A misfire caused by corroded core plugs?
This sounds strange so I will explain; in this case the vehicle is a 56 reg Ford Focus 1.6 ti vct.
Upon inspection it was found that coolant had passed through the core plugs in the cylinder head and was shorting out No.1 Spark Plug causing a misfire.
Solution, replace the core plugs? Not that simple, engine coolant should not corrode the core plugs. After further investigation the problem was
found to be water leaking from the bonnet where the washer jets attach and laying in the trough in the cylinder head which corroded the core plugs from the outside in.
Ford sell modified washer jets (they have a rubber seal at the base) to address this problem.
If you are going to tackle this job yourself please be aware that the spark plug or plugs may have corroded threads and are liable to snap.
If you require any further advice please contact RP Automotive Services.