Poor Fuel Mileage

Check for fuel leaks.
Check for clogged air filter.
Check compression.
Check fuel pressure.
Check cooling system.
Check ignition and valve timing.
Check spark plugs.
Check for faulty throttle position sensor (if equipped).
Check for faulty coolant (water) temperature sensor.
Check for faulty ECM.
Check for high idle RPM.
Check for malfunctioning electronic fuel injection system.
Check for malfunctioning distributor mechanical and vacuum advance (if equipped).
Check for malfunctioning EGR valve (if equipped).
Ensure sufficient secondary spark is available.