Looking for some opinions - I had my 2008 G37 tuned today. They were able to get everything smoothed out nicely (there were some odd bumps to start with...). The problem we ran into was the fuel trims around 2-3000 were around 100% on bank 2, and 85% bank 1. Seemed to even back out to 100% once you got about 4k. We weren't sure whether to tune to the high side or the low side. Bank 1 looked a little slow to respond, so I'm thinking maybe 02 sensor?

Mods:
Injen intake, cat back exhaust, Motordyne M370 intake manifold, Moroso catch can.

78,000 miles. Have not changed plugs, O2 sensors. I drive the car like it's supposed to be driven

I noticed when I did the intake one bank had a slight gold sheen on the lower manifold, while the other was very clean. I thought it was bank 2 with the gold oil sheen (nothing out of the ordinary), but am doubting my memory now. I'm going to try the brake cleaner trick to check for manifold leaks, but I don't think that is the problem. No check engine lights and idle has been perfect since manifold and clean throttle bodies.

Any suggestions where to start first? I'd rather not get into injectors and coils unless there is real chance those will be an issue.

 

Could be your upstream bank 1 sesor has a lot of carbon build up on it.

Remove it get a torch and put the flame on the tip till its red hot to burn off all the carbon build. You can also plug a volt meter to it and check if it's reading faster after that. that.

 

Thanks for the tip - hadn't thought about that. Amazon has one NTK sensor left for $180, so I may snap that up.

 

Our cars have a AFR sensor (pre-cat) not an O2 sensor... This means you can't measure the voltage to see how it responds.

BUT you can use an OBD2 dongle and phone app (torque or fusion) to read the AFR on bank 1 and 2. (and many more things...)

Fuel trims are normally around -10 to +10% (for long term & short term), not 100% or 85%.
The only thing that falls in that range is "Short term fuel trim (Bank 1 or 2, Sensor 2)" and both of mine are 99.22% on stock tune & car. Sensor 2 is a typical O2 sensor and you can read its value in volts using the app mentioned above.

Is it possible that one of the catch cans has a slight leak? Look at your OBD fuel trims for both banks and report back the numbers. They should be within 3-5% of each other.

 

I recall they said short term fuel trim, so that 's maybe what it would be? I'm confused now if it's getting signal from upstream or downstream sensor? There is only one catch can for the M370, since it tee's into one line. I checked for leaks by spraying carb cleaner and didn't hear or see any idle change. I don't think the problem is a leak.

Where can you get an OBD2 cable? I have some old OBD1 cables, but those are way out of date.