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1969 MUSTANG 351 MILD BUILD, MSD MECHANICAL DISTRIBUTOR, MSD #8222 HIGH VIBRATION COIL EPOXY FILLED, MSD 8.5 MIL WIRES ALL PARTS LIKE NEW. TOOK THE CAR OUT TO GET FUEL AND ONCE IT WARMED UP IT HAD A MISS THAT GOT WORSE. OPENED THE HOOD AND FOUND THE COIL SPARKING FROM THE TOWER TO THE POSITIVE AND NEGATIVE POSTS ON THE COIL CAUSING THE MISFIRE. I CHANGED THE COIL AND WHEN IT WARMED UP IT DID THE SAME THING. ANYBODY HAD A SIMILAR EXPERIENCE?
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There is an open circuit in one of the spark plug wires or plugs. The coil builds maximum voltage trying to over come it
Larry
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WHY ONLY AFTER THE ENGINE COMES TO FULL TEMPERATURE
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Because with heat either the wire goes open or the plug goes open. You already figured out it's not the coil
This can happen on a good system by pulling a wire off and not allowing it to jump to ground. The coil will build it's maximum voltage trying to bridge the gap and often will jump from the high voltage terminal to the primary terminals on the coil
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Run the car at night and see if you can see any other arching under there. Another thing to try would be to let it warm up. Shut the car off and pull off one of the plug wires. Start it back up and see if the miss gets worse or stays the same. If it gets worse now you have 2 cylinders missing, if it stays the same that could be your problem wire.
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I have to admit I thought this was going to be a thread about opening the hood and the engine was gone. Here's my thoughts. Electricity always takes the path of least resistance. So for some reason there is less resistance for the spark to just jump from the coil terminal to the power and ground terminals. Here is what I would do. First with the engine running use a spray bottle and mist the plug wires with water. If you hear a bunch of snapping there is spark jumping from a wire to the engine. Inspect the wire and make sure it/they are snapped on to the terminals securely. Probably would be a good time to put some dielectric grease in all the boots. Make sure plugs are gapped properly. Make sure the engine has a good chassis ground. Inspect the distributor cap and rotor button and make sure they have good contact. If all that fails and there is still spark jumping around, the wires may be faulty by not having the ends crimped on properly. I've seen it before on the DIY crimp kits that MSD sells.
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