Why different spark plugs- 99 Taurus?

Perfectly OK. When they build the engine they use platinum plugs almost like the aftermarket ones. Aftermarket plugs are platinum plated on both the point and electrode but half the originals are plated on one only. We assume this cuts costs a bit on the big scale but for replacements, it would likely cost more to supply both so we can only get the ones that have both sides plated. Something about the electrical characteristics of the spark transferring metal. Use the double platinum plugs, the one side is just getting a slightly better plug than specified.

Hope this Helps

PoD

Double platinum is just fine. I think what Ford did is save a bit of money on spark plugs. My understanding is that some of the plugs fire from center electrode to ground, while others of the plugs fire the opposite way due to the fact that some get +voltage, and some get -voltage. Where the platinum needs to be depends on which direction the spark is traveling. Ford, therefore, saved half on platinum by only putting it where it has to be.

If you get plugs with platinum at both ends of the gap, you're literally covered either way.

CJB

Platinum Ground electrode

but cylinders 4,5,6 have plugs suffix "P".

Platinum center electrode

Different potential biasing in the high tension spark circuits - one ground to center, the other center to ground.

The guide for Autolite plugs says Double platinum

Perfect. Stick with Autolites and you will be "blessed".

My question though - do you really have enough miles to need a plug change?

Rob

One bank has normal spark polarity, the other bank is reversed. Rob hit it right:)

Al

Sounds right if he's a long distance commuter

To further illuminate:

Count the coil packs, note you have 1 per each 2 cyls... unlike the legacy ign systems where one side of secondary (Hi-V) connected directly to ground, each coil fires once per rev and is wired:

Gnd --plug A -- coil neg &&& coil pos -- Plug B -- Gnd

Thus both plugs are fired on each trigger