I have a 2003 Ford Fiesta LX. This morning the engine was running very lumpy. After around 15 miles the engine management light came on. ( At least I think that`s what it is. Orange.. like an engine block and next to the 7 on the rev counter ) I changed the spark plugs. They had large gaps but ok apart from that. The lumpiness has gone but the light is still on. What might be causing it?