If you drain all the coolant from an 03 350z on level ground using the plug at the bottom of the radiator, how much coolant volume should you expect? I'm only getting about a gallon. I was reading that a complete fill should take about 2.5 gal (10 quarts) Is there 1.5 gallons that usually remains in the engine ?

 

Yes, 2.5 gal is when you drain the block also. Since your draining from the radiator, assuming your car isn't anywhere close to operating temperature, the thermostat is closed keeping the remain 1.5 gal in the block.

 

You can fill and drain the coolant twice.
Run the motor with the heater ON.
That should get 80 percent of the old coolant out of the block.