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Old 09-18-2013, 08:23 AM   #1
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Coil

How do you diagnose a bad coil?
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Old 09-18-2013, 09:09 AM   #2
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2 ways I know of. One is more technical and requires a multimeter, the other is less technical and fairly simple.

All you do is while engine is idling, disconnect each coil one by one, while listening to the idle once you disconnect a coil. If when you disconnect one and you don't really notice a change in the idle...there's your culprit.
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Old 09-18-2013, 10:04 AM   #3
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Originally Posted by FanOfSkynyrd View Post
2 ways I know of. One is more technical and requires a multimeter, the other is less technical and fairly simple.

All you do is while engine is idling, disconnect each coil one by one, while listening to the idle once you disconnect a coil. If when you disconnect one and you don't really notice a change in the idle...there's your culprit.
Thanks man I appreciate it. I'm having such a mystery right now. I'm having erratic idling problems. Sometimes it drives like a charm and sometimes it wants to shut off. It's only shut off once. But if I had a bad coil, wouldn't I notice a decrease in power?
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Old 09-18-2013, 10:08 AM   #4
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Well how many miles are on the motor? If its over 100k and you haven't done the COPs (replaced or rebuilt) then its time to go through them all.
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Old 09-18-2013, 10:12 AM   #5
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Well how many miles are on the motor? If its over 100k and you haven't done the COPs (replaced or rebuilt) then its time to go through them all.
It's just over 107k and it has the stock motocraft coils. Alright, and also, if I did have a vacuum leak, wouldn't run rough all the time not just sometimes?
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Old 09-18-2013, 12:46 PM   #6
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Most likely but you should check everything just to make sure. Vac lines are good to replace as well at this point since they'll likely all be dry rotted.
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Old 09-18-2013, 09:07 PM   #7
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Also, you could swap out the o2 sensors if the originals are still on there. A lazy o2 sensor could result in an erratic idle and stalling issues. Well already on mine it did. The o2's were cheap ($50 for a pair for the upstream sensors) and easy to install..about 10 minutes total for me with use of a lift
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