My '98 was running like it had a fouled plug when it had about 78,000 on the clock. So, I round up a set of (pricey) plugs and figure I can change them myself. Wrong. Can't even get to the back 2, really. So I took it to the dealer. Exchange went something like this.
Service Writer (SW): What can we do for you today?
DzlBenz (DB): My truck has a fouled plug.
SW: No it doesn't.
DB: Yes, it does.
SW: No, it doesn't.
DB: Hey, pal, this ain't my first rodeo. I know a fouled plug, or at least a bad plug wire, when I feel it. And I'm feelin' it in my truck right now. But since my truck doesn't have plug wires (coil-on-plug ignition) I know it's not a wire. It's a fouled plug.
SW: No, it isn't. It may be running like a fouled plug, but that isn't what's wrong with it. In fact, I'll guarantee it. If we find a fouled plug, the repair is free.
DB: You're on.
So we go 'round back and hook it up to the Sun machine (or whatever), and at first, I feel vindicated. The mechanic says, "Uh, oh. #8 not firing." Actually, it may have been #7. It was one of the back ones. I felt pretty good. Then the bottom dropped out. "Dead coil." Shit.
What had happened was I had a pinhole leak in the heater hose. It was blowing steam directly on the coil for the back passenger side plug. (Is that #7 or #8?) Changed the hose in about 5 minutes, and the coil in probably less than that and I was on my merry way. Still, the whole repair was less than a C-note.
Moral of the story: There are so GD many wires under the hoods of these cars nowadays that there's no tellin' what's wrong with them when they start running goofy.