Hi all. First time posting. I'd thought it might be helpful to share what I found out while fixing my random misfire on cylinder 1 (code P0301).
I had a coil pack fail on cylinder 1 last winter (after a very cold spell) so I replaced it and then the misfire came back a few months later. I swapped it out again and it fixed it for a week and then came back. I suspected a dirty connection at the coil end or a possible bad plug or dry joint in the ECU.
I found a few web sites with mention of testing the supply lines going to the coil connector from the DME ECU. Some techs had mentioned using an oscilloscope or a frequency meter but I was a tad suspicious so I checked these myself. I just used a cheap Digital Multimeter and tested the trigger voltage on both the 20VDC and 200VAC ranges (since, of course, it is not a constant steady value). I was expecting to see much less than 13.8V and indeed the results were approx 5.5 VDC or 10VAC. I verified these measurements were exactly the same as cylinder 2. (One might assume that these are typical values for any on-plug coil system depending on how they are being triggered or switched but please do your own tests.)
Looking into the triangular socket the pin layout is as follows (also see pic):
The right hand supply pin should be live when ignition key is switched on or engine running
In the photo you can see a length of stiff copper wire that I wrapped around the meter probe. Believe it or not, that is what fixed the problem. The action of poking the wire into the socket must have cleaned the connector just enough to make it work properly. Once I had confirmed that nothing else was causing any issues, I got some isopropyl alcohol and a small stiff brush and washed the connector as best as I could. I also made sure the blades in the sockets were tight enough. I think it would be better to replace the entire socket or loom if this is not thought to be reliable since a misfire can be dangerous if engine power drops off while overtaking.
So many of these misfire threads seem to go cold with no resolution, so I am pleased that I broke the spell. Like me, I'm sure a lot of you find it very frustrating when you read about a problem on a single cylinder, only to find the dealer has diagnosed it as something which would be bound to affect the whole engine. I always try to ensure that Occam's Razor is the first tool to come out of the box.
It might be worth mentioning that I also had a problem with the engine revs varying between 800 to 2500 while idling. Thanks to a three-word answer on a YouTube post, this was fixed by tightening down the oil filler cap which had been left loose (by yours truly) after the last investigation. One for the pub quiz there...
I'm more than happy to answer any follow-ups on this topic but I really hope the above information saves someone else some grief down the line.