They may give you a hard time even with the FI. If the dealer is to try to find the reason why you are running rich (which has to do with the info from the intake and various exhaust sensors) they may say, well you have an aftermarket exhaust which may be causing your condition. As you may already know, running rich is bad for your car. Fouls the plugs, bad for your CAT and of course really poor fuel economy.

The irony is if it were that simple I would be happy but my brother checked the sensors with this tool and they are ok. I have an extended warrantee untill 100k miles so I am all set in that regard. It's the not knowing that is killing me.

OP are you sure you didn't get the case of "my car is louder so my foot got heavier"? Theoretically a muffler delete really shouldn't have any plan in a/f ratios but I'd like to see what the outcome of this really is. I highly doubt it relates to the muffler delete

Maybe put the stocker back on and see if the mpg goes back?

i have done a muffler delete and tb bypass's.that was my first mods and when i bought the car,the computer said i was getting 18.5 miles to the gallon.after delete and bypasses,i havnt seen under 20.

I just did the muffler delete last week and had the tips custom welded back on and sometimes usually when I am getting on it I can smell gas. I mentioned it to my buddy and he said it means its running rich. I asked the shop before hand if it could have any negative impact and they said no it will just change the sound. So if it is causing a rich condition (which would suck...) then I will go have the muffler put back on.

I just got a tune this past weekend, and after several discussions with various people of varying knowledge, I believe that the car runs rich regardless. Has anyone ever seen a stock 370Z that isn't running rich? With an 11:1 compression, it's likely Nissan just being cautious. They have to run it even richer to account for uncaring drivers who use regular gas.

I amfully stock except for intakes (which, if anything, should make it leaner) and my tuner said I was running extremely rich.

Let me revise this and say it doesn't bluntly say your O2 sensor is messed up when you look it up. It was a ver common P0300 code and maybe something else that did nothing to help track it down. Data logging was the only way I saw it.

After installing my FI exhaust my gas mileage went back to normal.

It's your o2 sensor. Happened to my Z also

so you fixed it? What was it?

If your muffler delete was rattling, then yes, the knock sensors could have picked up vibrations.

Im not sure how loud it was, but if you car was crazy loud, then the knock sensors can pick up on that in general.

any updates?

Still running with stock gas mileage when I installed the FI exhaust. I honestly cannot give you a definitive answer as to why it ran rich with the muffler delete but perhaps the knock sensor theory has some merit? I don't think so but perhaps.

Shortly after the FI install I also installed the Takeda intake. The only change was a HP increase. Mileage while cruising on the highway is more or less the same. I have 15k + miles with both mods without any issues. So I consider the issue resolved.