The theme here is that a weak spark is suspected. Cap/rotor get corroded and bogging down occurs. Replace and all good for a while. Elsewhere in this string is the suggestion that that other items. like the coil, can be the culprit of a weak spark > if the coil is weak to start with a little corrosion on cap/rotor is enough to cause the issues. How to test?
They make a simple device to determine how strong the spark is > a spark tester. It’s a tool that goes in line with the spark plug (or coil) and has an adjustable gap. You visually watch the spark jump this gap and by adjusting the size of the gap get a sense of the voltage output. See pic below for what they look like. While they are inexpensive, I have found them difficult to use.
I recently ran across an inductive device that will determine the voltage range and its much easier to use. However, it pricy (about $80).
Link to the mfg. Always looking for an excuse to add to my tool collection I convinced myself to get one. And I have used it on my VP 5.0 GXi-E. It worked as advertised.
Unfortunately, the voltage ranges are wide. A green (good) light indicates 3,000 to 28,000 for large engines per this video I found on the device:
link. So, you may still get a green light but still have too low of voltage > I suspect (but do not know) the coil output should be about 25k. So, unless you spring for an expensive analyzer, I do not know of a way to test voltage the plugs are seeing.
Also let us know if the vents are plugged on the distributor.
