Hi, recently figured out a new test that I haven't seen before. On the new GDI systems they suffer from carbon build up on the back of the inlet valves (in my opinion due to the fuel now not being sprayed on to the back of the valves). Due to this carbon build up the cylinder can not draw all the air required to burn the mixture correctly. You will have no fault codes and as you raise the RPM to about 1200 -1500 the misfire will become most apparent.you should be able to pick this up on the misfire counter easily. At this point on my first experience I start swapping coils and plugs around and the misfire doesn't move, so I move injectors around still it stays on the same cylinder. So I do a relative compression and a cranking compression and still nothing. I then use an oscilloscope on the MAP and AC couple it and bingo I can see an annomilly every 4th time a valve opens.I then realised that every time I started the car for about 2 seconds the annomilly wasn't there. What could be causing that. Then I realised if I manually opened the intake runner flaps the annomilly would disappear and my misfire would go away. We have recently purchased the BG intake system cleaner. We run it through the engine and bingo the misfire was fixed. Then a different car comes in today with a GDI system with identical symptoms. So I whip out the vacuum pump pull the intake runner flaps open and boom misfire disappears. So I know what I will be selling the customer tomorrow. Summary:if you allow the cylinder to pull all the air in using both inlet valves and not just one, due to the intake runner flaps being shut the misfire will instantly disappear. I wouldn't really call it a misfire as such it's more of a slight engine judder just off of idle. Definitely worse when engine is cold. I hope this helps someone and it saves them the pain I went through. You never know Mr scanner danner may even come across it and make a great training video on it. Don't forget to give me a mention if you do