I had a '96 Ford Exploder with the 4.0L pushrod V6 that had a spark plug simply fail one day while driving. You talk about a wild ride! "Check Engine" light on, galloping idle, crazy acceleration behavior, you name it. I thought I dropped a valve or something the way it was acting, but no, just a dead spark plug.

When O2 sensors are involved, any unburnt fuel will cause the computer to attempt to lean out the mixture to "correct" the problem, only to go too lean and wind up swinging the other way. The result is erratic operation, stumbling, and poor power. Makes sense that a weak coil would cause similar behavior at lower RPM's.

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Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]