Poor Fuel Mileage

Ensure fuel pump is okay. Check fuel pressure and pressure regulator. Inspect for fuel leaks or restrictions in return line.
Check injector operation by individually unplugging connectors. Engine RPM should drop noticeably. Check opening duration. Inspect injectors for leaks.
Check coolant temperature sensor.
Check Manifold Absolute Pressure (MAP) sensor (if equipped).
Check intake air temperature sensor (if equipped).
Check throttle position sensor.
Verify mass airflow sensor filament is not broken and burn-off function is okay (if equipped).
Ensure electrical harness, connectors, and wires are not broken or loose. Check all input signals to ECM.