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Discussion Starter · #1 ·
I have a '07 Fiesta (1.25 16 valve) with 125k km (77k miles) that earlier this year developed a rough idle after starting up. If I would put the engine under load right after starting up it would misfire (low power, rough running, flashing check engine light). The problem would completely disappear after a few minutes, after warming up a bit.

The problem seemed to be more pronounced either when the weather was colder or when the humidity was higher (i.e. after raining). Not sure specifically which of the factors played a larger role, but my guess is humidity. In warm and dry weather it would stutter a bit after startup (10-15 seconds) and then be on its merry way without further issues.

Initially changed the very old spark plugs but this did next to nothing for the problem. Inspected air filter, had some oil in it. Car also seemed to have burned oil since last change (level was at minimum). This led me to believe I had the PCV valve stuck open. As a result during yearly service a few days ago I changed PCV valve, valve seal and hose, along with filters, oil and timing kit.

This made the car idle beautifully and all seemed great until today when it rained heavily and it manifested again.

Since the problem is so transient and tied to external factors I need ideas to debug it further. I'm not a proper gearhead but I have some tools (multimeter, spanners and such) and I'm fairly technically inclined (but haven't worked on cars almost at all so far).
 

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Discussion Starter · #2 ·
New information: immediately after a heavy rain I took off the air filter box/engine cover and discovered water pooling on the valve cover near one of the spark plug "holes". Ingress point seems to be the washer jet, and water trickles on the filter box bezel to the back of the engine. Stupid stupid stupid design.

Did not take out the plug boot as the rain was picking up again and I had nothing to collect the pooled water, but there is a possibility that water was in the sparkplug well also.

Will be changing leads and the stupid jets (part 1442176), hopefully that will stop the leak and also fix my issue. I'll keep y'all posted.
 

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Discussion Starter · #3 ·
Rain subsided so I could wrench at it. Took plug boots out and, while there was not much water inside the spark plug wells, it was definitely damp. I suspect this is actually the main issue.

Considering whether to change leads or not; on one hand I'd rather be safe than sorry, but on the other hand I'm very curious if the water ingress was the sole problem.

PS: This is fun!
 
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