jspringator wrote:
UPDATE: Ended up the problem was a badly corroded negative battery terminal.
Great... you seemed to have found it.
Did the lights dim at idle? It seems unreal that the ignition would be that sensitive to moderately low voltage. As an example, the alternator easily keeps up at 13.8vdc idle unless accessories draw too much. So did the truck run worse depending on large items like headlights, blower, AC being on?
There is a lot of misinformation out there. For example the website
http://www.magnecor.com/magnecor1/cop/fordv8v10.htm states that,
"Once the boot fails ignition coil failure is not far behind since it becomes overloaded by the excessive voltage grounding out on the cylinder head.". That is not true, a coil doesn't fail because of grounding out of a boot. The ignition charge might leak to ground but as long as it has somewhere to go there is not a "damage" problem.
The reasoning is the way that all ignition coils work. They usually work by a collapsing magnetic field that "generates" a high voltage within the windings (some work by the field still moving outward, but it must move). The ignition voltage builds up
only until it can jump a path to ground, hopefully through the spark plug. So the most elaborate, expensive, fancy ignition module in the world will fire (jump the gap) at exactly the same voltage in the same system conditions. If there is no suitable path, the internal voltage can build up so high it begans to break down the insulating quality of the coil wire coatings (or old time spark plug wires, i.e. the weak link).
Once a spark begans to burn through insulation it quickly forms a leaky carbon path that now shorts the coil or destroys solid state devices in newer CD type ignitions. This only gets worse as the inner spark discharge creates heat at the internal spark point, thereby causing the coil to continue failure even when new plugs (with proper gaps) are installed. Or the "transistor" is permanently blown by high volt peaks. This is the primary reason one should never ever let modern solid state type ignitions fire into nowhere, something we used to get away with back in the day of mechanical point distributors. Therefore it is generally ok, and least damaging to short spark to ground rather than prevent it jumping.
One other principle that ignitions have
always operated with, is that spark has a difficult time firing through pure compressed air, the higher the compression the worse. So most misfires occur when the throttle is wide open at low rpm and the maximum amount of air cylinder filling can occur. On the other hand at moderate speeds and power demand, the pistons are essentially compressing a partial vacuum which is to say the spark need jump through very little insulating air which it can easily do. As an example, the easiest way for electrons (spark) to jump a gap is found in the obsolete vacuum tubes. As these ancient tubes filled over time with insulating air leaks, or shorted from waste gas vapor from eroded metal grid compounds etc, they conducted less and less planned electricity until the intended circuit path failed to rule... until a clean new vacuum tube was installed.
But I said spark has difficulty firing through pure compressed air. It just so happens that gasoline vapor enhances electrical conduction. So even when the engine is working with high compression, the spark will fire when reasonable fuel is available. During cold weather extra fuel may be needed to promote not just burn but also spark conduction. By the same token a weak injector can cause a spark misfire because of lack of conducting spark path. So out of three things for proper cylinder firing, two may be missing. Enough fuel and subsequently, suitable spark path. The third, compression may be ok... or maybe not.
The third item for proper cylinder firing is compression. I do suspect that engines not used often may develop rusty valve stems. Once valves do this, they wear the guides, then become imprecisely seated. During cranking and low speeds they may not seal as well each and every cycle, which can cause erratic compression. Cylinder walls also can gather rust while parked for long periods. Modern gas engines do not ordinarily leave much oil residue on these surfaces. Diesel is a little better. So even if one has proper ignition and fuel, the engine may run erratically at low speeds because the third requirement for firing is unreliable... that of compression. This problem may show up in some dealship computer testing, or an actual mechanical compression test. Nobody like to pull plugs on these hard-to-service engines though.
Basically, unless spark is actually being released somewhere, the voltage is building above safe design levels and the coil is in danger of burning through either internal insulation or a solid state device. This is the primary reason coils finally fail after worn spark plugs develop large, hard-to-fire gaps. They do keep firing beyond design limits, but it is hard on them, and worn, widened-gap plugs should be changed before ignition voltage levels reach damaging levels. And barring frequent driving, long term engine storage begs that an engine be mothballed with extra protective oil added to the cylinders... unless it's already an oil-burner.
Wes
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