Black spark plug

mark_weaving · Post by **mark_weaving** » 20 Feb 2006, 13:44

I've just competed an head off rebuild due to cam belt failure. The car runs but is very rough on tickover, after 3K rpm it runs fine, I've fitted the belt over ten times now thinking the problem is with the timing. I've since noticed that the Spark plug on cylinder 2 is black ( sooty not damp)
Could this be the reason for the rough tickover? if so what is the problem? I don't want to take it to citroen garage if i can help it. Anyone have any suggestions. Its a Xantia 2.0 VSX R reg. Thanks in advance.

jeremy · Post by **jeremy** » 20 Feb 2006, 14:02

I'm not sure of the valve arrangement (or number for that matter) on your engine. Was there any valve damage when the belt failed? It could be that there is valve damage on no 2 cylinder and it doesn't fire properly at low revs but things get better when the engine runs faster. A compression test would reveal any problem.

Its worth checking the plug lead resistance (less than 20K ohm) and any distributor cap (probably hasn't got one.) I'm not sure if your engine has one of the suspect coil packs.

mark_weaving · Post by **mark_weaving** » 20 Feb 2006, 14:07

Jeremy, Yes it had 12 damaged valves and valve guides, They were all replaced profesionally, unfortunatly it doesn't have ht leads its all electronic and its just one unit that fits across all the plugs.

jeremy · Post by **jeremy** » 20 Feb 2006, 14:28

I'd still do a compression test to eliminate valve problems. Could be one has been adjusted tight - re-assembled with the wrong shim or some dirt under it.

alan s · Post by **alan s** » 20 Feb 2006, 20:53

Not just a faulty injector by any chance?
We had a BX gp yaya without warning which turned out to be an injector jammed open and it was giving some very rich mix to just one pot.
Rich mix = black plug.
An esy test would be to swap the injector out of that cylider with one in another to see if the problem follows it.
I suspect you are 100% certain it is soot and not oil on that plug? I know it doesn't bear thinking about, but if the piston or rings have been damaged on that pot due to the valves getting mixed up in there, it can be compression as Jeremy says or it could be oil getting past the rings too.

Alan S

jeremy · Post by **jeremy** » 20 Feb 2006, 21:03

I agree with Alan - could be injector.