Had some misfiring, specifically above 40 mph and when accelerating out of overdrive on my 99 4.6L.

Replace the spark plug wires, and didn't add any di-electric grease (wires were pre-greased). Mis-fires went away completely, but came back suddenly about 3 weeks later.

Read some posts on this forum about the di-electric grease, and added more. Mis-fires went away completetly, again.. but after a good rain last Thursday, came back.

Not sure what else to do, it's obvious where the problem is. Moisture is getting into the spark plug ports. But why does this not go away after a few dry days?

Any ideas on what I should do. Are the wires bad? Should I get a different set? got the Duralast Brand from Autozone.

TIA - Jesse

 

After doing alot of reading and research I just changed my plugs and wires on my 97 4.6l and went with the stock plug and Autolite Professional set of wires
runs like new

 

I swapped out the Duralast (Autozone) wires and went with the more expensive Bosch wires, and used quite a bit of grease. Bosch wires were even longer that the Duralast, so of course I was left with a long one for the #5 plug.. And had to move a few around.

So far, so good. Been through two rains, and quite a bit of humidity.

(Knock on wood!)