RPMs are running high on a 09 370z
 

I installed Borla cats and full exhaust w a set of Stillen headers on my buddy's 09 370z. We had to order new O2 sensors for the headers since the fact ones were sealed, breaking 2 o2 sensor sockets borrowed from a local parts store (LOL). The o2 senors were not factory ones meaning we spliced the w the fact. connectors. Now the car is running at about 300-500 RPMs higher than normal, in addition to the service engine light being on and this bad boy is burning fuel (read 15 MPG driving very conservatively). I placed an additional washer on the header's o2 sensor and have o2 spacers for the cat backs.... What are we overlooking? Any and all help will be greatly appreciated.

With the service engine soon light on, the car will alway idle high, as soon as the light is cleared it will go back down. It is the same as when you start the car cold, it will idle high and take a little to go down. I believe the reason is that when the car is started, the cats are not warmed up and the fuel/emissions system runs on an open loop system and does not use feedback from the O2 sensors to determine how much fuel should be added for complete combustion. This typically results in a running rich condition, as well as the decrease in gas mileage. When the check engine light isnt on, and the cats are up to operating temp the system will go to the closed loop operation and adjust fuel based on the feedback from O2 sensor. The check engine light forces the car to continue running in the open loop situation so this will have the effect of a high idle and poor gas mileage

I think your problems might be with the "universal" oxygen sensor that you "spliced" into the OEM harness. It's probably not the new generation "heated" oxygen sensor. IE: The device and the heater are integrated with each other, enabling the sensor to start within 5 seconds after the engine key is turned ON. They're known as "planer" sensors. I don't think universal sensors are this type. (even though manufacturers "say" they are).

Do yourself a favor, and take the $$$ hit, and buy the OEM sensors from Nissan. To date there is no aftermarket sensors available for the 3.7L engines in either the 370's or the G's.

:iagree:...Everything would seem to point to that..

yeah the header o2 sensors are actually factory widebands so i hope you installed a five wire wideband sensor instead of a five wire narrow band. what was the brand and part number of the sensor you bought? if you only have the store p/n tell us where you bought it from as well. i'll look it up for you and double check it with the manual. any chance of getting the output voltage reading?

edit: also check for gasket leaks and header bolts that have loosened up. i always get the car up to heat with a hard drive let it cool and retorque the bolts thenext day as they like to come loose. if you're getting air in there it would rn excessivly rich