Poor Fuel Economy

SYMPTOM DESCRIPTION:
From an actual road test the measured M.P.G. is noticeably lower than expected. At one time the fuel economy for this vehicle was at a greater average than current measurement.



SYSTEM OR COMPONENT TO CHECK

FUEL SYSTEM

CHECK:


- FUEL TYPE: Fuel quality and alcohol content (excessive alcohol content increases fuel consumption).
- FUEL PRESSURE: System pressure too high or too low. Check for restricted fuel return line or defective pressure regulator.
- FUEL INJECTOR: Leaking or sticking injector.
- FUEL TANK, HOSES AND CONNECTIONS: Fuel leaking.
- RESTRICTED AIR INTAKE: Air filter or filter screen dirty or restricted.


COMPUTER INPUTS/OUTPUTS

CHECK:

- CONTROL MODULE: Check for correct ECM and connections.
- HEATED OXYGEN SENSOR: False lean indication. Check for exhaust manifold leaks, check oxygen sensor installation, wiring and connections.

NOTE: Also check for plugged or restricted injector(s). If one or more injectors are restricted, or plugged, the corresponding cylinders pump oxygen into the exhaust manifold. The system will go to "full rich" in attempt to compensate for the apparent lean condition, with no effect on the problem cylinder(s). The oxygen sensor continues to indicate a lean condition and the system stays at "full rich".

- MASS AIRFLOW SENSOR: Defective sensor, poor connections or damaged wiring to ECM.

Refer to COMPUTERS AND CONTROL SYSTEMS.



IGNITION SYSTEM

CHECK:

- SPARK PLUGS: Remove spark plugs, check for wet plugs, cracks, wear, improper gap, burned electrodes, or heavy deposits. Repair or replace as necessary.
- IGNITION WIRES: check for cracking, hardness, and high secondary resistance or incorrect firing order.
- KNOCK SENSORS: Defective sensor, dirt between mating surfaces, poor connections to control module.
- IGNITION DISTRIBUTOR, CAP, AND ROTOR: Check for cracks, arcing, moisture, rust deposits or high resistance through rotor.

Refer to IGNITION SYSTEM.



COOLING SYSTEM

CHECK FOR OVERHEATED ENGINE:

- AUXILIARY COOLING FAN: Temperature switch defective or damaged wiring to control module.
- ENGINE COOLANT: Low engine coolant level and/or leaks, coolant solution - should be a 50/50 mix of anti-freeze (coolant) and water.
- WATER PUMP: Loose belt?
- RADIATOR: Restricted air flow to radiator or restricted coolant flow through radiator.
- THERMOSTAT: Faulty or incorrect.



MECHANICAL

CHECK:

- ENGINE COMPRESSION: Perform engine compression and cylinder leak down test. Check for leaking valves, worn piston rings (wet & dry test), or blown head gasket.
- ENGINE CAMSHAFT: Check for worn cam lobes &/or incorrect valve timing.
- TORQUE CONVERTER CLUTCH: Check operation.
- TRANSMISSION SHIFT POINTS (AUTO TRANS): Check for correct shift points/vehicle speed.
- CLUTCH OPERATION: Worn or damaged clutch disc or pressure plate.
- TIRES: Check tires for correct inflated pressure.