My 04 FX4 hasn't been running well for about a week or so, power has been off, missing, hear some knocking under power, etc. Check engine light only came on once blinked for a bit, never came on again. Really hate just spending money swapping out parts never knowing if its going to do any good but, Thought I'd start with easiest / cheapest things first. Replaced fuel filter- no improvement. It was about due for oil change, did that-no improvement, Checked the FPDM - it looked fine, Cleaned the MAF and throttle body - no Improvement. Tried Seafoam in the intake and fuel tank - no improvement. Already did the spark plugs just last year so I didn't think that could be it, With 80K miles, thought possibly one or more of the cops were going out. So thinking this will be the last thing I try before going to the dealer, I picked up a set, and while I was replacing those I thought, I'm this deep, I might as well pull the plugs and check them anyway. They were all looking fine till the very last one, some of you know which one Im talking about, the very back one on the passenger side, anyway,,, Found the cause of engine miss (YIPEE!!) I cant imagine what would cause the spark plug to be destroyed like this, Replaced it, Purrs like a kitten now! I guess as a bonus I've taken care of several tune-up / maintenance items along the way.

 

Is it possible that that cylinder is running HOT!!!! and it caused it to disintegrate like that??? I'd probably pull it again after 10k miles and check it to make sure it isn't something going on with that cylinder....JMO

 

Besides from the size of this threads page being all screwed up, I'd say you have a real lean condition on that cylinder, soon to have burned valve. Look to an air leak first but don't rule out the injector malf.

 

Judging by the looks of the intact plug you are running lean.

 

Wonder if it not being torqued to 25lbs could cause this? It look almost like the combustion was getting past the threads.... I'm no expert just kind of wondering....

 

The plug was not in tight when I removed it, likely didn't get it torqued properly, Haven't found any vacuum leaks, still going to keep an eye on that cylinder for any other possible issues.