I recently changed plugs, oil, filter etc and a few day's later, the engine started to misfire. I checked the plugs only to find No4 plug had broken up (ceramic surround). I cleaned out all of the debris (i beleive) and replaced with a new plug. Still misfiring and backfiring now.

I have now replaced the plug leads and a new set of plugs. Running better but still misfiring, no backfiring yet. Any ideas or do I need a diagnostic?

Chris Lee, August 2008

Sorted the issue. The replacement plug lead on No4 was not sitting properly and as a consequence was arcing (white residue on lead).

I re-seated the lead at a slight angle and the lead clicked into place. No more issue.

As for the plug disintergrating, I have no idea how this occurred. I am fortunate to have a car hoover with a pipe just slighlty less than that of the plug area, so I was able to remove all debris before removing the plug. I dread to think of the catastophy that would of caused. Anyhow, thank you for your response.

Chris Lee, August 2008

how the hell did a spark plug disintegrate?Id be worried that a portion of debris has dammaged the valve seat.only way to check,shot off strip down would be a compression leakage test.Normal comp test might or might not show up difference in cyl comps.A small bit of dammage would be enough to reduce comp and so missfire,that would tie in with backfire,ie valve not fully closed allowing unburnt fuel into exhaust and impropper valve seal allowing spark/hot gasses into exhaust and detonating same.Short answer,take off head and remove and inspect valves/seats.

caine, August 2008