I have a '99 Discovery II, recently it started to run rougher than usual and threw a code, P0304, cylinder 4 misfire. The truck would go from running fine to misfiring and then back again. The spark plugs looked ok except for cylinder 4 which was covered in fuel if the truck was running bad. I replaced the original spark plug wires and autolite spark plugs. The misfire is still there. If I pull the spark plug wire from the plug the truck runs the same, so I am pretty certain it is getting no spark to cylinder 4. I have no idea what else it could be, I have double checked the connections at the coil and spark plug and even swapped spark plug wires in case it was a bad wire, but nothing has worked. Also, I don't know if it is related, but during the middle of trying to fix this it threw a code for no signal from the front o2 sensor (p0154 or something). That code appeared once and has since went away when I cleared it once. If anyone has any ideas or suggestions, I am very open to them at this point.

 

What brand of plug wire are you using? Autolite plugs are not anywhere near the plug of choice when it comes to your engine.

 

I used Duralast auto zone spark plug wires and NGK plugs.

 

Tried replacing the #4 plug wire with an old one, it ran rough again for a few seconds and then ran fine. A few seconds later it ran rough again and threw codes p0300, p0304 and now p0150 (o2 sensor malfunction). I can only think this is related to the coil pack or the o2 sensors. It was my understanding however that when a coil pack goes, multiple cylinders misfire. For some reason, my #4 plug is not getting spark, the new #4 plug is wet with fuel. Any ideas?

Also, if it helps, the truck has 127k, I use 91 octane fuel and the o2 sensors look old/original. I Seafoamed the engine approx. 4k miles ago and use Chevron Techron fuel system cleaner every three or four tanks.

 

Auto Zone has great stuff and for **** plug wires, some people use the NGK plugs, I am not a big fan of them in a Rover, but I don't think they are the problem.
Switch the #4 plug wire to anothe cylinder and go drive it, see what shows up.

 

I don't think it is the plug wire(s). When it first misfired after the new plugs/wires I switched the #4 wire with another new plug wire. The #4 cylinder still misfired. I just tried swapping wires again, this time swapping the #4 wire with an old wire, it still misfired. I have had two different 02 sensor codes pop up know, both for the same sensor, saying it has no signal and another popping up later saying signal malfunction. I dunno what it could be other than the coil or 02 sensors now. Also, the exhaust has started smelling bad since all this happened, not sweet like coolant, but kind of smokey/smelly.

 

If you pull the no.4 lead off can you hear and see it sparking?Have replaced quite a few coilpacks with only one cylinder dead myself.
Regards
Marius.

 

Huh, I turned off all the lights in my garage and pulled the wire so the end of the boot was touching the spark plug. I could see spark. When I put the plug wire back on it still misfired. I pulled the plug and it was wet with fuel still. I am thouroughly confused now.

 

Well, I decided to pull the spark plug and crank the engine to see if the cylinder was flooded. I put a paper towel over the hole. When I bumped the engine a few times. I looked at the paper towel and it was wet with liquid that only slightly smelled of fuel. I put a new spark plug in and the truck ran great for a few seconds and then began misfiring. The new plug was wet, but it didn't smell strong enough to be just fuel, I'm pretty sure its water/coolant. I'm guessing the headgasket is leaking or went. It has never lost coolant and there is no water in the oil but I'm guessing the gasket is leaking into the cylinder. Does this sound about right?

 

Time to do a compression test and or a leak down, you may have a chipped exhaust valve.