corsa c no4 plug is black?

SOOTY BLACK SPARK PLUGS ARE NORMALLY DOWN TO UNBURNT FUEL OR OVER RICH MIXTURE.
MEANS THAT THE PETROL IN THAT COMBUSTION CHAMBER IS EXCESSIVE AND NOT BURNING.

Have you replaced the plugs with the correct type and temperature range. If plug range too cold plugs will soot up.

Also the engine will need to go on a longish journey(at higher engine speeds) to get plug colour to grey short runs at low speeds will lead to black plugs

IF ONE SPARK PLUG IS COLD COMPARED TO THE OTHERS THAN THE PLUG COULD BE FAULTY (SWAP IT WITH ANOTHER PLUG)

Might be faulty HT lead coil or coil pack

If plug is very oily and compression is low on that cylinder then you may have a valve stem seal problem or worn piston rings or worn burnt valve seats.

Maurice
January 2011

look at your previous listings

cdc
January 2011