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Old 11-08-2011, 09:35 AM   #1
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Angry Repeated coil failure

Fist off, thanks for any help anyone can give me. Everyone on Clubroadster only knows camber specs and thats about it.

I have a 97 non turbo miata. About 2 years ago the car started blowing coils. It would blow one coil and then be running on 2 cylinders. I have checked all the grounds and even added alot more grounding. The car kept doing this. So I took it to HP Motors, a miata racing shop, and they did not know what was causing this. So I thought it might had been the ECU and we changed it. This seemed to solve the issue for about 7k miles. It usually blows on about 1-1.5k. I was at an autocross on sunday pushing it hard and only got 2 runs in and it blew the coil.

Specs on the car
Just normal bolt-ons like intake and exhaust
18 degree timing advance with 93 octane always. NO pinging occurs
The motor seems to be on its last leg. Burns oil and runs rich. Im pretty sure it has low compression as well. The plugs always have a good bit of carbon on them. I will be doing a turbo build soon but want to fix the Coil issue b4 I do.

It has eaten at least 10 coils

Anyone have any ideas?
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Old 11-08-2011, 11:04 AM   #2
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Strange. Never heard of such a thing.

More vibration than stock - do you have poly engine mounts?
More heat than usual - did you relocate the coils?
Moisture - are they getting wet?

High system voltage - does the car eat headlights too? Do you know that the system voltage is <15V?
Do they coils tend to blow after extended high RPM running?
In hot weather when the engine is hot?
What plug wires and plugs are you running?
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Old 11-08-2011, 11:09 AM   #3
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All the electrical components have been checked...good voltage..good grounding...the motor has stock motor mounts but they are good. It doesnt eat headlights. Ive already replaced the alternator just in case..but still does it.. and the coil blowing always happens at cruising speed except for this last time just sitting in line to get my next run in. And it doesnt seem to be related to temps either and the coils have not been relocated.

Running NGK wires and 6953 NGK v plugs
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Old 11-09-2011, 05:08 PM   #4
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Always the same coil?
I think your ignitor is leaking a current to the coil(s).
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Old 11-09-2011, 07:09 PM   #5
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97 the ignitor is in the coil, but there could be a fault in the logic circuit in the ECU. Easy to verify with a scope.
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Old 11-09-2011, 10:23 PM   #6
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Originally Posted by RustyNWeathered View Post
the motor has stock motor mounts but they are good.
How do you know this? Have you actually removed the mounts and seen that they're not cracked and then re-installed? A bad motor mount looks the exact same as a good one from the outside. It's only when you remove it that you see it has broken in two pieces.

It really sounds like you've done a lot of electrical work. Changed alternators and ECUs, verified grounding and voltage, etc. That leaves me thinking there's some sort of mechanical reason for the coils going. Like being repeatedly smashed against the firewall somehow. Or being bathed in a steady gush of oil. Or anything that might explain the repeated failures.

One thing about that 97 ECU: it sucks. The chances you replaced a bad one with another bad one are not as remote as you might think.
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Old 11-10-2011, 03:07 PM   #7
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well the motor is very stable....It doesnt move as if a motor mount is bad. The only thing left now is to build a new harness for the completely rule out wiring. .and the ECU came out of a good running 96 car with 28k miles on it. Bought from HPMotors. The coils looks brand new when they are replaced.. NO oil on them.. NO signs of hitting anything.

I want to go to COP and a MS3. but not sure if this would stop it.. also the motor will be replaced with a built 99 motor.

I will check to see it it is the same coil that blows repeatedly

Im almost at the point to go with the original plan for the car and just put a LS1 in it and call it a day
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