Spark plug wire meltdown every 15 miles (always cyl. #8)

For the last six months this has baffled all comers. The boot attached to the plug disintegrates from the inside out. We have put in a new metal boot cover,Kevelar heat shield,cap, coil, spark plug, M.A.P. sensor, M.A.P. plug, Spark control module,ignition module,and finally a new computer. 1991 GMC/6500 w/366 throttle body fuel injected gas motor. Help Please!

I would run a compression test on at least the No. 8 cylinder. (Preferably test all cylinders)

If you have very low compression this will kill the spark plug as the compression pressure is simply not high enough to burn the air/fuel mix thoroughly.
This in turn leads to the spark plug becoming gas fouled very quickly. The spark that is produced by the coil is still going to attempt to go somewhere if it cannot jump the plug gap.

This means the spark is going to try and burn its way through the wire insulator to the closest ground it can find and that is the cylinder head.
Hope that helps.

I would guess the damage is due to the spark jumping through the insulator for some reason. The only thing I can see that would make that happen is the grounding for the plug isn’t there. Have you checked to see if the plug has good spark across the electrodes while it is set on the engine or checked to see if the spark jumpes through the plug cover to ground?

This brings up a good question, how good of an electrical insulator is gasoline? An overly rich mixture, of gasoline and air, may not pass an electrical spark, either. So, the spark would follow the path of the least resistance…the boot.
Overly lean, or, overly rich, that is the question.

The number 8 cylinder on Chevrolet V-8s have always been the first to indicate severe driving conditions. Broken pistons, stuck and broken rings, gauled cylinders, etc., occur at #8 first. How is the truck used?