I rebuilt my 94 Ranger 4.0L last September, 2018. Since then I have put less than 2000 miles on it. Recently I checked the plugs and found the plug in #5 cylinder looked pretty bad.

https://ibb.co/7bq41Xz
https://ibb.co/R7QhTpJ
https://ibb.co/VV86qjH
https://ibb.co/PQxYdsS

I realize this is a general issue of interpreting plugs, and not isolated to Ranger. If anyone has a clue what this means, let me know. To me it looks like deposits, but what they are and why they're confined to only the #5 cylinder is a mystery to me.

Kenny

 

Yes, looks like ash deposits from oil

Good look here at spark plug tips: https://www.boschautoparts.com/docum...2-1dadd0d5def6

The threads look bad as well, you have a valve cover leak above #5 spark plug hole?

You could have had a big wack of assembly lube in/at #5 intake

 

Thanks RonD. I installed brand new heads, and didn't noticed an excess of assembly lube. I did have a lot of oil in the cylinders when I assembled. Also I noticed all the plugs had loosened to where I barely had to budge to get them out. So I must not have tightened them enough, which could explain the oil on the threads. One plug, either 5 or 6, the spark plug wire had come off, which had been causing a miss. I replaced all the plugs with the recommended platinum tipped plug, and tightened them adequately this time. I suppose I'll drive another few hundred miles and then check #5 again.

 

Sounds like a good plan