Poor Fuel Mileage

Ensure fuel pump is okay. Check fuel pressure and pressure regulator. Check for fuel leaks or restrictions in return line.
Check injector operation by unplugging connectors one at a time. Engine RPM should drop noticeably. Check opening duration. Check injectors for leaks.
Check coolant temperature sensor.
Check throttle position sensor.
Ensure mass airflow meter filament is not broken and burn-off function is okay.
Ensure electrical harness, connectors and wires are not broken or loose. Check all input signals to ECU.