I will try to tell ya all what my test results are. I just tried the smoke test but kept blowing the duct tape off the throttle body. Going to stuff and tape and see what happens. Next is the compression test. I pulled all the plugs. Now I had to drive it yesterday because the wife and kids needed the good car. So I put a new plug in it. Here are some pics of #1-#3 cylinders. Going from left to right #3-#1. I will keep posted and any other thoughts would be great. Thanks everyone.

 

Compression test result:

6-150
5-145
4-150
3-150
2-145
1-140

So is that good or bad??

 

Okay. So just did the smoke test and the only smoke I got coming out was from the egr valve. Checked the bolts to the manifolds. All tight as a tick. I guess next I will check and make sure the dist. And cap are in the right spot.

 

I think spec is 160 PSI or so but all 6 cyls within 15 or 20 PSI of each other. Other people seem to be doing okay on 130. So looks good to me

 

Thanks salad. Makes feel a little better. Just checked my dist. And cap. I think I'm right on the money. Here is a pic of the rotor at TDC. Lines up with #1 cylinder post. So I think I'm good there. Now I did some reading last night And read some stuff that a bad knock sensor could possibly cause the cylinder to bust the plug. Any thoughts??

 

Coming out of where on the EGR valve?

Do you realize that the EGR can be sticking open and introducing exhaust gasses into the intake all the time? Equivalent of a vacuum leak.....

 

Its hard too tell. I think from the rod that sticks out from the egr and goes through the plate on the manifold.

 

That shouldn't be happening. What happens if you spray throttle body cleaner around the EGR valve with the engine idling?