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As I started typing this I realised I could test this easily myself by simply unplugging one coil while the engine is running, But I may as well keep typing now ive started...
Im having a intermittant problem on my skyline where the AFR's seem to be about 0.5-1 point leaner then normal... Ive checked fuel pressure and thats always been constant... Im thinking I may have a intermittant missfire on one or more cylinders... On a 6 cylinder say at cruise AFR is normally 14.7:1 if one cylinder was missfiring what would you expect AFR's to be (assuming no closed loop)
Misifire would cause a lean reading because of unburned oxygen is making its way thru the engine into the exhaust stream.
I would say every engine is different, but just yesterday we've had a Golf MK VI GTI on our dyno and it misfiring on one cylinder every now and then. The spike was about 3% leaner than the regaluar AFR, just to give you an idea of what I experienced ;)
It would really depend on rpm and duration of the cylinder drop but if your dropping from 14.7-15.2 or 15.7 then I'd look at a triggering issue.
Is there any other symptoms that could help narrow it down?
I'm not convinced you could specify exactly how lean the AFR would read due to a misfire. For example is it just a single engine cycle that the miss occurred, or did it miss for multiple engine cycles?
I know it's tricky but usually you will be able to feel and/or hear a misfire event that correlates to a lean reading on the wideband. Nissans are notorious for problems with ignition coils so it's quite likely your issue may be around this.