It could be a wiring problem. Make sure the wiring to the coil is in good condition and not corroded or damaged. Also, the crank sensor sends a signal VIA the computer, to the coil. It may have a weak signal.
Before I go further, the most common cause of a rough idle is a vacuum leak or a bad idle air control valve. In your case, check for vacuum leaks. If the idle is not changing up and down, the IAC is most likely fine.
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Friday, December 9th, 2011 AT 2:50 AM