Question about Subaru Legacy

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1990 Subaru Legacy shorted out the ignition coil. I replaced it and in less than 1 mile,it shorted out the new one.

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Subaru engines (including Legacy and engines of many other automobiles, GM, etc.) have returned to so-called wasted spark ignition systems since advent of the electronic distributor-less ignition (having been used historically in early engines with magneto ignitions). With this approach, the ignition systems are less expensive and usually quite reliable. At the same time, however, a shorted spark plug (cracked or carbon clogged, etc.) (or bad ignition spark plug wire) grounds both the ignition spark and the false spark being used. This means that it also grounds the corresponding respective ignition spark of the other 180 degree out of phase cylinder (that uses the same the same coil because the spark current is connected to the spark plugs of two cylinders). Since such a defect allows more current to flow in the spark coil to two plugs when the resistance of the spark plug gap disappears due to a cracked plug, etc, it seems likely to hasten coil burn out if the spark plug/plug wire defect is not corrected before installing the new spark coil.

To explain further, wasted spark means that when a cylinder is ready to fire, it gets a spark to its spark plug while part of the spark (from the same coil and current) as a sort of copy is also sent to another cyclinder at the same time but when the piston in that cylinder is exhausting its ignited gases 180 degrees out of phase from when its spark is needed. When the second cylinder is ready to fire and receives its "spark, the first cyclinder the also receivesa "wasted" spark "copy" of the one needed by the second cylinder. Economy is achieved in manufacture because otherwise electronics would have to turn on and off four different spark coils to distribute four different sparks as needed to the four cylinders. With wasted sparks (which are cheap), because the spark current of one coil is connected (internally in the ignition coil unit) to two cylinders that are in the same position but 180 degrees out of phase in firing, the system can turn on and off two spark coils twice as fast with half the electronics and fire two cylinders that are 180 degrees out of phase. A four cylinder engine thus uses only two coils, a six cylinder engine, three coils, etc., in a wasted spark system.

Posted on Sep 15, 2008

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Subaru engines (including Legacy and engines of many other automobiles, GM, etc.) have returned to so-called wasted spark ignition systems since advent of the electronic distributor-less ignition (having been used historically in early engines with magneto ignitions). With this approach, the ignition systems are less expensive and usually quite reliable. At the same time, however, a shorted spark plug (cracked or carbon clogged, etc.) (or bad ignition spark plug wire) grounds both the ignition spark and the false spark being used. This means that it also grounds the corresponding respective ignition spark of the other 180 degree out of phase cylinder (that uses the same the same coil because the spark current is connected to the spark plugs of two cylinders). Since such a defect allows more current to flow in the spark coil to two plugs when the resistance of the spark plug gap disappears due to a cracked plug, etc, it seems likely to hasten coil burn out if the spark plug/plug wire defect is not corrected before installing the new spark coil.

To explain further, wasted spark means that when a cylinder is ready to fire, it gets a spark to its spark plug while part of the spark (from the same coil and current) as a sort of copy is also sent to another cyclinder at the same time but when the piston in that cylinder is exhausting its ignited gases 180 degrees out of phase from when its spark is needed. When the second cylinder is ready to fire and receives its "spark, the first cyclinder the also receivesa "wasted" spark "copy" of the one needed by the second cylinder. Economy is achieved in manufacture because otherwise electronics would have to turn on and off four different spark coils to distribute four different sparks as needed to the four cylinders. With wasted sparks (which are cheap), because the spark current of one coil is connected (internally in the ignition coil unit) to two cylinders that are in the same position but 180 degrees out of phase in firing, the system can turn on and off two spark coils twice as fast with half the electronics and fire two cylinders that are 180 degrees out of phase. A four cylinder engine thus uses only two coils, a six cylinder engine, three coils, etc., in a wasted spark system. old_subaru_man

Posted on Sep 15, 2008

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