coil

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Could be the coil.

An easy way to find out, if it is an aftermarket coil, is you can put the stocker back on, if you have it, and see if the problem goes away.

On the other hand, a miss fire could be the coil, the wires, the plugs, a dirty MAF, bad/worn cap or rotor. The list goes on.

Do some visual inspections and diagnositc testing to try and find the source of the miss fire.
 
The magic number for failing coils seems to often be about 4K RPM.

Earley hit the major players I'd have noted too. If you have plat plugs, that can cause a 4K miss as well.
 
can the coil going bad cuase a miss fire at certian rpms?

Yes it can but also check your spark plugs and wires .I changed my coil and had the same issue but a little bit better and it was arcing so I insulated the coil mustang ran better but still had the miss.I replaced the spark plug wires and the spark plugs and the problem went away .It ended being the spark plugs had the autolites for 2,000 miles and they sucked :notnice:
 
Every coil I ever had go bad on me, just died, no warning. I've gone through at least 7 of them.

Kurt
dame kurt. Were they all OEM or aftermarket? Mine went bad once when i installed the digital 6 in my car. And yes Magic rpm miss fire was 4k. First i thought it was MSD box but it was the coil. It was one of those new MSD ones too. I switched abck to the old MSD one and it's trouble free. Older ones are better IMO for MSD coil
 
Most of them were Crane Cams PS91 coils, which I generally like, and they seem to work fine with the Digital 6 I use. A few of them were stock coils. I've had 3 Mustangs in my life. Seven seems like a lot, but I've owned a Mustang for the last 13 years.

Kurt
 
I worked for a company that built OEM coils for 8 years. I was in every phase of the creation of an ignition coil. I have fired coils in tests that would mimic you driving at top speed, while sitting in a thermal shock chamber bouncing in temps from -20F to 250F for over a week. I have never seen a coil fail. Failures only come back to us and were related to manufacturing defects. Defects usually are from contamination, poor solder connections, or wiring that has a cut in the coating (pinholes) and over time will eventually short. We are talking like 5 a year here at the most. 750,000 coils a year. The only thing I can tell you is that if your going through 9 coils, you may want to check your ignition module. A coil fires from a square-wave signal and that signals varies in frequency depending on your RPM. Your module sounds like either it has lost the ability to control the voltage and or frequency. Regardless, most of your ignition problems seem to come in this order. #1 Worn Plugs, #2 Rotor and Cap wear #3 Plug Wires (usually burned on headers) #4 Ignition Module #5 Ignition Coil
 
Well, whatever the reason for the failure, factory ignition coils seem to fail with reasonable frequency. I don't mind buying an ignition coil every few years. MSD makes ****ty products, no doubt about it, but I've had this Digital 6 for about 5 years. I have gone through at least 3 Digital 6s. I bought one when they first came out in 98/99 whenever it was. I am on my second ignition module. There's not much you can do to make everything work perfect. It's much easier to just change the coil every few years.

Kurt