So i´ve already posted about this issue a while back (not sure what happened to my old account). These problems first started in the year 2014 (66,000 miles), just a couple of times the car misfired after startup but it quickly went away. at the end of 2016 (74,000 miles) the misfiring suddenly appeared after a brutal wheel hop when accelerating in wet conditions. That misfire was fixed with new plugs, cap and rotor.
Then in the first drive of 2020 (77,000 miles) the misfire had come back. The weather was quite damp and cold, so i immediately thought that could play a role with the misfire. Funny enough the misfire cleared itself after i drove the car to its operating temperature. Then we got a two month streak of warm and dry weather here in Finland, and during that period i got zero problems with misfires. Then came the first proper rainy day at the end of June and there it was again...Weird thing is, even in extremely wet conditions, the misfire goes away when the car reaches its operating temperature.
The misfire has always situated at a rev range of idle -> 2500 rpm. At least there it is at its worst. Car has had a major service by a local Porsche Center at 66,000 miles where they did a variety of the "bigger" services i.e adjusted the valves. After that the plugs, cap and rotor were changed at 74,000 miles and now at 78,000 miles i´ve changed the ignition wires, ignition coil, plugs (Bosch WRR4C) and lastly the DME relay. The parts have made 0 difference to the misfire, but the car runs noticeably better when warmed up and has more power after replacing the old 33 year old parts so that´s something.
At the moment i am running out of ideas what could be the cause of these persistent misfires after so many parts have been replaced. Tried to search the forums for same kind of problems (misfiring only when engine is cold or in wet weather) but did not find any threads. So does anyone have any idea what could be be the reason for this weirdness? Any help would be greatly appreciated.
The attached picture shows the terrible condition of the 4000 miles driven spark plugs that i just changed for new ones.
Oh, i´ve also installed a steve wong chip (correct model, yes) at 70,000 miles and adjusted the ignition for RON 98 fuel from the Motronic box.