How come it only lets me fill up 8-10gallons from empty 2004 350z enthusiast

 

Welcome- but components on a car that's 18 years old have been known to go south. The floats for the sending unit on the fuel pump may be defective. Only you can decide if it's worth it to pull the pump and replace/repair it.

 

There was a TSB for this back in the day.
See Page 7 for the relevant part numbers. You have to drop the tank to replace the hoses.

https://static.nhtsa.gov/odi/tsbs/20...44508-9999.pdf

 

I have a 2006 Roadster and the problem you describe happens to me 1-3 time a year. If I drive a few miles I can go back to the gas station and then I can fill it. I've tried bouncing the car when the pump stops and it works some times. I called the dealer about the TSB but they want to charge $150 for a diagnostic and will only waive the charge if the problem is the Evaporative emission vapor vent tube.

 

I’m sure you reset your trip A or Trip B when you fill. How many miles are you getting and since you reset your mpg tracker when filling , what is your average mpg for that tank of fuel. With that data and basic math skills you should know how many gallons were burned out of the 20 gallons tank and know how many you will put in to fill.

 

My 2003 does this. I've just learned to deal with it

 

The work around is turning the pump handle upside down.