Ford Focus misfire

My Ford Focus has a misfire during warm up and at some other infrequent times. No dashboard lights showing. First diagnostic showed no fault. ECU temperature sensor changed. Second diagnostic again showed no fault. Missfire persists. No acceleration or other problems whilst running. Suggestions for cure please?

Asked on 5 June 2015 by B J Freeman

Answered by Honest John
This is usually due to rainwater collecting in the spark plug valley on top of the engine. On a well used Focus 1.6 petrol it is evaporated off quickly, but causes problems on little used cars where it can eventually rot away the cylinder head core plugs.
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