So here's what I have done so far:

Engine was running ok, but on ancient champion pypb4 plugs, so I decided to change them since I am going through the high-mileage checklist on my new-to-me Disco.

Put in Autolite platinum plugs. I noticed in the process that the wires looked pretty bad, and one boot was cracked. When I put everything back together, it was idling rough.

Next day I got a P0307 and CEL - went to the NAPA and got their plug wires (I know, only 7mm, but I knew I had the cracked boot, and needed to do something. A search here turned up a couple of people who used them successfully, so I gave 'em a go.)

Replaced wires. Today it is idling rough again, hesitates just a little bit sometimes, and it is throwing the P0307 code and CEL again. So no change with the new wires.

Is it possible that I happened to get a bad plug? Could a loose plug cause it - I used the torque wrench on it.

The old plugs looked normal when I pulled them, BTW - just a regular greyish, no signs heat damage, excess carbon or oil fouling.

Any ideas? I'm going to pull the plug when I get home, and I need a new air filter badly, so I will do that, too, but I am looking for any ideas what I should check for.

Frustrating, especially since it was running ok with the old plugs...

Thanks!

 

Swap plugs and see if the missfire follows that plug. Do the same for the wire on that plug. If still just that cylinder, it could be the coil, fuel injector or lack of compression or something else causing the missfire such as a coolant leak into that cylinder.

PS. if it were running fine with the old plugs and only now has the symptoms, the more then likly its a bad plug, or wire. You could have also crossthreaded the plug or cracked the porcelin. In the dark (as dark as it can get) watch all the plugs and wires closly, you might see a blue flash or hear a louder then normal snap intermittenly on 1 of the plugs or wires against the head where its grouding out against the motor instead of pushing the arc across the gap on the plug tip causing a missfire from a weak or abscent spark.

 

Good advice from GrandKodiak.
I would first reseat the wire at the coil and at the plug.

You probably just have the wire mis-seated on the coil end.
Or
wire is bad
or
Plug is bad
or wire is mis-seated on the plug..

You can get one of the old plugs - pull the wire off of the plug on #7.
Then - try it on a spare plug and look for the spark with the old plug solidly on metal and plugged into the wire.

 

Picked up another plug on the way home. After it cools down a bit, I will pull that plug wire and see what I get against the block with a fresh plug and/or the old plug from that cylinder - hopefully it is just a bum plug, and I'll be able to swap it out for the new one.

 

Bingo - started it up once it got darker out, and there was a nice light show going on in cylinder 7 - it was like the plug was throwing a little rave in there.

I pulled the boot and the insulator was completely cracked in two. Blew the pieces out of the plug well with some compressed air, pulled the plug and put in the new one I picked up on the way home, and finally I had a smooth running motor.

The funny part is that I heard the plug arcing yesterday, but mistakenly thought the motor was just ticking a little - now with the new plug all is quiet...nice.

Thanks for the help guys - you all keep me sane.

 

haha without this forum, or lots of alcohol, its hard to own anything english and remain sane... women, gin and movies included.