How is OBD data transmitted? We would need to find out if it goes to the Navi unit.

I doubt it does.

Over the Canbus line you should be able to pick up everything you need. I still think Canbus would be the way to go. Just way too complicated for someone to just 'hack' in one afternoon. The more I think about it a small microcontroller would probably be the easiest way. Grab the injector signal and it can calculate the duty cycle. Now, if we can just figure out fuel pressure? I've never heard of any fuel pressure guage in the car, but I would imagine there is one somewhere?

 

The NAV definitly doesn't use the OBD II stuff for mpg. Internally it probably transmits everything using nissan's propriatary consult II protocol. This may be some form of CAN bus at the hardware level, but theres at least another proprietary layer over it since our OBD II port can not be used with CAN based OBD II readers. More on the consult II specifics can be found here: http://www.mp3car.com/vbulletin/showthread.php?t=18690 Its a start, but its definitly going to take a lot of work to get it fully working, but if you did fully hack this you could get absolutly everything about the car systems as well as set things.

Fuel pressure is available through PID 0x0A in OBD II but not anything dealing with fuel injection.