2012 R56 S with N18 engine manual transmissions.H flow filter with decat downpipe and a working (for now) spacer for second o2 sensor. Brand new fuel filter (issue existed before filter). Checked spark plugs, checked coils, so that's out of the way.

Issue is this; if I try to give some sudden power in anything but first gear (and if I try to really hit it in first gear) I will get a misfire right away. Codes thrown are misfires on all cylinders and random misfire code. Throws the car into limp mode most of the time unless I'm quick to correct the issue. Misfire sounds like a deep exhaust sound, and yes an odd example, but perhaps like a major wet fart. Can't think of any other way to describe it.

Known issues; occasional "check gas cap" on dash, gas cap is secured. Occasional o2 sensor CEL due to the decat. Mild coolant leak that I've yet to track down. A slight but noticable amount of white smoke rising from oil cap when it is opened after the engine runs for a minute.

A few things; replaced a bad coil that was causing a constant misfire on 4th cylinder. Spark plugs are slight dark and are Bosch brand. Will be testing another minis spark plugs tomorrow so we will see what that does. On the Bosch ones I did check the gaps, .22. The rumbling misfire is worse the higher the gear and even worse in sport which is directly related to the power being fed to it.

Anyone else seen this and, more importantly, found a fix? At this point any ideas are welcome.

 

First, I would do a compression test and perhaps even a leak down to get a status of engine condition. Second, I would check fuel pressure at idle and at high rpm under load. Third, assuming ignition is good bad ECU ? (guessing)

 

We may have tracked it to a crack in the intake line where it meets the turbo. Replacing later today. We will see what happens!