Hi.. you can see there's a variety of proposed answers - here's my experience (Fiesta 1.4 Zetec 2004).
- 35k miles : misfire at high revs; on inspection i found rusty water around spark plugs and i immediately assumed this was leaking core plugs and had these replaced...in error
- 40k miles : problem reoccurred the following winter and i found that it was the washer jet sealing as mentioned elsewhere; should've realised first time as water was rusty, not antifreeze coloured
- 60k miles: really serious misfire on light throttle which turned out to be the coil pack and not connected in anyway to water
- 65k miles: blown head gasket, broken front coil spring (all joints/nuts seized making replacement a nightmare) and starter motor not engaging...all occurred within 50 miles of each other. Not a good advert for Ford !
Jon
June 2011