These cars can be really sensitive to getting wet. In my experience, the spark plug boots are likely the cause of a misfire when the engine gets wet. Mine used to do that when it'd get wet, either driving in heavy rain or washing the engine. New plugs and boots seems to have solved it.

Obviously, this has nothing to do with the idle air control motor, just something to be aware of.

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Brian Rogers
'96 E320 traded in at 292,000 miles
'09 C350