Coil on plugs keep going bad

I have a 03 mustang gt while driving it started driving a little rough and the engine light came on. When I got home I checked the code and it said that my #3 coil plug was bad. When I pulled the #3 coil there was some kind of hard black stuff that came out of it. So I got some new coils and changed then. Right when i finished changing them i decided to go give it a test ride and only after about 30 seconds of driving I smelt a burning smell and the car was running bad again. The same code popped up and when I looked at the #3 coil it had the same problem. And ideas on what could be the problem?

Yeah, you should have changed the spark plug too.

Coils work by running current through the primary winding which causes a magnetic field to form around the primary. That field also go around the secondary. When the control (in your case a transistor in the PCM) cuts off the current flow, the field collapses. The collapsing field will try to keep the same current flowing in the primary, and in doing so, it also induces current in the secondary.

One misconception that people have is the voltage developed in the secondary during the field collapse is controlled by the ratio of the number of windings in the secondary to the number of winding in the primary. This is only partially true. The voltage developed is also controlled by the resistance it meets and the time of the collapse. The spark plug has a high resistance so the secondary voltage can be anywhere from 20-40k.

If the spark plug fails, then the current in the secondary has no where to go and the voltage will keep building until it overcomes the insulation resistance of the coil and arcs either within the coil or to the nearest ground outside the coil. New coils are epoxy insulated so when they arc, the epoxy is damaged and the coil burns up.

BTW, when the coil arcs, a percentage of the voltage developed in the secondary is induced into the primary and that is determined by the ratio between the secondary and the primary. Most coils have a ratio of 100:1 to if it took 45kV to arc the coil, then 450V was induced to the primary and that could be felt on the switching transistor in the PCM. Too much of that would not be good for the PCM.

Make sure #3 spark plug isn’t loose, allowing combustion gases past it.


As a rule I have always replaced spark plugs when replacing failed COP coils and whether it’s blow by or excess resistance the plug is the most likely cause.

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Yea I forgot to mention that I changed the spark plugs. I’ve owned the car for about a month and havent had any problems with it till now. I just did a road trip to come see my family. It was about 650 miles. And the car has been tuned with an SCT X4 tuner could that have anything to do with the issue?