1999 Ford Explorer XLT engine question

[jimwelsh]

Hello. I have a 1999 Ford Explorer XLT with the 4.0 v6 engine. My rig has 243000 miles on it which isn't bad for a 20 year old vehicle. I bought it with 143000. My question is about the engine. It seems that after about every 14000 miles or so the #6 spark plug fouls out. All the rest of the spark plugs look good. Replace the fouled spark plug and it will run for another 14k miles or so. It doesn't burn oil or otherwise smoke. Coolant is good. It gets anywhere between 14 and 16 miles a gallon. This is the only quirk and quite frankly, I really don't like changing the spark plugs (donate a knuckle every time). Anybody else have this issue, and if so what did you do to take care of it once and for all?

Thanks.

 

[410Fortune]

You could have a weak injector or have valve seal issues with that cylinder
What does the plug look like when you replace it?
I am going out on a limb here and guessing you have a OHV 4.0L engine in that 99? (not the SOHC 4.0)

I say that because the ohv engine is known for having oiling/valvetrain issues on the rear two cylinders 3 & 6
But they have also been known to make it past 400K mile with those same issues.
Could also be a weak injector, intake manifold gasket leak, bad plug wire, piston rings, etc
ONly real way to know for sure is to do some tests