Hello XXXXXXXXX
Thank you for choosing Just Answers, I will try to help you make sense of the scanner readings.
Long and short term fuel trim numbers, taken as a snapshot like you have, are actually not real helpful in determing a problem. The computer system uses a rich/lean averaging system to try to keep the fuel ratio within its designed ratios, so it may be that you caught those readings at a rich or lean time. More helpful is an average of the long term and short term fuel trims.
I don't think that the SCV system is causing any problems, but there are specific tests to ensure that it is operating as designed, and I am researching the system to ensure I am not steering you wrong.
The one parameter that is often overlooked is Fuel Pressure. If it is too high, due to a restricted return line or bad regulator, then it can cause your problems. But according to your posts, you have high hydrocarbons, which instead of indicating a mixture problem, indicate lack of combustion, most likely an ignition issue. As a professional technician, even though I know it SHOULDN't make a difference, I only will install NGK spark plugs in Nissan vehicles to avoid having problems down the road. I have fought a slight misfire/rough idle problem in the past on Nissans that was corrected, after hours of searching for a reason, by simply replacing the spark plugs.
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ASE MAstertech
ASE Mastertech, 30+ years working on all makes and models in independant repair shops.