A 1997 Ford Expedition V8 has only
eight spark plugs, not nine. There isn't an "8th" one in the sense of a separate numbering beyond the eight cylinders. The location of each spark plug depends on the engine's orientation in the vehicle, and which bank of the V8 (left or right, as you're facing the engine) you're looking at. There's no standard "plug 8" designation.
To find them, you need a repair manual specific to your 1997 Ford Expedition V8 engine (likely a 4.6L or 5.4L). The manual will have diagrams showing the location of each spark plug. Otherwise, using online forums dedicated to Ford Expeditions and searching for images or videos showing spark plug replacement may assist you visually.
