2000 Ford Ranger Spark Plug Problem

Misfiring on cylinder 3, your issue is a bad knock sensor. If the sensor detects a knock or goes bad, it changes timing. Your bad sensor changed timing so much that it seriously corroded one bank, yours being the right bank. The knock sensor is easy to replace, and the part only cost $125.

Thanks for the for the input.

I'm not sure the knock sensor is the culprit.
the engine runs fine, there is no knock or lack of performance. my gas mileage is 17 mpg in city.

if the knock sensor is bad why only cyl 3 is bad instead of the whole bank of spark plugs?

the bad plug has a white-ish light brown-ish color on the tip and insulator. the electrode seems fine no discoloration.

what else can i check?

Engine knock is also known as pre-ignition. It's when the gasoline in a cylinder detonates before it's supposed to. It can happen if you have fuel with too low an octane rating, if the engine is running hot or the outside temperature is very high, or if the engine is traveling under a heavy load like climbing a hill or accelerating hard. The knock sensor alerts the engine-management computer that knock is occurring, and the computer then retards the ignition timing to correct for it.

Sadly there is no easy way to test a knock sensor. They are a piezo-electric sensor, which can only be tested by exciting them with a frequency sweep (and typically high voltage). Each differently type of sensor has different frequency at which they will resonate.

By the picture you gave only other thing that I could can think of that would cause a "LEAN MIXTURE" or pre-ignition in your # 3 cylinder, and corrode your plug is carbon buildup, intake manifold vacuum leak, or sticking valves.

Maybe someone else has a different theory... Anyways good luck