help ford techs ...idles rough ..no miss fire codes

Hi all ...any ford techs out here... i had a windstar in the shop few weeks ago , running on 5 cylinders , no spark on one cylinder (3.8 lite engine )..
but PC wouldn't set a missfire code , customer said its been like that for over 2 months , he fiduard it was do to having over 260,000 miles , but i found a bad plug wire on one cylinder ...so my question is why didn't the pc pick up the missfire and set a code ? what triggers a missfire code , i thought it was off the crank sensor , seeing RPM decrease ...when it should be an increase .... i know fords are slower at setting missfire codes than other manufactures but this was 2 months , guy drives alot , other fords with 4.6 and 5.4 with coil on plug pick up missfire faster with in minuets long minuets have had them take ove 5 mins to set a code , dodge i hate to say is way faster , any ideas ?
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Not knowing the year makes a bit more of a guess. I have seen a couple of Windstars that would not set misfire codes because misfire profile wasn't learned. You'd have to check the PIDs and see the "MP_LEARN" status. The PCM learns the progile of the crankshaft trigger wheel to take into account any manufacturing variations.
The two that I saw would not learn this profile. The cure for these was to replace the factory ground that runs from the alternator mount to the rad support with a continuous wire ground strap - the factory strap has a plastic "box" containing what I assume is a capacitor.

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well thanks Jim , year i don't remember , 97-99 , once i find the mp learn , can it be reset from scan tool ?..or just go and check the ground strap as you mentioned ... doubt i'll see this van again , he was trying to sell it .. Thanks or your advice

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