On my trip south through France, every so often the engine suffered a short bout of misfiring on one cylinder. This tended to occur after a longish period of 4000+ rpm running, followed by a potter through a 50kph zone. Giving it some beans tended to clear things again.
The plugs are NGK BR7EFS, fitted new about 3k miles ago, and gapped to 0.9mm (37thou).
This morning I took the plugs out. #1 (and #2 less so) showed a slight black "shadow" on one side of the ceramic insulator:
This was quite hard-baked and would not simply scrape off.
The other plugs were the same colour, but with no "shadow".
Is this evidence of anything untoward (the spark tracking sideways to earth, for example)?
So far, I've replaced plug #1, and will test things out at the earliest opportunity (Monday).
The plug loom itself (connectors, wires etc) looks just fine. The coils are clean and grip the plugs firmly. The plug cavities are also clean and dry as a bone. In every other respect the car goes exactly like an R400D should.