OT-Automotive ignition coil

If the coil is leaking internally, such as from degraded insulation, it may spark in air but not in the cumbustion chamber. This is because the air, at atmospheric pressure, has much less resistance than an air/fuel mixture when it is compressed. So, the insulation inside the coil may be OK when just firing through air but when it tries to fire through the compressed air/fuel mixture the insulation breaks down and the coil arcs internally. ERS

One more mystery of the Universe solved! Thanks, I thought it was a "carma" thing.

Well, it takes more voltage to fire the spark through compressed air, and that's what's in the cylinder near TDC. When the coils start to break down internally, the spark at the plug is competing with internal arcing in the coil. If the voltage required to make a spark at the plug goes up, then the spark will move to inside the coil. Usually, you get a weakened spark that is enough to start the engine, but then it runs really badly and will just quit when you open the throttle.

Jon

I've been led down this path myself. Does anyone know of an easy way to test that the plug is indeed firing through compression?

If you have a timing light connect it to the suspect cylinder. If the light flashes at idle and stops when the throttle is opened then it's not firing in that cylinder. There also inductive pickups that use a neon light for checking this problem. ERS

Inductive timing light will show this - if the plug is not firing, you won't get the lamp to flash.

Jim

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You said that the plug wire laying on the valve cover had a blue spark. But how far did it have to jump? It it only had to jump 25 to 40 thou, that's one thing. But if you pull the wire a half inch away, and it doesn't fire, that's one way to get an idea that the coil might be the culprit. Just for reference, try that now that you have replaced the coil and see how far away you can hold the wire. If you really want to check it out, put the old coil back on and try the same thing. Back in the days of the magneto, we were told to make a little adjustable spark-gap tester the evaluate the mag before and after repair.

Pete Stanaitis

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Tom Gardner wrote:

Testing mag coils can be tricky, if the gap is too large the coil *will* break down internally. Same with modern ignition coils in cars, they will fail if the secondary is left open and the primary pulsed.

Jim

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Old spark plug testers had a place to put your plug under a glass window and put compressed air to it... You sure could see the difference under pressure ...

Like if a coil wire goes bad, even.

Or if somebody is 'testing' the ignition in some way by seeing how large a gap the spark will jump. At some point it will no longer break down the gap and then the secondary insulation is at risk.

Jim

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Often overlooked items, for sure, are those ignition leads. They can break down internally and cause problems. I once spent the better part of a year trying to figure out why my van would run fine when accelerating, but at cruise would start missing. It turned out, after I sold the thing to my brother-in-law, that he found a bad plug wire. The fuel/air mixture in cruise is lean and not easily ignited, and the weak spark that reaches the plug through that bad wire isn't enough to fire it reliably. We use an old Champion sparkplug tester for our aircraft spark plugs. The spark will brighten as the pressure rises through about 40 psi, and then decreases as the pressure rises further. If they won't spark consistently at 120 psi, they get replaced. We can sometimes hear the plug arcing internally to the metal shielding (aircraft plugs are differently made, with metal all the way up and the shielded lead attached with a nut). Champions, by the way, are the worst plug I have ever worked with. Aircraft or automotive. My mechanics teacher in High School 35 years ago told me that 90% of all engine problems would be electrical. I have since found that to be true. Unless it's a diesel, of course.

Dan

I see warnings about ignition modules related to that.