Cylinder 8 not firing

nm9stheham said

Not really..... being able to fire a spark plug gap in open air is meaningless. The spark voltage needed to fire a spark in a compressed fuel-air mixture is many times higher than in open air. So, it can fire fine in open air but not at all in actual operation. You have MAYBE proven that the spark plug can fire, but not that the plug wire is OK. So actually swap the plug AND wire with another set from another cylinder. Or, connect the #8 plug wire to a screw driver and lay it 1/4"-to 3/8" from metal and see if it jumps a gap of that size with a nice blue spark.

Your spark may in deed be fine but you just have not proven that yet. Then move on with other other checks.

Click to expand...

Exactly.
Believe it or not, cylinder compression makes it harder for the spark to jump the plug gap.
The higher the compression the more it resists, and when it resists the spark can actually choose a different path than going to the plug.

A wire that the core is burning out of will do exactly this, because outside of the cylinder there is no resistance and the spark jumps the gap caused by the core burning away and also jumps the gap of the plug just fine.

When in the cylinder that big gap inside the wire is harder for the spark to cross.

If you look close at the ends of that wire it will be evident that the core material isn't there and making contact to the metal terminal on the end of the wire.
It will probably look black in the center of the wire also, if that's what it is.

Swapping two wires is a pretty easy way to check this, and the new cylinder that wire is on would not run.