Replaced wires today because I had an arcing wire on cylinder 4 which was throwing P0304 code. Now, AFTER changing wires and plugs (I used Kingsborne 8 mm wires). I still have a misfire on cylinder 4... Original plug removed from cylinder 4 looked normal (no oil or coolant).

The other weird thing is it stops misfiring above 2500 RPM and runs like a bat outta hell

Any ideas? This is frustrating

 

Make sure that you are fully clipping the wire onto the spark plug end, as well as into the coil. Especially check the coil connection. Some wires will be harder to clip in than others, and you may think its in but it really isn't.

 

Try some dielectric grease too on the connectors.

 

Used Dielectric grease and connections were double checked yesterday. All Good. Getting spark though wire

 

What year and model, how many miles on it and what octane are you burning? Might need to do a leak down test on the cylinder, may have a valve sticking or burned.

 

Disco Mke,

2001 Disco II 127K miles. Running 92 Octane (always) plus added a can of seafoam on last fill

Ran cold compression test on cylinder 4 and it logged 150 PSI

 

A leak down may tell you more.

 

What plugs did you use?

 

Bosch Platinum 4

 

I am starting to see a pattern, we have had a rash of people do a tune up using those plugs and they all have had a single cylinder mis fire that cannot be tracked down or cured.
Do yourself a favor and go and buy Champion plugs, the cheap copper ones and put those in the truck, clear the code and see what happens.
I am betting that your mis fire goes away.