Even though it's Mechanic 101 I'll point something out since I see people doing it.
Never run a plug wire open circuit in a High Energy Ignition system like ours. While such a practice used to be a handy old school troubleshooting tool for finding leaky secondary components on lower voltage systems it's risky on HEI systems. A turboed engine ignition system needs to be HEI because boost pressure increases the need for higher voltages but what I'm about to say is good practice when working with any ignition system.
In all ignition systems secondary voltage rises as high as it needs to (within limits of the design) in order to ionize the gases in the plug gap that will serve as the conductor for current flow. Ionized gas is called plasma. Plasma is electrically charged gas and is thus conductive. I'll spare you the physics behind this but whether it's the spark in a plug, a welding arc, neon sign, lightning, or the Aurora seen in the Alaskan sky, it's all plasma. In fact plasma, often called the fourth state of matter, is the most common form of matter in the universe.
Anyway, the resistance of a "normal" A/F mixture, even under compression, generally requires only around 10-15 kilovolts to "ignite" a plasma between the plugs electrodes. I'm talking about the true firing event on a DLI system here, not the waste spark. After the plasma is established and current begins to flow across the plug gap secondary voltage will drop to about 4-7 kv during the time it takes the mixture to either be consumed or until the coil pack runs out of juice, at which point the plasma will extinguish. Anyone familiar with secondary waveform analysis using an ignition scope will recognize what I'm talking about.
Since secondary voltage will rise to whatever is required when you run the secondary open circuit the voltage will go all the way to it's limit. This can be 50 kv or more in a HEI system. In doing so it'll stress every portion of the secondary circuit but the weakest link is the secondary winding. The secondary winding in the coil pack is comprised of thousands of turns of very fine wire wound around itself on a bobbin. This wire is insulated by a very thin layer of varnish-like coating that has limits to it's dielectric properties and when the voltage is allowed to near instantaneously rise to maximum, as it will when the pack is fired open circuit, the voltage can easily punch through the varnish and short adjacent windings.
Depending on how many windings this happens to the result can be anything from not firing a plug at all to a slight and irritating misfire that'll only show up on an ignition scope. You can't measure this damage to a coil pack without special equipment. An accurate low range ohmmeter is required and the high voltage diodes integral to GTE coil packs make measurement very difficult.
Even if you get away with doing it once the stress on the insulation will weaken it and in time a winding may short during normal operation. All it'll take is a wide plug gap or a lean mixture in that cylinder to raise the voltage high enough for it to punch through the weakened area. And every time you pull this stunt you increase the odds it'll fail in the future.
This type of coil damage can cause frustrating to resolve driveabilty and emissions problems and without an ignition scope the only way to run them down is by replacing the coil pack(s) in question. Those who say "I've done it and everything was fine" are either lucky or clueless. The bottom line is never allow a secondary output to fire open circuit. Either ground it, use a spare plug, or buy a tester. They look like a spark plug with a big alligator clip. You connect the plug wire to it and ground the clip. The tester has a window to observe the spark. They cost all of $5 at "Harbor Fright" and a few bucks more elsewhere. It's just not worth the risk to do testing any other way because of the unseen damage that can occur and the hassle involved in tracking it down.
-Jetjock, MKIIItech.com
