Engine running rough

[PottsIan]

I have a 2016 Yaris Hybrid which has been faultless until recently when the engine started running rough.

I took it to my local garage who were puzzled as to the cause as there was no engine warning light appearing nor any error codes on the computer. Then after driving it around a bit more the engine warning light came on and further computer analysis showed that No1 cylinder was misfiring. Further investigation revealed that the coil which sits above each spark plug was faulty, the fault was caused by water getting down into the space between the coil and the spark plug and causing it to short. The water must have got in to the engine bay during the recent downpours we have been having and unfortunately on most new cars the spark plugs are situated on the top of the cylinder head, hence making it easy for water to gain access. The problem was resolved with a new coil but the question remains as to how to stop it happening again now we are experiencing very unpredictable weather.

[Gray86]

Have you checked the water drains on the scuttle at the bottom of the windscreen - usually on both sides to allow water to clear away rather than get into the engine bay. They can block up with leaves & foliage