Hey I recently got 1980 280zx from my dad and one of the first things I noticed is that I was getting really bad gas mileage (10 mpg). So I did all the normal things to try and remedy the problem such as: new air filter, new fuel filter, new spark plugs and wires, checked the cap and rotor, adjusted the ignition timing, tuned the valves. And for some reason I'm still getting 10 miles to the gallon. I also checked my spark plugs again recently, and they are very black. I believe that means the engine is running rich. Does anyone  have any ideas of why my car could be running so rich and getting bad gas mileage? Or a way to test? Thanks you so much!!

Two off the bat:

1. High fuel pressure
2. Cold start injector always firing

Better answer:

You can find the factory service manual at http://xenonzcar.com/s130/FSM/1980fsm.php. In the EF section there is a troubleshooting section. Unfortunately it is a little vague.

The sensors are definitely good things to test, though.

Two more might be blown FPR  diaphragm and bad/disconnected/high resistance coolant temperature sensor.

This book is good.  You really really really need a multimeter to work on EFI.  Hope you have one.  There's an Electrical Test section.