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).
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).
