I put the old plug back in and same code, but I'll go get the Champion and try it. It's a cheap experiment and I hope this problem is that simple. I'll let you know. Thanks for the help

 

We just need to rule out the cheap stuff first, so replace all of your plugs, it will cost you $12 or so and then we can go deeper if we need to.
But always start cheap and simple first.

 

Spike,I would suggest before you start making statement causing people to purchase new plugs look at 2 things, he had the misfire before changing plugs and wires and you should suggest he switch plugs with another hole to see if the misfire follows, if not, it is not the plugs fault.

 

Hold off on buying a new set of plugs, just rotate that #4 plug to another hole and see if the fault code shows up for that hole.
I'll put money on it, it is not the Plus 4 causing the problem.
Need to look at a leak down on that hole to rule out a burned valve for a bad valve guide hanging up.

 

OK guys - will do. Thanks for the help. didn't get to it today because it's raining like crazy here. Will do tomorrow. I'll be doing a leak down test this weekend and my brother has a video borescope I can use if need be.

 

Quick and maybe crazy question - Do any other cylinders on the 4.0 fire at the same time as cylinder 4? Reason I am asking is because if so, I want to swap wires to see if the problem moves.

Tried plug on different cylinder - no change. Still need to do leak down test but that won't happen until next week.

 

The wires are in pairs of equal lengths so just swap wires across the cylinder. So swap 4 & 3. You will need to switch it at the coil also.

 

So I swapped spark plug wires on 3 and 4 (on coil and actual wire). Now I'm throwing P0303 and P0304. Still need to do the leak down test, but wanted to try to figure this out before throwing money at it.

So let's go over the history here:

2001 Disco II 127K miles - Premium fuel only - Ran Seafoam

Originally running rough with flashing CEL

Checked code - P0304 (misfire cylinder 4)

Replaced wire set (Kingsborne and Bosch Platinum plugs) after finding visible arcing (at night with water mist) on cylinder 4

No unusual residue on cylinder 4 plug

After wire/plug replace - still threw P0304 code

Swapped plug on cylinder 6 with cylinder 4 (to isolate potential bad plug)

Again - P0304

Swapped cylinder 3 and 4 wires and switched at coil

Now throwing P0303 and P0304

Quadruple checked wires to coil routing - wired properly

What are the flipping odds of having an original P0304 code, visible arching, replacing wire, swapping new wires on 3/4 wire and now have P0303 code (indicating a bad new wire from the original cylinder 4 PLUS having another issue at the root of the P0304 code)?

I should have played the lottery instead....

 

Here is what I would do...go to AutoZone and buy 8 el' cheapo (DII)spark plugs, it will set you back $12.
Clear the codes, install the new plugs and drive it like normal and see what happens.
That is what I would do.

 

After tat, the look at compression and a possible leak down test.